Re: [PATCH] c++: Improve static_assert diagnostic [PR97518]

Tue, 10 Nov 2020 08:32:23 -0800 

On 11/9/20 7:21 PM, Marek Polacek wrote:

Currently, when a static_assert fails, we only say "static assertion failed".
It would be more useful if we could also print the expression that
evaluated to false; this is especially useful when the condition uses
template parameters.  Consider the motivating example, in which we have
this line:

   static_assert(is_same<X, Y>::value);

if this fails, the user has to play dirty games to get the compiler to
print the template arguments.  With this patch, we say:

   static assertion failed due to requirement 'is_same<int*, int>::value'
I'd rather avoid the word "requirement" here, to avoid confusion with Concepts.
Maybe have the usual failed error, and if we're showing the expression, have a second inform to say e.g. "%qE evaluates to false"?
Also, if we've narrowed the problem down to a particular subexpression, let's only print that one. 


which I think is much better.  However, always printing the condition that
evaluated to 'false' wouldn't be very useful: e.g. noexcept(fn) is
always parsed to true/false, so we would say "static assertion failed due
to requirement 'false'" which doesn't help.  So I wound up only printing
the condition when it was instantiation-dependent, that is, we called
finish_static_assert from tsubst_expr.

Moreover, this patch also improves the diagnostic when the condition
consists of a logical AND.  Say you have something like this:

   static_assert(fn1() && fn2() && fn3() && fn4() && fn5());

where fn4() evaluates to false and the other ones to true.  Highlighting
the whole thing is not that helpful because it won't say which clause
evaluated to false.  With the find_failing_clause tweak in this patch
we emit:

   error: static assertion failed
     6 | static_assert(fn1() && fn2() && fn3() && fn4() && fn5());
       |                                          ~~~^~

so you know right away what's going on.  Unfortunately, when you combine
both things, that is, have an instantiation-dependent expr and && in
a static_assert, we can't yet quite point to the clause that failed.  It
is because when we tsubstitute something like is_same<X, Y>::value, we
generate a VAR_DECL that doesn't have any location.  It would be awesome
if we could wrap it with a location wrapper, but I didn't see anything
obvious.
Hmm, I vaguely remember that at first we were using location wrappers less in templates, but I thought that was fixed. I don't see anything setting suppress_location_wrappers, and it looks like tsubst_copy_and_build should preserve a location wrapper. 


Further tweak could be to print the failing clause of the condition if
possible, whether or not it was instantiation-dependent.


If not instantiation-dependent, underlining the clause seems sufficient.


In passing, I've cleaned up some things:
* use iloc_sentinel when appropriate,
* it's nicer to call contextual_conv_bool instead of the rather verbose
   perform_implicit_conversion_flags,
* no need to check for INTEGER_CST before calling integer_zerop.

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

gcc/cp/ChangeLog:

        PR c++/97518
        * cp-tree.h (finish_static_assert): Adjust declaration.
        * parser.c (cp_parser_static_assert): Pass false to
        finish_static_assert.
        * pt.c (tsubst_expr): Pass true to finish_static_assert.
        * semantics.c (find_failing_clause_r): New function.
        (find_failing_clause): New function.
        (finish_static_assert): Add a bool parameter.  Use
        iloc_sentinel.  Call contextual_conv_bool instead of
        perform_implicit_conversion_flags.  Don't check for INTEGER_CST before
        calling integer_zerop.  Call find_failing_clause and maybe use its
        location.  Print the original condition if SHOW_EXPR_P.

gcc/testsuite/ChangeLog:

        PR c++/97518
        * g++.dg/diagnostic/pr87386.C: Adjust expected output.
        * g++.dg/diagnostic/static_assert1.C: New test.
        * g++.dg/diagnostic/static_assert2.C: New test.

libcc1/ChangeLog:

        PR c++/97518
        * libcp1plugin.cc (plugin_add_static_assert): Pass false to
        finish_static_assert.

libstdc++-v3/ChangeLog:

