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 <nathanieloshead@gmail.com>
Reviewed-by: Patrick Palka <ppalka@redhat.com>
Reviewed-by: Jason Merrill <jason@redhat.com>

2 files changed, 3 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/std/format/arguments/args_neg.cc b/libstdc++-v3/testsuite/std/format/arguments/args_neg.cc
index ded56fe..83c7b22 100644
--- a/libstdc++-v3/testsuite/std/format/arguments/args_neg.cc
+++ b/libstdc++-v3/testsuite/std/format/arguments/args_neg.cc
@@ -42,3 +42,4 @@ void test_const_arg()
 }
 
 // { dg-prune-output "no matching function for call to .*::basic_format_arg<" }
+// { dg-prune-output "use of deleted function" }
diff --git a/libstdc++-v3/testsuite/std/format/string_neg.cc b/libstdc++-v3/testsuite/std/format/string_neg.cc
index 09cc9a2..acae88e 100644
--- a/libstdc++-v3/testsuite/std/format/string_neg.cc
+++ b/libstdc++-v3/testsuite/std/format/string_neg.cc
@@ -8,3 +8,5 @@ auto s = std::format(" {9} "); // { dg-error "call to consteval function" }
 struct X { };
 std::format_string<X> str(""); // { dg-error "here" }
 // { dg-error "std::formatter must be specialized" "" { target *-*-* } 0 }
+
+// { dg-prune-output "use of deleted function" }