On Fri, Oct 23, 2020 at 09:33:38PM -0400, Jason Merrill via Gcc-patches wrote:
> On 10/23/20 3:01 PM, Marek Polacek wrote:
> > This patch implements the -Wvexing-parse warning to warn about the
> > sneaky most vexing parse rule in C++: the cases when a declaration
> > looks like a variable definition, but the C++ language requires it
> > to be interpreted as a function declaration.  This warning is on by
> > default (like clang++).  From the docs:
> > 
> >    void f(double a) {
> >      int i();        // extern int i (void);
> >      int n(int(a));  // extern int n (int);
> >    }
> > 
> >    Another example:
> > 
> >    struct S { S(int); };
> >    void f(double a) {
> >      S x(int(a));   // extern struct S x (int);
> >      S y(int());    // extern struct S y (int (*) (void));
> >      S z();         // extern struct S z (void);
> >    }
> > 
> > You can find more on this in [dcl.ambig.res].
> > 
> > I spent a fair amount of time on fix-it hints so that GCC can recommend
> > various ways to resolve such an ambiguity.  Sometimes that's tricky.
> > E.g., suggesting default-initialization when the class doesn't have
> > a default constructor would not be optimal.  Suggesting {}-init is also
> > not trivial because it can use an initializer-list constructor if no
> > default constructor is available (which ()-init wouldn't do).  And of
> > course, pre-C++11, we shouldn't be recommending {}-init at all.
> 
> What do you think of, instead of passing the type down into the declarator
> parse, adding the paren locations to cp_declarator::function and giving the
> diagnostic from cp_parser_init_declarator instead?

Interesting idea.  I suppose it's better, and makes the implementation
more localized.  The approach here is that if the .function.parens_loc
is UNKNOWN_LOCATION, we've not seen a vexing parse.

The tests haven't changed at all in this patch.

Thanks,

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --
This patch implements the -Wvexing-parse warning to warn about the
sneaky most vexing parse rule in C++: the cases when a declaration
looks like a variable definition, but the C++ language requires it
to be interpreted as a function declaration.  This warning is on by
default (like clang++).  From the docs:

  void f(double a) {
    int i();        // extern int i (void);
    int n(int(a));  // extern int n (int);
  }

  Another example:

  struct S { S(int); };
  void f(double a) {
    S x(int(a));   // extern struct S x (int);
    S y(int());    // extern struct S y (int (*) (void));
    S z();         // extern struct S z (void);
  }

You can find more on this in [dcl.ambig.res].

I spent a fair amount of time on fix-it hints so that GCC can recommend
various ways to resolve such an ambiguity.  Sometimes that's tricky.
E.g., suggesting default-initialization when the class doesn't have
a default constructor would not be optimal.  Suggesting {}-init is also
not trivial because it can use an initializer-list constructor if no
default constructor is available (which ()-init wouldn't do).  And of
course, pre-C++11, we shouldn't be recommending {}-init at all.

I also uncovered a bug in cp_parser_declarator, where we were setting
*parenthesized_p to true despite the comment saying the exact opposite.

gcc/c-family/ChangeLog:

        PR c++/25814
        * c.opt (Wvexing-parse): New option.

gcc/cp/ChangeLog:

        PR c++/25814
        * cp-tree.h (struct cp_declarator): Add the function::parens_loc
        member.
        * parser.c (make_call_declarator): New location_t parameter.  Use it
        to set declarator->u.function.parens_loc.
        (cp_parser_lambda_declarator_opt): Pass UNKNOWN_LOCATION to
        make_call_declarator.
        (warn_about_ambiguous_parse): New function.
        (cp_parser_init_declarator): Call warn_about_ambiguous_parse.
        (cp_parser_declarator): Set *parenthesized_p to false rather than to
        true.
        (cp_parser_direct_declarator): Have
        cp_parser_parameter_declaration_clause set a flag if it encountered
        a vexing parse.  Crate a location for the function's parentheses and
        pass it to make_call_declarator.
        (cp_parser_parameter_declaration_clause): New bool * parameter with
        a default value.  Set it.  Pass it to
        cp_parser_parameter_declaration_list.
        (cp_parser_parameter_declaration_list): New bool * parameter.  Set it.
        (cp_parser_cache_defarg): Adjust the call to
        cp_parser_parameter_declaration_list.

gcc/ChangeLog:

        PR c++/25814
        * doc/invoke.texi: Document -Wvexing-parse.

gcc/testsuite/ChangeLog:

        PR c++/25814
        * g++.dg/cpp2a/fn-template16.C: Add a dg-warning.
        * g++.dg/cpp2a/fn-template7.C: Likewise.
        * g++.dg/lookup/pr80891-5.C: Likewise.
        * g++.dg/lto/pr79050_0.C: Add extern.
        * g++.dg/lto/pr84805_0.C: Likewise.
        * g++.dg/parse/pr58898.C: Add a dg-warning.
        * g++.dg/template/scope5.C: Likewise.
        * g++.old-deja/g++.brendan/recurse.C: Likewise.
        * g++.old-deja/g++.jason/template4.C: Likewise.
        * g++.old-deja/g++.law/arm4.C: Likewise.
        * g++.old-deja/g++.mike/for2.C: Likewise.
        * g++.old-deja/g++.other/local4.C: Likewise.
        * g++.old-deja/g++.pt/crash3.C: Likewise.
        * g++.dg/warn/Wvexing-parse.C: New test.
        * g++.dg/warn/Wvexing-parse2.C: New test.
        * g++.dg/warn/Wvexing-parse3.C: New test.
        * g++.dg/warn/Wvexing-parse4.C: New test.
        * g++.dg/warn/Wvexing-parse5.C: New test.
        * g++.dg/warn/Wvexing-parse6.C: New test.

libstdc++-v3/ChangeLog:

