c++: DR2237, cdtor and template-id tweaks [PR107126]

Since my r11-532 changes to implement DR2237, for this test:

  template<typename T>
  struct S {
    S<T>();
  };

in C++20 we emit the ugly:

q.C:3:8: error: expected unqualified-id before ')' token
    3 |   S<T>();

which doesn't explain what the problem is.  This patch improves that
diagnostic, reduces the error to a pedwarn, and adds a -Wc++20-compat
diagnostic.  We now say:

q.C:3:7: warning: template-id not allowed for constructor in C++20 [-Wtemplate-id-cdtor]
    3 |   S<T>();
q.C:3:7: note: remove the '< >'

This patch also fixes
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97202#c8>
where the C++20 diagnostic was missing altogether:  The problem was that I checked
for CPP_TEMPLATE_ID too early, at a point at which cp_parser_template_id may not
have been called yet.  So let's check for it at the end of the function, after
the tentative parse and rollback.

-Wc++20-compat triggered in libitm/; I sent a patch for that.

	DR 2237
	PR c++/107126
	PR c++/97202

gcc/c-family/ChangeLog:

	* c-opts.cc (c_common_post_options): In C++20 or with -Wc++20-compat,
	turn on -Wtemplate-id-cdtor.
	* c.opt (Wtemplate-id-cdtor): New.

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_unqualified_id): Downgrade the DR2237 error to
	a pedwarn.
	(cp_parser_constructor_declarator_p): Likewise.

gcc/ChangeLog:

	* doc/invoke.texi: Document -Wtemplate-id-cdtor.

gcc/testsuite/ChangeLog:

	* g++.dg/DRs/dr2237.C: Adjust dg-error.
	* g++.dg/parse/constructor2.C: Likewise.
	* g++.dg/template/error34.C: Likewise.
	* g++.old-deja/g++.pt/ctor2.C: Likewise.
	* g++.dg/DRs/dr2237-2.C: New test.
	* g++.dg/DRs/dr2237-3.C: New test.
	* g++.dg/DRs/dr2237-4.C: New test.
	* g++.dg/DRs/dr2237-5.C: New test.
	* g++.dg/warn/Wtemplate-id-cdtor-1.C: New test.
	* g++.dg/warn/Wtemplate-id-cdtor-2.C: New test.
	* g++.dg/warn/Wtemplate-id-cdtor-3.C: New test.
	* g++.dg/warn/Wtemplate-id-cdtor-4.C: New test.

1 files changed, 18 insertions, 1 deletions

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 71339b8..0de184f 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -270,7 +270,7 @@ in the following sections.
 -Wno-non-template-friend  -Wold-style-cast
 -Woverloaded-virtual  -Wno-pmf-conversions -Wself-move -Wsign-promo
 -Wsized-deallocation  -Wsuggest-final-methods
--Wsuggest-final-types  -Wsuggest-override
+-Wsuggest-final-types  -Wsuggest-override  -Wno-template-id-cdtor
 -Wno-terminate  -Wno-vexing-parse  -Wvirtual-inheritance
 -Wno-virtual-move-assign  -Wvolatile  -Wzero-as-null-pointer-constant}
 
@@ -4604,6 +4604,23 @@ namespaces, and this may be used to enforce that rule.  The warning is
 inactive inside a system header file, such as the STL, so one can still
 use the STL.  One may also use using directives and qualified names.
 
+@opindex Wtemplate-id-cdtor
+@opindex Wno-template-id-cdtor
+@item -Wno-template-id-cdtor @r{(C++ and Objective-C++ only)}
+Disable the warning about the use of simple-template-id as the declarator-id
+of a constructor or destructor, which became invalid in C++20 via DR 2237.
+For example:
+
+@smallexample
+template<typename T> struct S @{
+  S<T>(); // should be S();
+  ~S<T>();  // should be ~S();
+@};
+@end smallexample
+
+@option{-Wtemplate-id-cdtor} is enabled by default with
+@option{-std=c++20}; it is also enabled by @option{-Wc++20-compat}.
+
 @opindex Wterminate
 @opindex Wno-terminate
 @item -Wno-terminate @r{(C++ and Objective-C++ only)}