c++: Unwrap type traits defined in terms of builtins within diagnostics [PR117294]

Currently, concept failures of standard type traits just report
'expression X<T> evaluates to false'.  However, many type traits are
actually defined in terms of compiler builtins; we can do better here.
For instance, 'is_constructible_v' could go on to explain why the type
is not constructible, or 'is_invocable_v' could list potential
candidates.

Apart from concept diagnostics, this is also useful when using such
traits in a 'static_assert' directly, so this patch also adjusts the
diagnostics in that context.

As a first step to supporting that we need to be able to map the
standard type traits to the builtins that they use.  Rather than adding
another list that would need to be kept up-to-date whenever a builtin is
added, this patch instead tries to detect any variable template defined
directly in terms of a TRAIT_EXPR.

This patch also adjusts 'diagnose_trait_expr' to provide more helpful
diagnostics for these cases.  Not all type traits have yet been updated,
this patch just updates those that seem particularly valuable or
straight-forward.  The function also gets moved to cp/semantics.cc to be
closer to 'trait_expr_value'.

Various other parts of the compiler are also adjusted here to assist in
making clear diagnostics, such as making more use of 'is_stub_object' to
refer to a type directly rather than in terms of 'std::declval<T>()'.
Additionally, since there are now more cases of nesting within a
'static_assert'ion I felt it was helpful for the experimental-nesting
mode to nest here as well.

	PR c++/117294
	PR c++/113854

gcc/cp/ChangeLog:

	* call.cc (implicit_conversion_error): Hide label when printing
	a stub object.
	(convert_like_internal): Likewise, and nest candidate
	diagnostics.
	* constexpr.cc (diagnose_failing_condition): Nest diagnostics,
	attempt to provide more helpful diagnostics for traits.
	* constraint.cc (satisfy_atom): Pass result before constant
	evaluation to diagnose_atomic_constraint.
	(diagnose_trait_expr): Adjust diagnostics for clarity and
	detail.
	(maybe_diagnose_standard_trait): New function.
	(diagnose_atomic_constraint): Attempt to provide more helpful
	diagnostics for more traits.
	* cp-tree.h (explain_not_noexcept): Declare new function.
	(is_trivially_xible): Add parameter.
	(is_nothrow_xible): Likewise.
	(is_xible): Likewise.
	(is_convertible): Likewise.
	(is_nothrow_convertible): Likewise.
	(diagnose_trait_expr): Declare new function.
	(maybe_diagnose_standard_trait): Declare new function.
	* error.cc (dump_type) <case TREE_VEC>: Handle trait types.
	* except.cc (explain_not_noexcept): New function.
	* method.cc (build_trait_object): Add complain parameter.
	(build_invoke): Propagate complain parameter.
	(assignable_expr): Add explain parameter to show diagnostics.
	(constructible_expr): Likewise.
	(destructible_expr): Likewise.
	(is_xible_helper): Replace trivial flag with explain flag,
	add diagnostics.
	(is_trivially_xible): New explain flag.
	(is_nothrow_xible): Likewise.
	(is_xible): Likewise.
	(is_convertible_helper): Add complain flag.
	(is_convertible): New explain flag.
	(is_nothrow_convertible): Likewise.
	* typeck.cc (cp_build_function_call_vec): Add handling for stub
	objects.
	(convert_arguments): Always return -1 on error.
	* typeck2.cc (cxx_readonly_error): Add handling for stub
	objects.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/any/misc/any_cast_neg.cc: Adjust
	diagnostics.
	* testsuite/20_util/expected/illformed_neg.cc: Likewise.
	* testsuite/20_util/optional/monadic/or_else_neg.cc: Likewise.
	* testsuite/23_containers/array/creation/3_neg.cc: Likewise.
	* testsuite/24_iterators/range_generators/lwg3900.cc: Likewise.
	* testsuite/29_atomics/atomic/requirements/types_neg.cc:
	Likewise.
	* testsuite/30_threads/stop_token/stop_callback/invocable_neg.cc:
	Likewise.
	* testsuite/30_threads/stop_token/stop_callback/destructible_neg.cc:
	Likewise.
	* testsuite/std/format/arguments/args_neg.cc: Likewise.
	* testsuite/std/format/string_neg.cc: Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-traits3.C: Adjust diagnostics.
	* g++.dg/cpp2a/concepts-traits4.C: New test.
	* g++.dg/diagnostic/static_assert5.C: New test.
	* g++.dg/ext/has_virtual_destructor2.C: New test.
	* g++.dg/ext/is_assignable2.C: New test.
	* g++.dg/ext/is_constructible9.C: New test.
	* g++.dg/ext/is_convertible7.C: New test.
	* g++.dg/ext/is_destructible3.C: New test.
	* g++.dg/ext/is_invocable6.C: New test.
	* g++.dg/ext/is_virtual_base_of_diagnostic2.C: New test.

