c++: Don't emit deprecated warnings or unavailable errors on lambda declarations

On the following testcase, we emit deprecated warnings or unavailable errors
even on merge declarations of those lambdas (the dg-bogus directives), while
IMHO we should emit them only when something actually calls those lambdas.

The following patch temporarily disables that diagnostics during
maybe_add_lambda_conv_op.

PR2173R1 also says that ambiguity between attribute-specifier-seq at the
end of requires-clause and attribute-specifier-seq from lambda-expression
should be resolved to attribute-specifier-seq for the latter.  Do we need
to do anything about that?  I mean, can a valid requires-clause end with
an attribute-specifier-seq?  Say operator int [[]] is valid primary
expression, but requires operator int [[]] isn't valid, nor is
requires operator int, no?

2022-05-04  Jakub Jelinek  <jakub@redhat.com>

	* lambda.cc: Include decl.h.
	(maybe_add_lambda_conv_op): Temporarily override deprecated_state to
	UNAVAILABLE_DEPRECATED_SUPPRESS.

	* g++.dg/cpp23/lambda-attr1.C: New test.
	* g++.dg/cpp23/lambda-attr2.C: New test.

1 files changed, 9 insertions, 3 deletions

diff --git a/gcc/cp/lambda.cc b/gcc/cp/lambda.cc
index 10834d6..afac53b 100644
--- a/gcc/cp/lambda.cc
+++ b/gcc/cp/lambda.cc
@@ -31,6 +31,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "toplev.h"
 #include "gimplify.h"
 #include "target.h"
+#include "decl.h"
 
 /* Constructor for a lambda expression.  */
 
@@ -1192,9 +1193,14 @@ maybe_add_lambda_conv_op (tree type)
 	}
     }
   else
-    call = build_call_a (callop,
-			 direct_argvec->length (),
-			 direct_argvec->address ());
+    {
+      /* Don't warn on deprecated or unavailable lambda declarations, unless
+	 the lambda is actually called.  */
+      auto du = make_temp_override (deprecated_state,
+				    UNAVAILABLE_DEPRECATED_SUPPRESS);
+      call = build_call_a (callop, direct_argvec->length (),
+			   direct_argvec->address ());
+    }
 
   CALL_FROM_THUNK_P (call) = 1;
   SET_EXPR_LOCATION (call, UNKNOWN_LOCATION);