        PR c++/25814
        * testsuite/20_util/reference_wrapper/lwg2993.cc: Add a dg-warning.
        * testsuite/25_algorithms/generate_n/87982_neg.cc: Likewise.
---
 gcc/c-family/c.opt                            |   4 +
 gcc/cp/cp-tree.h                              |   1 +
 gcc/cp/parser.c                               | 169 ++++++++++++++++--
 gcc/doc/invoke.texi                           |  34 +++-
 gcc/testsuite/g++.dg/cpp2a/fn-template16.C    |   2 +-
 gcc/testsuite/g++.dg/cpp2a/fn-template7.C     |   2 +-
 gcc/testsuite/g++.dg/lookup/pr80891-5.C       |   2 +-
 gcc/testsuite/g++.dg/lto/pr79050_0.C          |   2 +-
 gcc/testsuite/g++.dg/lto/pr84805_0.C          |   2 +-
 gcc/testsuite/g++.dg/parse/pr58898.C          |   4 +-
 gcc/testsuite/g++.dg/template/scope5.C        |   2 +-
 gcc/testsuite/g++.dg/warn/Wvexing-parse.C     | 110 ++++++++++++
 gcc/testsuite/g++.dg/warn/Wvexing-parse2.C    |  24 +++
 gcc/testsuite/g++.dg/warn/Wvexing-parse3.C    | 129 +++++++++++++
 gcc/testsuite/g++.dg/warn/Wvexing-parse4.C    |  74 ++++++++
 gcc/testsuite/g++.dg/warn/Wvexing-parse5.C    |  14 ++
 gcc/testsuite/g++.dg/warn/Wvexing-parse6.C    |  24 +++
 .../g++.old-deja/g++.brendan/recurse.C        |   2 +-
 .../g++.old-deja/g++.jason/template4.C        |   2 +-
 gcc/testsuite/g++.old-deja/g++.law/arm4.C     |   2 +-
 gcc/testsuite/g++.old-deja/g++.mike/for2.C    |   2 +-
 gcc/testsuite/g++.old-deja/g++.other/local4.C |   2 +-
 gcc/testsuite/g++.old-deja/g++.pt/crash3.C    |   2 +
 .../20_util/reference_wrapper/lwg2993.cc      |   2 +-
 .../25_algorithms/generate_n/87982_neg.cc     |   2 +-
 25 files changed, 582 insertions(+), 33 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/warn/Wvexing-parse.C
 create mode 100644 gcc/testsuite/g++.dg/warn/Wvexing-parse2.C
 create mode 100644 gcc/testsuite/g++.dg/warn/Wvexing-parse3.C
 create mode 100644 gcc/testsuite/g++.dg/warn/Wvexing-parse4.C
 create mode 100644 gcc/testsuite/g++.dg/warn/Wvexing-parse5.C
 create mode 100644 gcc/testsuite/g++.dg/warn/Wvexing-parse6.C

diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 1009defbf16..bcd1db2b13a 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -1265,6 +1265,10 @@ Wvarargs
 C ObjC C++ ObjC++ Warning Var(warn_varargs) Init(1)
 Warn about questionable usage of the macros used to retrieve variable 
arguments.
 
+Wvexing-parse
+C++ ObjC++ Warning Var(warn_vexing_parse) Init(1)
+Warn about the most vexing parse syntactic ambiguity.
+
 Wvla
 C ObjC C++ ObjC++ Var(warn_vla) Init(-1) Warning
 Warn if a variable length array is used.
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 5c06ac3789e..2054c2a8672 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -6052,6 +6052,7 @@ struct cp_declarator {
       tree late_return_type;
       /* The trailing requires-clause, if any. */
       tree requires_clause;
+      location_t parens_loc;
     } function;
     /* For arrays.  */
     struct {
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 7ec7d42773c..3e414ce07e2 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -1438,7 +1438,8 @@ clear_decl_specs (cp_decl_specifier_seq *decl_specs)
    VAR_DECLs or FUNCTION_DECLs) should do that directly.  */
 
 static cp_declarator *make_call_declarator
-  (cp_declarator *, tree, cp_cv_quals, cp_virt_specifiers, cp_ref_qualifier, 
tree, tree, tree, tree);
+  (cp_declarator *, tree, cp_cv_quals, cp_virt_specifiers, cp_ref_qualifier,
+   tree, tree, tree, tree, location_t);
 static cp_declarator *make_array_declarator
   (cp_declarator *, tree);
 static cp_declarator *make_pointer_declarator
@@ -1621,7 +1622,8 @@ make_call_declarator (cp_declarator *target,
                      tree tx_qualifier,
                      tree exception_specification,
                      tree late_return_type,
-                     tree requires_clause)
+                     tree requires_clause,
+                     location_t parens_loc)
 {
   cp_declarator *declarator;
 
@@ -1635,6 +1637,7 @@ make_call_declarator (cp_declarator *target,
   declarator->u.function.exception_specification = exception_specification;
   declarator->u.function.late_return_type = late_return_type;
   declarator->u.function.requires_clause = requires_clause;
+  declarator->u.function.parens_loc = parens_loc;
   if (target)
     {
       declarator->id_loc = target->id_loc;
@@ -2279,9 +2282,9 @@ static tree cp_parser_type_id_1
 static void cp_parser_type_specifier_seq
   (cp_parser *, cp_parser_flags, bool, bool, cp_decl_specifier_seq *);
 static tree cp_parser_parameter_declaration_clause
-  (cp_parser *, cp_parser_flags);
+  (cp_parser *, cp_parser_flags, bool * = nullptr);
 static tree cp_parser_parameter_declaration_list
-  (cp_parser *, cp_parser_flags);
+  (cp_parser *, cp_parser_flags, bool *);
 static cp_parameter_declarator *cp_parser_parameter_declaration
   (cp_parser *, cp_parser_flags, bool, bool *);
 static tree cp_parser_default_argument
@@ -11234,7 +11237,8 @@ cp_parser_lambda_declarator_opt (cp_parser* parser, 
tree lambda_expr)
                                       tx_qual,
                                       exception_spec,
                                        return_type,
-                                      trailing_requires_clause);
+                                      trailing_requires_clause,
+                                      UNKNOWN_LOCATION);
     declarator->std_attributes = std_attrs;
 
     fco = grokmethod (&return_type_specs,
@@ -20603,6 +20607,100 @@ strip_declarator_types (tree type, cp_declarator 
*declarator)
   return type;
 }
 