Signed-off-by: Nathaniel Shead <[email protected]>
Reviewed-by: Patrick Palka <[email protected]>
Reviewed-by: Jason Merrill <[email protected]>

Author: wreien

gcc/cp/call.cc

@@ -4927,6 +4927,11 @@ implicit_conversion_error (location_t loc, tree type, tree expr)
 	   && !CP_AGGREGATE_TYPE_P (type))
     error_at (loc, "designated initializers cannot be used with a "
 	      "non-aggregate type %qT", type);
+  else if (is_stub_object (expr))
+    /* The expression is generated by a trait check, we don't have
+       a useful location to highlight the label.  */
+    error_at (loc, "could not convert %qH to %qI",
+	      TREE_TYPE (expr), type);
   else
     {
       range_label_for_type_mismatch label (TREE_TYPE (expr), type);
@@ -8698,6 +8703,7 @@ convert_like_internal (conversion *convs, tree expr, tree fn, int argnum,
   diagnostic_t diag_kind;
   int flags;
   location_t loc = cp_expr_loc_or_input_loc (expr);
+  const bool stub_object_p = is_stub_object (expr);
 
   if (convs->bad_p && !(complain & tf_error))
     return error_mark_node;
@@ -8774,7 +8780,10 @@ convert_like_internal (conversion *convs, tree expr, tree fn, int argnum,
 				      "from %qH to %qI", TREE_TYPE (expr),
 				      totype);
 	      if (complained)
-		print_z_candidate (loc, N_("candidate is:"), t->cand);
+		{
+		  auto_diagnostic_nesting_level sentinel;
+		  print_z_candidate (loc, N_("candidate is:"), t->cand);
+		}
 	      expr = convert_like (t, expr, fn, argnum,
 				   /*issue_conversion_warnings=*/false,
 				   /*c_cast_p=*/false, /*nested_p=*/true,
@@ -8799,7 +8808,14 @@ convert_like_internal (conversion *convs, tree expr, tree fn, int argnum,
 	  else if (t->kind == ck_identity)
 	    break;
 	}
-      if (!complained && expr != error_mark_node)
+      if (!complained && stub_object_p)
+	{
+	  /* An error diagnosed within a trait, don't give extra labels.  */
+	  error_at (loc, "invalid conversion from %qH to %qI",
+		    TREE_TYPE (expr), totype);
+	  complained = 1;
+	}
+      else if (!complained && expr != error_mark_node)
 	{
 	  range_label_for_type_mismatch label (TREE_TYPE (expr), totype);
 	  gcc_rich_location richloc (loc, &label, highlight_colors::percent_h);

gcc/cp/constexpr.cc

@@ -2883,10 +2883,15 @@ diagnose_failing_condition (tree bad, location_t cloc, bool show_expr_p,
   if (TREE_CODE (bad) == CLEANUP_POINT_EXPR)
     bad = TREE_OPERAND (bad, 0);
 
+  auto_diagnostic_nesting_level sentinel;
+
   /* Actually explain the failure if this is a concept check or a
      requires-expression.  */
   if (concept_check_p (bad) || TREE_CODE (bad) == REQUIRES_EXPR)
     diagnose_constraints (cloc, bad, NULL_TREE);
+  /* Similarly if this is a standard trait.  */
+  else if (maybe_diagnose_standard_trait (cloc, bad))
+    ;
   else if (COMPARISON_CLASS_P (bad)
 	   && ARITHMETIC_TYPE_P (TREE_TYPE (TREE_OPERAND (bad, 0))))
     {