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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>
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