+/* Warn about the most vexing parse syntactic ambiguity, i.e., warn when
+   a construct looks like a variable definition but is actually a function
+   declaration.  TYPE is the return type of such a function; DECLARATOR is
+   the declarator for this function declaration.  */
+
+static void
+warn_about_ambiguous_parse (tree type, const cp_declarator *declarator)
+{
+  if (declarator->kind != cdk_function
+      || !declarator->declarator
+      || declarator->declarator->kind != cdk_id
+      || !identifier_p (get_unqualified_id
+                       (const_cast<cp_declarator *>(declarator))))
+    return;
+
+  location_t loc = declarator->u.function.parens_loc;
+
+  if (loc == UNKNOWN_LOCATION)
+    return;
+
+  if (TREE_CODE (type) == TYPE_DECL)
+   type = TREE_TYPE (type);
+
+  /* If the return type is void there is no ambiguity.  */
+  if (same_type_p (type, void_type_node))
+    return;
+
+  auto_diagnostic_group d;
+  tree params = declarator->u.function.parameters;
+  const bool has_list_ctor_p = CLASS_TYPE_P (type) && TYPE_HAS_LIST_CTOR 
(type);
+
+  /* The T t() case.  */
+  if (params == void_list_node)
+    {
+      if (warning_at (loc, OPT_Wvexing_parse,
+                     "empty parentheses were disambiguated as a function "
+                     "declaration"))
+       {
+         /* () means value-initialization (C++03 and up); {} (C++11 and up)
+            means value-initialization or aggregate--initialization, nothing
+            means default-initialization.  We can only suggest removing the
+            parentheses/adding {} if T has a default constructor.  */
+         if (!CLASS_TYPE_P (type) || TYPE_HAS_DEFAULT_CONSTRUCTOR (type))
+           {
+             gcc_rich_location iloc (loc);
+             iloc.add_fixit_remove ();
+             inform (&iloc, "remove parentheses to default-initialize "
+                     "a variable");
+             if (cxx_dialect >= cxx11 && !has_list_ctor_p)
+               {
+                 if (CP_AGGREGATE_TYPE_P (type))
+                   inform (loc, "or replace parentheses with braces to "
+                           "aggregate-initialize a variable");
+                 else
+                   inform (loc, "or replace parentheses with braces to "
+                           "value-initialize a variable");
+               }
+           }
+       }
+    }
+  /* The T t(X()) case.  */
+  else if (list_length (params) == 2)
+    {
+      if (warning_at (loc, OPT_Wvexing_parse,
+                     "parentheses were disambiguated as a function "
+                     "declaration"))
+       {
+         gcc_rich_location iloc (loc);
+         /* {}-initialization means that we can use an initializer-list
+            constructor if no default constructor is available, so don't
+            suggest using {} for classes that have an initializer_list
+            constructor.  */
+         if (cxx_dialect >= cxx11 && !has_list_ctor_p)
+           {
+             iloc.add_fixit_replace (get_start (loc), "{");
+             iloc.add_fixit_replace (get_finish (loc), "}");
+             inform (&iloc, "replace parentheses with braces to declare a "
+                     "variable");
+           }
+         else
+           {
+             iloc.add_fixit_insert_after (get_start (loc), "(");
+             iloc.add_fixit_insert_before (get_finish (loc), ")");
+             inform (&iloc, "add parentheses to declare a variable");
+           }
+       }
+    }
+  /* The T t(X(), X()) case.  */
+  else
+    warning_at (loc, OPT_Wvexing_parse,
+               "parentheses were disambiguated as a function "
+               "declaration");
+}
+
 /* Declarators [gram.dcl.decl] */
 
 /* Parse an init-declarator.
@@ -20797,6 +20895,13 @@ cp_parser_init_declarator (cp_parser* parser,
            }
        }
 
+      if (decl_specifiers->storage_class == sc_none
+         && at_function_scope_p ()
+         && !member_p
+         && !decl_spec_seq_has_spec_p (decl_specifiers, ds_typedef)
+         && !cp_parser_error_occurred (parser))
+       warn_about_ambiguous_parse (decl_specifiers->type, declarator);
+
       /* Check to see if the token indicates the start of a
         function-definition.  */
       if (cp_parser_token_starts_function_definition_p (token))
@@ -21191,7 +21296,7 @@ cp_parser_declarator (cp_parser* parser,
       /* If a ptr-operator was found, then this declarator was not
         parenthesized.  */
       if (parenthesized_p)
-       *parenthesized_p = true;
+       *parenthesized_p = false;
       /* The dependent declarator is optional if we are parsing an
         abstract-declarator.  */
       if (dcl_kind != CP_PARSER_DECLARATOR_NAMED)
@@ -21338,6 +21443,7 @@ cp_parser_direct_declarator (cp_parser* parser,
                cp_parser_parse_tentatively (parser);
 
              /* Consume the `('.  */
+             const location_t parens_start = token->location;
              matching_parens parens;
              parens.consume_open (parser);
              if (first)
@@ -21355,8 +21461,16 @@ cp_parser_direct_declarator (cp_parser* parser,
                current_binding_level->immediate_fn_ctx_p = true;
 
              /* Parse the parameter-declaration-clause.  */
+             bool vexing_parse_p = false;
              params
-               = cp_parser_parameter_declaration_clause (parser, flags);
+               = cp_parser_parameter_declaration_clause (parser, flags,
+                                                         &vexing_parse_p);
+             location_t parens_loc = UNKNOWN_LOCATION;
+             if (vexing_parse_p && !parser->in_type_id_in_expr_p)
+               {
+                 location_t e = cp_lexer_peek_token (parser->lexer)->location;
+                 parens_loc = make_location (parens_start, parens_start, e);
+               }
 
              /* Consume the `)'.  */
              parens.require_close (parser);
@@ -21430,7 +21544,8 @@ cp_parser_direct_declarator (cp_parser* parser,
                                                     tx_qual,
                                                     exception_specification,
                                                     late_return,
-                                                    requires_clause);
+                                                    requires_clause,
+                                                    parens_loc);
                  declarator->std_attributes = attrs;
                  declarator->attributes = gnu_attrs;
                  /* Any subsequent parameter lists are to do with
@@ -22638,11 +22753,14 @@ function_being_declared_is_template_p (cp_parser* 
parser)
 
