From: Matthias Kretz <kretz@kde.org>

This attribute overrides the diagnostics output string for the entity it
appertains to. The motivation is to improve QoI for library TS
implementations, where diagnostics have a very bad signal-to-noise ratio
due to the long namespaces involved.

On Tuesday, 27 April 2021 11:46:48 CEST Jonathan Wakely wrote:
> I think it's a great idea and would like to use it for all the TS
> implementations where there is some inline namespace that the user
> doesn't care about. std::experimental::fundamentals_v1:: would be much
> better as just std::experimental::, or something like std::[LFTS]::.

With the attribute, it is possible to solve PR89370 and make
std::__cxx11::basic_string<_CharT, _Traits, _Alloc> appear as
std::string in diagnostic output without extra hacks to recognize the
type.

gcc/ChangeLog:

	PR c++/89370
	* doc/extend.texi: Document the diagnose_as attribute.
	* doc/invoke.texi: Document -fno-diagnostics-use-aliases.

gcc/c-family/ChangeLog:

	PR c++/89370
	* c.opt (fdiagnostics-use-aliases): New diagnostics flag.

gcc/cp/ChangeLog:

	PR c++/89370
	* error.c (dump_scope): When printing the name of a namespace,
	look for the diagnose_as attribute. If found, print the
	associated string instead of calling dump_decl.
	(dump_decl_name_or_diagnose_as): New function to replace
	dump_decl (pp, DECL_NAME(t), flags) and inspect the tree for the
	diagnose_as attribute before printing the DECL_NAME.
	(dump_aggr_type): If the type has a diagnose_as attribute, print
	the associated string instead of printing the original type
	name.
	(dump_simple_decl): Call dump_decl_name_or_diagnose_as instead
	of dump_decl.
	(dump_decl): Ditto.
	(lang_decl_name): Ditto.
	(dump_function_decl): Ensure complete replacement of the class
	template diagnostics if a diagnose_as attribute is present.
	(dump_function_name): Replace the function diagnostic output if
	the diagnose_as attribute is set.
	* name-lookup.c (handle_namespace_attrs): Handle the diagnose_as
	attribute. Ensure exactly one string argument. Ensure previous
	diagnose_as attributes used the same name.
	* tree.c (cxx_attribute_table): Add diagnose_as attribute to the
	table.
	(check_diagnose_as_redeclaration): New function; copied and
	adjusted from check_abi_tag_redeclaration.
	(handle_diagnose_as_attribute): New function; copied and
	adjusted from handle_abi_tag_attribute. If the given *node is a
	TYPE_DECL and the TREE_TYPE is an implicit class template
	instantiation, call decl_attributes to add the diagnose_as
	attribute to the TREE_TYPE.
---
 gcc/c-family/c.opt   |   4 ++
 gcc/cp/error.c       |  85 ++++++++++++++++++++++++++++---
 gcc/cp/name-lookup.c |  27 ++++++++++
 gcc/cp/tree.c        | 117 +++++++++++++++++++++++++++++++++++++++++++
 gcc/doc/extend.texi  |  37 ++++++++++++++
 gcc/doc/invoke.texi  |   9 +++-
 6 files changed, 270 insertions(+), 9 deletions(-)


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 Dr. Matthias Kretz                           https://mattkretz.github.io
 GSI Helmholtz Centre for Heavy Ion Research               https://gsi.de
 std::experimental::simd              https://github.com/VcDevel/std-simd
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