        * testsuite/20_util/scoped_allocator/69293_neg.cc: Adjust dg-error.
        * testsuite/20_util/uses_allocator/69293_neg.cc: Likewise.
        * testsuite/20_util/uses_allocator/cons_neg.cc: Likewise.
        * testsuite/26_numerics/random/pr60037-neg.cc: Likewise.
---
  gcc/cp/cp-tree.h                              |  2 +-
  gcc/cp/parser.c                               |  3 +-
  gcc/cp/pt.c                                   |  6 +-
  gcc/cp/semantics.c                            | 79 ++++++++++++++++---
  gcc/testsuite/g++.dg/diagnostic/pr87386.C     |  2 +-
  .../g++.dg/diagnostic/static_assert1.C        | 20 +++++
  .../g++.dg/diagnostic/static_assert2.C        | 68 ++++++++++++++++
  libcc1/libcp1plugin.cc                        |  2 +-
  .../20_util/scoped_allocator/69293_neg.cc     |  2 +-
  .../20_util/uses_allocator/69293_neg.cc       |  2 +-
  .../20_util/uses_allocator/cons_neg.cc        |  2 +-
  .../26_numerics/random/pr60037-neg.cc         |  4 +-
  12 files changed, 168 insertions(+), 24 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/diagnostic/static_assert1.C
  create mode 100644 gcc/testsuite/g++.dg/diagnostic/static_assert2.C

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index b98d47a702f..230a1525c63 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -7234,7 +7234,7 @@ extern bool cxx_omp_create_clause_info            (tree, 
tree, bool, bool,
                                                 bool, bool);
  extern tree baselink_for_fns                    (tree);
  extern void finish_static_assert                (tree, tree, location_t,
-                                                 bool);
+                                                bool, bool);
  extern tree finish_decltype_type                (tree, bool, tsubst_flags_t);
  extern tree finish_trait_expr                 (location_t, enum 
cp_trait_kind, tree, tree);
  extern tree build_lambda_expr                   (void);
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 4c819ea1c5d..1ef14279ff7 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -14886,7 +14886,8 @@ cp_parser_static_assert(cp_parser *parser, bool 
member_p)
/* Complete the static assertion, which may mean either processing 
       the static assert now or saving it for template instantiation.  */
-  finish_static_assert (condition, message, assert_loc, member_p);
+  finish_static_assert (condition, message, assert_loc, member_p,
+                       /*show_expr_p=*/false);
  }
/* Parse the expression in decltype ( expression ). */ 
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index a2655a0ff52..6ba114c9da3 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -18492,8 +18492,8 @@ tsubst_expr (tree t, tree args, tsubst_flags_t 
complain, tree in_decl,
        tree condition;
++c_inhibit_evaluation_warnings; 
-        condition =
-          tsubst_expr (STATIC_ASSERT_CONDITION (t),
+       condition =
+         tsubst_expr (STATIC_ASSERT_CONDITION (t),
                         args,
                         complain, in_decl,
                         /*integral_constant_expression_p=*/true);
@@ -18502,7 +18502,7 @@ tsubst_expr (tree t, tree args, tsubst_flags_t 
complain, tree in_decl,
          finish_static_assert (condition,
                                STATIC_ASSERT_MESSAGE (t),
                                STATIC_ASSERT_SOURCE_LOCATION (t),
-                              /*member_p=*/false);
+                             /*member_p=*/false, /*show_expr_p=*/true);
        }
        break;
diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c 
index a550db69488..1cc5118748f 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -9823,13 +9823,53 @@ init_cp_semantics (void)
  {
  }
  