    Returns a representation for the parameter declarations.  A return
    value of NULL indicates a parameter-declaration-clause consisting
-   only of an ellipsis.  */
+   only of an ellipsis.  This function might set VEXING_PARSE_P if it
+   detects a syntactic ambiguity between a variable definition and a
+   function declaration.  */
 
 static tree
 cp_parser_parameter_declaration_clause (cp_parser* parser,
-                                       cp_parser_flags flags)
+                                       cp_parser_flags flags,
+                                       bool *vexing_parse_p/*=nullptr*/)
 {
   tree parameters;
   cp_token *token;
@@ -22671,8 +22789,12 @@ cp_parser_parameter_declaration_clause (cp_parser* 
parser,
       return NULL_TREE;
     }
   else if (token->type == CPP_CLOSE_PAREN)
-    /* There are no parameters.  */
-    return void_list_node;
+    {
+      /* There are no parameters.  */
+      if (vexing_parse_p)
+       *vexing_parse_p = true;
+      return void_list_node;
+    }
   /* Check for `(void)', too, which is a special case.  */
   else if (token->keyword == RID_VOID
           && (cp_lexer_peek_nth_token (parser->lexer, 2)->type
@@ -22685,7 +22807,8 @@ cp_parser_parameter_declaration_clause (cp_parser* 
parser,
     }
 
   /* Parse the parameter-declaration-list.  */
-  parameters = cp_parser_parameter_declaration_list (parser, flags);
+  parameters = cp_parser_parameter_declaration_list (parser, flags,
+                                                    vexing_parse_p);
   /* If a parse error occurred while parsing the
      parameter-declaration-list, then the entire
      parameter-declaration-clause is erroneous.  */
@@ -22732,10 +22855,14 @@ cp_parser_parameter_declaration_clause (cp_parser* 
parser,
 
    Returns a representation of the parameter-declaration-list, as for
    cp_parser_parameter_declaration_clause.  However, the
-   `void_list_node' is never appended to the list.  */
+   `void_list_node' is never appended to the list.  This function might
+   set VEXING_PARSE_P if it detects a syntactic ambiguity between a variable
+   definition and a function declaration.  */
 
 static tree
-cp_parser_parameter_declaration_list (cp_parser* parser, cp_parser_flags flags)
+cp_parser_parameter_declaration_list (cp_parser* parser,
+                                     cp_parser_flags flags,
+                                     bool *vexing_parse_p)
 {
   tree parameters = NULL_TREE;
   tree *tail = &parameters;
@@ -22812,7 +22939,14 @@ cp_parser_parameter_declaration_list (cp_parser* 
parser, cp_parser_flags flags)
          || cp_lexer_next_token_is (parser->lexer, CPP_SEMICOLON)
          || cp_lexer_next_token_is (parser->lexer, CPP_OPEN_BRACE))
        /* The parameter-declaration-list is complete.  */
-       break;
+       {
+         /* If the parameters were parenthesized, it's the case of
+            T foo(X(x)) which looks like a variable definition but
+            is a function declaration.  */
+         if (vexing_parse_p)
+           *vexing_parse_p = parenthesized_p;
+         break;
+       }
       else if (cp_lexer_next_token_is (parser->lexer, CPP_COMMA))
        {
          cp_token *token;
@@ -31860,7 +31994,8 @@ cp_parser_cache_defarg (cp_parser *parser, bool nsdmi)
                  cp_lexer_consume_token (parser->lexer);
                  begin_scope (sk_function_parms, NULL_TREE);
                  tree t = cp_parser_parameter_declaration_list
-                           (parser, CP_PARSER_FLAGS_NONE);
+                           (parser, CP_PARSER_FLAGS_NONE,
+                            /*vexing_parse_p=*/nullptr);
                  if (t == error_mark_node)
                    error = true;
                  pop_bindings_and_leave_scope ();
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index edea7ee25ba..3dfccdaa0e3 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -252,7 +252,8 @@ in the following sections.
 -Woverloaded-virtual  -Wno-pmf-conversions -Wsign-promo @gol
 -Wsized-deallocation  -Wsuggest-final-methods @gol
 -Wsuggest-final-types  -Wsuggest-override  @gol
--Wno-terminate  -Wuseless-cast  -Wvirtual-inheritance  @gol
+-Wno-terminate  -Wuseless-cast  -Wno-vexing-parse  @gol
+-Wvirtual-inheritance  @gol
 -Wno-virtual-move-assign  -Wvolatile  -Wzero-as-null-pointer-constant}
 
 @item Objective-C and Objective-C++ Language Options
@@ -3848,6 +3849,37 @@ use the STL.  One may also use using directives and 
qualified names.
 Disable the warning about a throw-expression that will immediately
 result in a call to @code{terminate}.
 
