C++ PATCH for c++/90572 - wrong disambiguation in friend declaration

[Marek Polacek]

Here we were parsing 

  template <typename T> struct C {
    friend C(T::fn)(); // this
  };

wrongly in C++2a since my P0634 patch.  T::fn is actually a function
declaration, but cp_parser_constructor_declarator_p was thinking it's
a constructor.  That function actually has code to prevent the compiler
from thinking that

  S (f)(int);

is a constructor: it checks if after 'S' there's '(' followed by ')', '...',
or a type.  If parsing the type succeeds, it looks like a ctor.  In this
case it succeeded, because T::fn is a qualified name and we are parsing
a member-declaration, so CP_PARSER_FLAGS_TYPENAME_OPTIONAL was set.

We can either look further and say it's not a constructor if what comes
after "(T::fn)" is '('.  Or we can prevent cp_parser_type_specifier from
assuming a type if friend_p, because while constructors can be friends,
they need to use a qualified-id, and at this point we know that the name
isn't qualified, so it can't be a valid friend constructor.

Bootstrapped/regtested on x86_64-linux, ok for trunk and 9?

2019-05-24  Marek Polacek  <pola...@redhat.com>

        PR c++/90572 - wrong disambiguation in friend declaration.
        * parser.c (cp_parser_constructor_declarator_p): Don't allow missing
        typename for friend declarations.

        * g++.dg/cpp2a/typename16.C: New test.
        * g++.dg/parse/friend13.C: New test.

diff --git gcc/cp/parser.c gcc/cp/parser.c
index 1440fdb0d18..08b4105c97f 100644
--- gcc/cp/parser.c
+++ gcc/cp/parser.c
@@ -27593,9 +27593,19 @@ cp_parser_constructor_declarator_p (cp_parser *parser, 
cp_parser_flags flags,
          parser->num_template_parameter_lists = 0;
 
          /* Look for the type-specifier.  It's not optional, but its typename
-            might be.  */
+            might be.  Unless this is a friend declaration; we don't want to
+            treat
+
+              friend S (T::fn)(int);
+
+            as a constructor, but with P0634, we might assume a type when
+            looking for the type-specifier.  It is actually a function named
+            `T::fn' that takes one parameter (of type `int') and returns a
+            value of type `S'.  Constructors can be friends, but they must
+            use a qualified name.  */
          cp_parser_type_specifier (parser,
-                                   (flags & ~CP_PARSER_FLAGS_OPTIONAL),
+                                   (friend_p ? CP_PARSER_FLAGS_NONE
+                                    : (flags & ~CP_PARSER_FLAGS_OPTIONAL)),
                                    /*decl_specs=*/NULL,
                                    /*is_declarator=*/true,
                                    /*declares_class_or_enum=*/NULL,
diff --git gcc/testsuite/g++.dg/cpp2a/typename16.C 
gcc/testsuite/g++.dg/cpp2a/typename16.C
new file mode 100644
index 00000000000..7f4242a1dba
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/typename16.C
@@ -0,0 +1,11 @@
+// PR c++/90572
+// { dg-do compile { target c++2a } }
+
+struct X { X(int); };
+
+template<typename T> struct S {
+  friend X::X(T::t);
+};
+
+struct W { using t = int; };
+S<W> s;
diff --git gcc/testsuite/g++.dg/parse/friend13.C 
gcc/testsuite/g++.dg/parse/friend13.C
new file mode 100644
index 00000000000..d716247a5da
--- /dev/null
+++ gcc/testsuite/g++.dg/parse/friend13.C
@@ -0,0 +1,16 @@
+// PR c++/90572
+
+template <typename T> struct C {
+  friend C(T::fn)();
+  friend C(T::fn)(int);
+  friend C(T::fn)(int, int);
+};
+
+struct X { };
+
+template<typename T>
+struct B {
+  friend X(T::fn)();
+  friend X(T::fn)(int);
+  friend X(T::fn)(int, int);
+};