+
+/* If we have a condition in conjunctive normal form (CNF), find the first
+   failing clause.  In other words, given an expression like
+
+     true && true && false && true && false
+
+   return the first 'false'.  EXPR is the expression.  */
+
+static tree
+find_failing_clause_r (tree expr)
+{
+  if (TREE_CODE (expr) == TRUTH_ANDIF_EXPR)
+    {
+      /* First check the left side...  */
+      tree e = find_failing_clause_r (TREE_OPERAND (expr, 0));
+      if (e == NULL_TREE)
+       /* ...if we didn't find a false clause, check the right side.  */
+       e = find_failing_clause_r (TREE_OPERAND (expr, 1));
+      return e;
+    }
+  tree e = contextual_conv_bool (expr, tf_none);
+  e = fold_non_dependent_expr (e, tf_none, /*manifestly_const_eval=*/true);
+  if (integer_zerop (e))
+    /* This is the failing clause.  */
+    return expr;
+  return NULL_TREE;
+}
+
+/* Wrapper for find_failing_clause_r.  */
+
+static tree
+find_failing_clause (tree expr)
+{
+  if (TREE_CODE (expr) != TRUTH_ANDIF_EXPR)
+    return NULL_TREE;
+  return find_failing_clause_r (expr);
+}
+
  /* Build a STATIC_ASSERT for a static assertion with the condition
     CONDITION and the message text MESSAGE.  LOCATION is the location
     of the static assertion in the source code.  When MEMBER_P, this
-   static assertion is a member of a class.  */
+   static assertion is a member of a class.  If SHOW_EXPR_P is true,
+   print the condition (because it was instantiation-dependent).  */
+
  void
  finish_static_assert (tree condition, tree message, location_t location,
-                      bool member_p)
+                     bool member_p, bool show_expr_p)
  {
    tsubst_flags_t complain = tf_warning_or_error;
@@ -9867,8 +9907,7 @@ finish_static_assert (tree condition, tree message, location_t location, 
    tree orig_condition = condition;
/* Fold the expression and convert it to a boolean value. */ 
-  condition = perform_implicit_conversion_flags (boolean_type_node, condition,
-                                                complain, LOOKUP_NORMAL);
+  condition = contextual_conv_bool (condition, complain);
    condition = fold_non_dependent_expr (condition, complain,
                                       /*manifestly_const_eval=*/true);
@@ -9877,21 +9916,38 @@ finish_static_assert (tree condition, tree message, location_t location, 
      ;
    else
      {
-      location_t saved_loc = input_location;
+      iloc_sentinel ils (location);
- input_location = location; 
-      if (TREE_CODE (condition) == INTEGER_CST
-          && integer_zerop (condition))
+      if (integer_zerop (condition))
        {
          int sz = TREE_INT_CST_LOW (TYPE_SIZE_UNIT
                                     (TREE_TYPE (TREE_TYPE (message))));
          int len = TREE_STRING_LENGTH (message) / sz - 1;
+
+         /* See if we can find which clause was failing (for logical AND).  */
+         tree bad = find_failing_clause (orig_condition);
+         /* If not, or its location is unusable, fall back to the previous
+            location.  */
+         location_t cloc = location;
+         if (cp_expr_location (bad) != UNKNOWN_LOCATION)
+           cloc = cp_expr_location (bad);
+
            /* Report the error. */
          if (len == 0)
-            error ("static assertion failed");
+           {
+             if (show_expr_p)
+               error_at (cloc, "static assertion failed due to "
+                         "requirement %qE", orig_condition);
+             else
+               error_at (cloc, "static assertion failed");
+           }
+         else if (show_expr_p)
+           error_at (cloc, "static assertion failed due to "
+                     "requirement %qE: %s", orig_condition,
+                     TREE_STRING_POINTER (message));
          else
-            error ("static assertion failed: %s",
-                  TREE_STRING_POINTER (message));
+           error_at (cloc, "static assertion failed: %s",
+                     TREE_STRING_POINTER (message));
/* Actually explain the failure if this is a concept check or a 
             requires-expression.  */
@@ -9905,7 +9961,6 @@ finish_static_assert (tree condition, tree message, 
location_t location,
          if (require_rvalue_constant_expression (condition))
            cxx_constant_value (condition);
        }
-      input_location = saved_loc;
      }
  }
  