+@item -Wno-vexing-parse @r{(C++ and Objective-C++ only)}
+@opindex Wvexing-parse
+@opindex Wno-vexing-parse
+Warn about the most vexing parse syntactic ambiguity.  This warns about
+the cases when a declaration looks like a variable definition, but the
+C++ language requires it to be interpreted as a function declaration.
+For instance:
+
+@smallexample
+void f(double a) @{
+  int i();        // extern int i (void);
+  int n(int(a));  // extern int n (int);
+@}
+@end smallexample
+
+Another example:
+
+@smallexample
+struct S @{ S(int); @};
+void f(double a) @{
+  S x(int(a));   // extern struct S x (int);
+  S y(int());    // extern struct S y (int (*) (void));
+  S z();         // extern struct S z (void);
+@}
+@end smallexample
+
+The warning will suggest options how to deal with such an ambiguity; e.g.,
+it can suggest removing the parentheses or using braces instead.
+
+This warning is enabled by default.
+
 @item -Wno-class-conversion @r{(C++ and Objective-C++ only)}
 @opindex Wno-class-conversion
 @opindex Wclass-conversion
diff --git a/gcc/testsuite/g++.dg/cpp2a/fn-template16.C 
b/gcc/testsuite/g++.dg/cpp2a/fn-template16.C
index becaff1e3fb..8ee783ad577 100644
--- a/gcc/testsuite/g++.dg/cpp2a/fn-template16.C
+++ b/gcc/testsuite/g++.dg/cpp2a/fn-template16.C
@@ -7,7 +7,7 @@ struct undeclared<int> { }; // { dg-error "not a class 
template" }
 int
 main ()
 {
-  int foo ();
+  int foo (); // { dg-warning "empty parentheses" }
   int foo (int);
   int foo (int, int);
   int a, b = 10;
diff --git a/gcc/testsuite/g++.dg/cpp2a/fn-template7.C 
b/gcc/testsuite/g++.dg/cpp2a/fn-template7.C
index d048606c0d6..2c5ee120dcd 100644
--- a/gcc/testsuite/g++.dg/cpp2a/fn-template7.C
+++ b/gcc/testsuite/g++.dg/cpp2a/fn-template7.C
@@ -7,7 +7,7 @@ struct undeclared<int> { }; // { dg-error "not a class 
template" }
 int
 main ()
 {
-  int foo ();
+  int foo (); // { dg-warning "empty parentheses" }
   int a, b = 10;
   a = foo<; // { dg-error "invalid template-argument-list|invalid" }
   a = foo < b; // { dg-error "invalid template-argument-list|invalid" }
diff --git a/gcc/testsuite/g++.dg/lookup/pr80891-5.C 
b/gcc/testsuite/g++.dg/lookup/pr80891-5.C
index e018922d68b..10d1ce3f3d5 100644
--- a/gcc/testsuite/g++.dg/lookup/pr80891-5.C
+++ b/gcc/testsuite/g++.dg/lookup/pr80891-5.C
@@ -14,7 +14,7 @@ template <typename, typename, typename, typename,
 struct B {
   B(A, A, int, int, int, int);
   void m_fn1(SubGraphIsoMapCallback p1) {
-    __normal_iterator __trans_tmp_1();
+    __normal_iterator __trans_tmp_1(); // { dg-warning "empty parentheses" }
     p1(__trans_tmp_1, 0);
   }
 };
diff --git a/gcc/testsuite/g++.dg/lto/pr79050_0.C 
b/gcc/testsuite/g++.dg/lto/pr79050_0.C
index 1f31b5d0020..464f5594769 100644
--- a/gcc/testsuite/g++.dg/lto/pr79050_0.C
+++ b/gcc/testsuite/g++.dg/lto/pr79050_0.C
@@ -3,5 +3,5 @@
 
 int main ()
 {
-  auto foo ();
+  extern auto foo ();
 }
diff --git a/gcc/testsuite/g++.dg/lto/pr84805_0.C 
b/gcc/testsuite/g++.dg/lto/pr84805_0.C
index 1509eae4845..668ba362aed 100644
--- a/gcc/testsuite/g++.dg/lto/pr84805_0.C
+++ b/gcc/testsuite/g++.dg/lto/pr84805_0.C
@@ -149,5 +149,5 @@ public:
 class XclImpRoot : XclRoot {}; 
 class XclImpColRowSettings : XclImpRoot {};
 void lcl_ExportExcelBiff() {
-XclRootData aExpData();
+extern XclRootData aExpData();
 }
diff --git a/gcc/testsuite/g++.dg/parse/pr58898.C 
b/gcc/testsuite/g++.dg/parse/pr58898.C
index e788c913c66..e67011d2fe7 100644
--- a/gcc/testsuite/g++.dg/parse/pr58898.C
+++ b/gcc/testsuite/g++.dg/parse/pr58898.C
@@ -5,12 +5,12 @@ struct Foo
 {
   Foo()
   {
-    int t(int()); // Error
+    int t(int()); // { dg-warning "parentheses were disambiguated" }
   }
 };
 
 int main()
 {
-  int t(int()); // OK
+  int t(int()); // { dg-warning "parentheses were disambiguated" }
   Foo<> a; // Error
 }
diff --git a/gcc/testsuite/g++.dg/template/scope5.C 
b/gcc/testsuite/g++.dg/template/scope5.C
index cf23a0837bd..b20d897b49f 100644
--- a/gcc/testsuite/g++.dg/template/scope5.C
+++ b/gcc/testsuite/g++.dg/template/scope5.C
@@ -59,7 +59,7 @@ template <typename av> struct ac : ao<av> { typedef 
c::e<am::an> aq; };
 template <typename aw, typename i, typename ax> void ay(aw, i, ax) {
   // Not sure if this has been creduced from an initialization of a
   // variable to a block-scope extern function decl
-  au<c::e<ap<typename ak<i>::o>::f> > az2();
+  au<c::e<ap<typename ak<i>::o>::f> > az2(); // { dg-warning "empty 
parentheses" }
 }
 void v() {
   ad a;
diff --git a/gcc/testsuite/g++.dg/warn/Wvexing-parse.C 
b/gcc/testsuite/g++.dg/warn/Wvexing-parse.C
new file mode 100644
index 00000000000..b02e904fa83
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wvexing-parse.C
@@ -0,0 +1,110 @@
+// PR c++/25814
+// { dg-do compile }
+// Test -Wvexing-parse.
+
+struct T { };
+
+struct X {
+  X();
+};
+
+struct S {
+  S(int);
+  S foo (int (int));
+  S(T);
+  int m;
+};
+
+struct W {
+  W();
+  W(X, X);
+  int m;
+};
+
+int g;
+int g1(int(g));
+int g2(int());
+void fg(int);
+
+void
+fn1 (double (a))
+{
+  extern int f0();
+  extern int f1(int(a));
+  int f2(int(a)); // { dg-warning "parentheses were disambiguated as a 
function declaration" }
+  int (*f3)(int(a));
+  int f4(int a);
+  int f5(int()); // { dg-warning "parentheses were disambiguated as a function 
declaration" }
+  int f6(...);
+  int f7((int(a)));
+  int (f8);
+  int f9(S(s)); // { dg-warning "parentheses were disambiguated as a function 
declaration" }
+  int(f10) __attribute__(());
+  int(f11(int()));
+  if (int(a) = 1) { }
+  int j, k, l(); // { dg-warning "empty parentheses were disambiguated as a 
function declaration" }
+  int m, f12(int(j)); // { dg-warning "parentheses were disambiguated as a 
function declaration" }
+
+  T t1(); // { dg-warning "empty parentheses were disambiguated as a function 
declaration" }
+  T t2(T()); // { dg-warning "parentheses were disambiguated as a function 
declaration" }
+  /* Declares a variable t3.  */
+  T(t3);
+  T t4(), // { dg-warning "empty parentheses were disambiguated as a function 
declaration" }
+    t5(); // { dg-warning "empty parentheses were disambiguated as a function 
declaration" }
+
+  extern S s1(int(a));
+  S s2(int(a)); // { dg-warning "parentheses were disambiguated as a function 
declaration" }
+  S s3(int a);
+  S s4(int()); // { dg-warning "parentheses were disambiguated as a function 
declaration" }
+  S s5(int(int)); // { dg-warning "parentheses were disambiguated as a 
function declaration" }
+  S s6(...);
+  S s7((int(a)));
+  S s8((int)a);
+  S s9 = int(a);
+  S(T());
+  S s10(S()); // { dg-warning "parentheses were disambiguated as a function 
declaration" }
+  S s11(T());
+  S s12(X()); // { dg-warning "parentheses were disambiguated as a function 
declaration" }
+  S s13 = S(T());
+  S(T()).foo(0);
+  S (S::*foo)(int (int));
+  S(*s14)(int(a));
+  S s15(); // { dg-warning "empty parentheses were disambiguated as a function 
declaration" }
+  S s16(void);
+
+  /* Don't warn here.  */
+  void fv1(int(a));
+  void fv2(int());
+  void (fv3)();
+  void (fv4)(void);
+  void (fv5)(int);
+
+  int n(); // { dg-warning "empty parentheses were disambiguated as a function 
declaration" }
+  int (n2)(); // { dg-warning "empty parentheses were disambiguated as a 
function declaration" }
+  int n3(void);
+
+  typedef int F(const char*);
+  typedef int F2();
+  typedef int F3() const;
+  typedef int F4(int(a)) const;
+
+  W w(X(), X()); // { dg-warning "parentheses were disambiguated as a function 
declaration" }
+}
+
+struct C1 {
+  C1(int);
+};
+
+struct C2 {
+  C2(C1, int);
+};
+
+template<int N> int value() { return N; }
+
+void
+fn2 ()
+{
+  int i = 0;
+  C2 c2(C1(int(i)), i);
+  C1(value<0>());
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wvexing-parse2.C 
b/gcc/testsuite/g++.dg/warn/Wvexing-parse2.C
new file mode 100644
index 00000000000..0dbeb7255cc
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wvexing-parse2.C
@@ -0,0 +1,24 @@
+// PR c++/25814
+// { dg-do compile { target c++11 } }
+// Test -Wvexing-parse.  C++11 features.
+
+struct X { };
+struct T {
+  T(X);
+};
+
+void
+fn1 (double (a))
+{
+  auto l = [](){
+    int f(int(a)); // { dg-warning "parentheses were disambiguated as a 
function declaration" }
+  };
+
+  [[noreturn]] int(e)(); // { dg-warning "empty parentheses were disambiguated 
as a function declaration" }
+
+  T t1{X()};
+  T t2(X{});
+  T t3{X{}};
+
+  using U = int();
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wvexing-parse3.C 
b/gcc/testsuite/g++.dg/warn/Wvexing-parse3.C
new file mode 100644
index 00000000000..43fcdf29f61
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wvexing-parse3.C
@@ -0,0 +1,129 @@
+// PR c++/25814
+// { dg-do compile { target c++11 } }
+// { dg-additional-options "-fdiagnostics-show-caret" }
+// Test -Wvexing-parse's fix-it hints in C++11.
+
+#include <initializer_list>
+
+struct X { };
+
+struct S {
+  S(X);
+  S(std::initializer_list<X>);
+  int m;
+};
+
+struct T {
+  T(X);
+  int m;
+};
+
+struct W {
+  W();
+  W(std::initializer_list<X>);
+  int m;
+};
+
+struct U {
+  U();
+  int m;
+};
+
+int
+main ()
+{
+  /*
+     Careful what we're suggesting:
+     S a((X())) -> S(X)
+     S a({X()}) -> (std::initializer_list<X>)
+     S a{X()} -> (std::initializer_list<X>)
+   */
+  S a(X()); // { dg-warning "6:parentheses were disambiguated as a function 
declaration" }
+  /* { dg-begin-multiline-output "" }
+   S a(X());
+      ^~~~~
+     { dg-end-multiline-output "" } */
+  // { dg-message "6:add parentheses to declare a variable" "" { target *-*-* 
} 41 }
+  /* { dg-begin-multiline-output "" }
+   S a(X());
+      ^~~~~
+       (  )
+     { dg-end-multiline-output "" } */
+
+  T t(X()); // { dg-warning "6:parentheses were disambiguated as a function 
declaration" }
+  /* { dg-begin-multiline-output "" }
+   T t(X());
+      ^~~~~
+     { dg-end-multiline-output "" } */
+  // { dg-message "6:replace parentheses with braces to declare a variable" "" 
{ target *-*-* } 53 }
+  /* { dg-begin-multiline-output "" }
+   T t(X());
+      ^~~~~
+      -
+      {   -
+          }
+     { dg-end-multiline-output "" } */
+
+  int n(   ); // { dg-warning "8:empty parentheses were disambiguated as a 
function declaration" }