diff --git a/gcc/testsuite/g++.dg/diagnostic/pr87386.C 
b/gcc/testsuite/g++.dg/diagnostic/pr87386.C
index 85726af9f01..679a5177f64 100644
--- a/gcc/testsuite/g++.dg/diagnostic/pr87386.C
+++ b/gcc/testsuite/g++.dg/diagnostic/pr87386.C
@@ -14,5 +14,5 @@ static_assert (foo::test<int>::value, "foo");         // { 
dg-error "static assertion f
  static_assert (foo::test<int>::value && true, "bar"); // { dg-error "static 
assertion failed: bar" }
  /* { dg-begin-multiline-output "" }
   static_assert (foo::test<int>::value && true, "bar");
-                ~~~~~~~~~~~~~~~~~~~~~~^~~~~~~
+                ~~~~~~~~~~~~~~~~^~~~~
     { dg-end-multiline-output "" } */
diff --git a/gcc/testsuite/g++.dg/diagnostic/static_assert1.C 
b/gcc/testsuite/g++.dg/diagnostic/static_assert1.C
new file mode 100644
index 00000000000..5cfb66a5ad7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/diagnostic/static_assert1.C
@@ -0,0 +1,20 @@
+// PR c++/97518
+// { dg-do compile { target c++17 } }
+
+template <typename T, typename U> struct is_same { static constexpr bool value 
= false; };
+template <typename T> struct is_same<T, T> { static constexpr bool value = 
true; };
+
+template <typename T> using some_metafunction_t = T;
+
+template <typename T>
+void foo(T ) {
+    using X = T*;
+    using Y = some_metafunction_t<T>;
+
+    static_assert(is_same<X, Y>::value); // { dg-error {static assertion failed due 
to requirement .is_same<int\*, int>::value.} }
+    static_assert(is_same<X, Y>::value, "foo"); // { dg-error {static assertion 
failed due to requirement .is_same<int\*, int>::value.: foo} }
+}
+
+void bar() {
+    foo(0);
+}
diff --git a/gcc/testsuite/g++.dg/diagnostic/static_assert2.C 
b/gcc/testsuite/g++.dg/diagnostic/static_assert2.C
new file mode 100644
index 00000000000..542697f99de
--- /dev/null
+++ b/gcc/testsuite/g++.dg/diagnostic/static_assert2.C
@@ -0,0 +1,68 @@
+// PR c++/97518
+// { dg-do compile { target c++11 } }
+// { dg-options "-fdiagnostics-show-caret" }
+
+constexpr bool yes () { return true; }
+constexpr bool no () { return false; }
+constexpr bool yay = true;
+constexpr bool nay = false;
+
+void
+bar ()
+{
+  static_assert (true && true && no(), ""); // { dg-error "static assertion 
failed" }
+/* { dg-begin-multiline-output "" }
+   static_assert (true && true && no(), "");
+                                  ~~^~
+   { dg-end-multiline-output "" } */
+  static_assert (yay && nay, ""); // { dg-error "static assertion failed" }
+/* { dg-begin-multiline-output "" }
+   static_assert (yay && nay, "");
+                         ^~~
+   { dg-end-multiline-output "" } */
+  static_assert (yes() && no(), ""); // { dg-error "static assertion failed" }
+/* { dg-begin-multiline-output "" }
+   static_assert (yes() && no(), "");
+                           ~~^~
+   { dg-end-multiline-output "" } */
+  static_assert (no() && yes(), ""); // { dg-error "static assertion failed" }
+/* { dg-begin-multiline-output "" }
+   static_assert (no() && yes(), "");
+                  ~~^~
+   { dg-end-multiline-output "" } */
+  static_assert (no() && no() && yes(), ""); // { dg-error "static assertion 
failed" }
+/* { dg-begin-multiline-output "" }
+   static_assert (no() && no() && yes(), "");
+                  ~~^~
+   { dg-end-multiline-output "" } */
+  static_assert (yes() && yes() && yes () && no() && yes(), ""); // { dg-error 
"static assertion failed" }
+/* { dg-begin-multiline-output "" }
+   static_assert (yes() && yes() && yes () && no() && yes(), "");
+                                              ~~^~
+   { dg-end-multiline-output "" } */
+  static_assert (yes() && yes() && yes () && (no() && yes()), ""); // { dg-error 
"static assertion failed" }
+/* { dg-begin-multiline-output "" }
+   static_assert (yes() && yes() && yes () && (no() && yes()), "");
+                                               ~~^~
+   { dg-end-multiline-output "" } */
+  static_assert ((yes() && no()) && no(), ""); // { dg-error "static assertion 
failed" }
+/* { dg-begin-multiline-output "" }
+   static_assert ((yes() && no()) && no(), "");
+                            ~~^~
+   { dg-end-multiline-output "" } */
+  static_assert ((yes() && no()) && no(), ""); // { dg-error "static assertion 
failed" }
+/* { dg-begin-multiline-output "" }
+   static_assert ((yes() && no()) && no(), "");
+                            ~~^~
+   { dg-end-multiline-output "" } */
+  static_assert ((no() || no()) && yes(), ""); // { dg-error "static assertion 
failed" }
+/* { dg-begin-multiline-output "" }
+   static_assert ((no() || no()) && yes(), "");
+                  ~~~~~~^~~~~~~~
+   { dg-end-multiline-output "" } */
+  static_assert ((yes() || no()) && no(), ""); // { dg-error "static assertion 
failed" }
+/* { dg-begin-multiline-output "" }
+   static_assert ((yes() || no()) && no(), "");
+                                     ~~^~
+   { dg-end-multiline-output "" } */
+}
diff --git a/libcc1/libcp1plugin.cc b/libcc1/libcp1plugin.cc
index bab2751a5ce..67a235f9095 100644
--- a/libcc1/libcp1plugin.cc
+++ b/libcc1/libcp1plugin.cc
@@ -3642,7 +3642,7 @@ plugin_add_static_assert (cc1_plugin::connection *self,
bool member_p = at_class_scope_p (); - finish_static_assert (condition, message, loc, member_p); 
+  finish_static_assert (condition, message, loc, member_p, false);
return 1; 
  }
diff --git a/libstdc++-v3/testsuite/20_util/scoped_allocator/69293_neg.cc 
b/libstdc++-v3/testsuite/20_util/scoped_allocator/69293_neg.cc
index 6f3cd0a2521..d28a4330504 100644
--- a/libstdc++-v3/testsuite/20_util/scoped_allocator/69293_neg.cc
+++ b/libstdc++-v3/testsuite/20_util/scoped_allocator/69293_neg.cc
@@ -46,7 +46,7 @@ test01()
    scoped_alloc sa;
    auto p = sa.allocate(1);
    sa.construct(p);  // this is required to be ill-formed
-  // { dg-error "failed: .* uses_allocator is true" "" { target *-*-* } 0 }
+  // { dg-error "failed due to requirement \[^\n\r\]*: .* uses_allocator is true" 
"" { target *-*-* } 0 }
  }
// Needed because of PR c++/92193 
diff --git a/libstdc++-v3/testsuite/20_util/uses_allocator/69293_neg.cc 
b/libstdc++-v3/testsuite/20_util/uses_allocator/69293_neg.cc
index 423ecc4d90e..b6d925edcd3 100644
--- a/libstdc++-v3/testsuite/20_util/uses_allocator/69293_neg.cc
+++ b/libstdc++-v3/testsuite/20_util/uses_allocator/69293_neg.cc
@@ -44,6 +44,6 @@ test01()
  {
    alloc_type a;
    std::tuple<X> t(std::allocator_arg, a); // this is required to be ill-formed
-  // { dg-error "failed: .* uses_allocator is true" "" { target *-*-* } 0 }
+  // { dg-error "failed due to requirement \[^\n\r\]*: .* uses_allocator is true" 
"" { target *-*-* } 0 }
    // { dg-error "no matching function for call" "" { target c++2a } 0 }
  }
diff --git a/libstdc++-v3/testsuite/20_util/uses_allocator/cons_neg.cc 
b/libstdc++-v3/testsuite/20_util/uses_allocator/cons_neg.cc
index c3ff05aa72a..20190feaa0b 100644
--- a/libstdc++-v3/testsuite/20_util/uses_allocator/cons_neg.cc
+++ b/libstdc++-v3/testsuite/20_util/uses_allocator/cons_neg.cc
@@ -43,5 +43,5 @@ void test01()
tuple<Type> t(allocator_arg, a, 1); 
  }
-// { dg-error "failed: .* uses_allocator is true" "" { target *-*-* } 0 }
+// { dg-error "failed due to requirement \[^\n\r\]*: .* uses_allocator is true" 
"" { target *-*-* } 0 }
  // { dg-error "no matching function for call" "" { target c++2a } 0 }
diff --git a/libstdc++-v3/testsuite/26_numerics/random/pr60037-neg.cc 
b/libstdc++-v3/testsuite/26_numerics/random/pr60037-neg.cc
index ba252ef34fe..e2066cd59ee 100644
--- a/libstdc++-v3/testsuite/26_numerics/random/pr60037-neg.cc
+++ b/libstdc++-v3/testsuite/26_numerics/random/pr60037-neg.cc
@@ -10,6 +10,6 @@ std::__detail::_Adaptor<std::mt19937, unsigned long> 
aurng(urng);
  auto x = std::generate_canonical<std::size_t,
                        std::numeric_limits<std::size_t>::digits>(urng);
-// { dg-error "static assertion failed: template argument must be a floating point type" "" { target *-*-* } 167 } 
+// { dg-error "static assertion failed due to requirement \[^\n\r\]*: template argument must 
be a floating point type" "" { target *-*-* } 167 }
-// { dg-error "static assertion failed: template argument must be a floating point type" "" { target *-*-* } 3346 } 
+// { dg-error "static assertion failed due to requirement \[^\n\r\]*: template argument must 
be a floating point type" "" { target *-*-* } 3346 }

base-commit: 04126e46eb2d829d7b4149d394b667e878912cc8

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