+  /* { dg-begin-multiline-output "" }
+   int n(   );
+        ^~~~~
+     { dg-end-multiline-output "" } */
+  // { dg-message "8:remove parentheses to default-initialize a variable" "" { 
target *-*-* } 67 }
+  /* { dg-begin-multiline-output "" }
+   int n(   );
+        ^~~~~
+        -----
+     { dg-end-multiline-output "" } */
+  // { dg-message "8:or replace parentheses with braces to value-initialize a 
variable" "" { target *-*-* } 67 }
+
+  S s(); // { dg-warning "6:empty parentheses were disambiguated as a function 
declaration" }
+  /* { dg-begin-multiline-output "" }
+   S s();
+      ^~
+     { dg-end-multiline-output "" } */
+
+  X x(); // { dg-warning "6:empty parentheses were disambiguated as a function 
declaration" }
+  /* { dg-begin-multiline-output "" }
+   X x();
+      ^~
+     { dg-end-multiline-output "" } */
+  // { dg-message "6:remove parentheses to default-initialize a variable" "" { 
target *-*-* } 86 }
+  /* { dg-begin-multiline-output "" }
+   X x();
+      ^~
+      --
+     { dg-end-multiline-output "" } */
+  // { dg-message "6:or replace parentheses with braces to 
aggregate-initialize a variable" "" { target *-*-* } 86 }
+
+  W w(); // { dg-warning "6:empty parentheses were disambiguated as a function 
declaration" }
+  /* { dg-begin-multiline-output "" }
+   W w();
+      ^~
+     { dg-end-multiline-output "" } */
+  // { dg-message "6:remove parentheses to default-initialize a variable" "" { 
target *-*-* } 99 }
+  /* { dg-begin-multiline-output "" }
+   W w();
+      ^~
+      --
+     { dg-end-multiline-output "" } */
+
+  T t2(); // { dg-warning "7:empty parentheses were disambiguated as a 
function declaration" }
+  /* { dg-begin-multiline-output "" }
+   T t2();
+       ^~
+     { dg-end-multiline-output "" } */
+
+  U u(); // { dg-warning "6:empty parentheses were disambiguated as a function 
declaration" }
+  /* { dg-begin-multiline-output "" }
+   U u();
+      ^~
+     { dg-end-multiline-output "" } */
+  // { dg-message "6:remove parentheses to default-initialize a variable" "" { 
target *-*-* } 117 }
+  /* { dg-begin-multiline-output "" }
+   U u();
+      ^~
+      --
+     { dg-end-multiline-output "" } */
+  // { dg-message "6:or replace parentheses with braces to value-initialize a 
variable" "" { target *-*-* } 117 }
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wvexing-parse4.C 
b/gcc/testsuite/g++.dg/warn/Wvexing-parse4.C
new file mode 100644
index 00000000000..3e010aaba3d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wvexing-parse4.C
@@ -0,0 +1,74 @@
+// PR c++/25814
+// { dg-do compile { target c++98_only } }
+// { dg-additional-options "-fdiagnostics-show-caret" }
+// Test -Wvexing-parse's fix-it hints in C++98.
+
+struct X { };
+
+struct T {
+  T(X);
+  int m;
+};
+
+struct U {
+  U();
+  int m;
+};
+
+int
+main ()
+{
+  T t(X()); // { dg-warning "6:parentheses were disambiguated as a function 
declaration" }
+  /* { dg-begin-multiline-output "" }
+   T t(X());
+      ^~~~~
+     { dg-end-multiline-output "" } */
+  // { dg-message "6:add parentheses to declare a variable" "" { target *-*-* 
} 21 }
+  /* { dg-begin-multiline-output "" }
+   T t(X());
+      ^~~~~
+       (  )
+     { dg-end-multiline-output "" } */
+
+  int n(   ); // { dg-warning "8:empty parentheses were disambiguated as a 
function declaration" }
+  /* { dg-begin-multiline-output "" }
+   int n(   );
+        ^~~~~
+     { dg-end-multiline-output "" } */
+  // { dg-message "8:remove parentheses to default-initialize a variable" "" { 
target *-*-* } 33 }
+  /* { dg-begin-multiline-output "" }
+   int n(   );
+        ^~~~~
+        -----
+     { dg-end-multiline-output "" } */
+
+  T y(); // { dg-warning "6:empty parentheses were disambiguated as a function 
declaration" }
+  /* { dg-begin-multiline-output "" }
+   T y();
+      ^~
+     { dg-end-multiline-output "" } */
+
+  X x(); // { dg-warning "6:empty parentheses were disambiguated as a function 
declaration" }
+  /* { dg-begin-multiline-output "" }
+   X x();
+      ^~
+     { dg-end-multiline-output "" } */
+  // { dg-message "6:remove parentheses to default-initialize a variable" "" { 
target *-*-* } 51 }
+  /* { dg-begin-multiline-output "" }
+   X x();
+      ^~
+      --
+     { dg-end-multiline-output "" } */
+
+  U u(); // { dg-warning "6:empty parentheses were disambiguated as a function 
declaration" }
+  /* { dg-begin-multiline-output "" }
+   U u();
+      ^~
+     { dg-end-multiline-output "" } */
+  // { dg-message "6:remove parentheses to default-initialize a variable" "" { 
target *-*-* } 63 }
+  /* { dg-begin-multiline-output "" }
+   U u();
+      ^~
+      --
+     { dg-end-multiline-output "" } */
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wvexing-parse5.C 
b/gcc/testsuite/g++.dg/warn/Wvexing-parse5.C
new file mode 100644
index 00000000000..3422e706160
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wvexing-parse5.C
@@ -0,0 +1,14 @@
+// PR c++/25814
+// { dg-do compile }
+// Test -Wvexing-parse in a template.
+
+struct X { };
+
+template<typename T>
+void fn ()
+{
+  T t(); // { dg-warning "empty parentheses were disambiguated as a function 
declaration" }
+  T a(X()); // { dg-warning "parentheses were disambiguated as a function 
declaration" }
+  X x(T()); // { dg-warning "parentheses were disambiguated as a function 
declaration" }
+  int i(T()); // { dg-warning "parentheses were disambiguated as a function 
declaration" }
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wvexing-parse6.C 
b/gcc/testsuite/g++.dg/warn/Wvexing-parse6.C
new file mode 100644
index 00000000000..58fa725a2ee
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wvexing-parse6.C
@@ -0,0 +1,24 @@
+// PR c++/25814
+// { dg-do compile }
+// Test from Wikipedia.
+
+class Timer {
+ public:
+  Timer();
+};
+
+class TimeKeeper {
+ public:
+  TimeKeeper(const Timer& t);
+
+  int get_time();
+};
+
+void f(double adouble) {
+  int i(int(adouble)); // { dg-warning "parentheses were disambiguated as a 
function declaration" }
+}
+
+int main() {
+  TimeKeeper time_keeper(Timer()); // { dg-warning "parentheses were 
disambiguated as a function declaration" }
+  return time_keeper.get_time(); // { dg-error "request for member" }
+}
diff --git a/gcc/testsuite/g++.old-deja/g++.brendan/recurse.C 
b/gcc/testsuite/g++.old-deja/g++.brendan/recurse.C
index de20a073221..0af1c147cd8 100644
--- a/gcc/testsuite/g++.old-deja/g++.brendan/recurse.C
+++ b/gcc/testsuite/g++.old-deja/g++.brendan/recurse.C
@@ -73,7 +73,7 @@ public:
 
 int main()
 {
-  DBpathrec a(), b();
+  DBpathrec a(), b(); // { dg-warning "empty parentheses" }
 
   a = b;// { dg-error "" }  non-lvalue in assignment.*
 }
diff --git a/gcc/testsuite/g++.old-deja/g++.jason/template4.C 
b/gcc/testsuite/g++.old-deja/g++.jason/template4.C
index de7d3312a71..1cf5a614411 100644
--- a/gcc/testsuite/g++.old-deja/g++.jason/template4.C
+++ b/gcc/testsuite/g++.old-deja/g++.jason/template4.C
@@ -17,5 +17,5 @@ template <class T>
 ccList <T> cc_List<T>::copy (){}
 
 int main (int, char **) {
-  ccList <int> size1();
+  ccList <int> size1(); // { dg-warning "empty parentheses" }
 }
diff --git a/gcc/testsuite/g++.old-deja/g++.law/arm4.C 
b/gcc/testsuite/g++.old-deja/g++.law/arm4.C
index bbcf7df2391..59492ca952c 100644
--- a/gcc/testsuite/g++.old-deja/g++.law/arm4.C
+++ b/gcc/testsuite/g++.old-deja/g++.law/arm4.C
@@ -20,7 +20,7 @@ int main(void)
 {
   double a = 2.0;
 
-  S x(int (a));
+  S x(int (a)); // { dg-warning "parentheses were disambiguated" }
   if (count > 0)
     { printf ("FAIL\n"); return 1; }
   else
diff --git a/gcc/testsuite/g++.old-deja/g++.mike/for2.C 
b/gcc/testsuite/g++.old-deja/g++.mike/for2.C
index 6eb5d66675e..4a7c3042544 100644
--- a/gcc/testsuite/g++.old-deja/g++.mike/for2.C
+++ b/gcc/testsuite/g++.old-deja/g++.mike/for2.C
@@ -14,7 +14,7 @@ void bar() {
 
 void bee () {
   int i = 0;
-  for (int fun() = 0; i != 2; ++i) {   // { dg-warning "extern" "extern" }
+  for (int fun() = 0; i != 2; ++i) {   // { dg-warning "extern|empty 
parentheses" "extern" }
   // { dg-error "initialized" "init" { target *-*-* } .-1 }
   }
 }
diff --git a/gcc/testsuite/g++.old-deja/g++.other/local4.C 
b/gcc/testsuite/g++.old-deja/g++.other/local4.C
index b5514a54b46..492ce2b7e70 100644
--- a/gcc/testsuite/g++.old-deja/g++.other/local4.C
+++ b/gcc/testsuite/g++.old-deja/g++.other/local4.C
@@ -6,6 +6,6 @@ int f (int);
 
 int main ()
 {
-  int f ();
+  int f (); // { dg-warning "empty parentheses" }
   return f ();
 }
diff --git a/gcc/testsuite/g++.old-deja/g++.pt/crash3.C 
b/gcc/testsuite/g++.old-deja/g++.pt/crash3.C
index 52701b776ef..d1d9b12738c 100644
--- a/gcc/testsuite/g++.old-deja/g++.pt/crash3.C
+++ b/gcc/testsuite/g++.old-deja/g++.pt/crash3.C
@@ -7,11 +7,13 @@ public:
     {
       // local-extern :)
       CVector<int> v(); // { dg-message "old declaration" }
+      // { dg-warning "empty parentheses" "" { target *-*-* } .-1 }
        return v;               // { dg-error "convert" }
     }
     CVector<long> g() const
     {
       CVector<long> v(); // { dg-error "ambiguating new" }
+      // { dg-warning "empty parentheses" "" { target *-*-* } .-1 }
        return v;               // { dg-error "convert" }
     }
 };
diff --git a/libstdc++-v3/testsuite/20_util/reference_wrapper/lwg2993.cc 
b/libstdc++-v3/testsuite/20_util/reference_wrapper/lwg2993.cc
index d1f0cafe688..c30511cab11 100644
--- a/libstdc++-v3/testsuite/20_util/reference_wrapper/lwg2993.cc
+++ b/libstdc++-v3/testsuite/20_util/reference_wrapper/lwg2993.cc
@@ -43,7 +43,7 @@ test01()
 void
 test02()
 {
-  std::reference_wrapper<int> purr();
+  std::reference_wrapper<int> purr(); // { dg-warning "empty parentheses" }
 
   // error, ambiguous: ICS exists from int prvalue to
   // reference_wrapper<int> and from reference_wrapper<int> to int
diff --git a/libstdc++-v3/testsuite/25_algorithms/generate_n/87982_neg.cc 
b/libstdc++-v3/testsuite/25_algorithms/generate_n/87982_neg.cc
index 4236b5a820d..73934638ee3 100644
--- a/libstdc++-v3/testsuite/25_algorithms/generate_n/87982_neg.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/generate_n/87982_neg.cc
@@ -23,7 +23,7 @@
 
 void test01()
 {
-  int gen();
+  int gen(); // { dg-warning "empty parentheses" }
   int a[2];
   std::generate_n(a, a+2, &gen);
 }

base-commit: 1aeb7d7d67d167297ca2f4a97ef20f68e7546b4c
-- 
2.26.2

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