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.

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);

gcc/testsuite/g++.dg/cpp23/lambda-attr1.C

@@ -0,0 +1,37 @@
+// P2173R1 - Attributes on Lambda-Expressions
+// { dg-do compile { target c++11 } }
+
+void
+foo (bool x, bool y)
+{
+  auto a = [][[noreturn]] () {};	// { dg-warning "'noreturn' function does return" }
+  if (x)
+    a ();
+  auto b = [][[noreturn]] {};		// { dg-warning "'noreturn' function does return" }
+  if (y)
+    b ();
+  auto c = [] [[ deprecated ]] () {};	// { dg-bogus "is deprecated" }
+  c ();					// { dg-warning "'foo\\\(bool, bool\\\)::<lambda\\\(\\\)>' is deprecated" }
+  auto d = [][[deprecated]] {};		// { dg-bogus "is deprecated" }
+  d ();					// { dg-warning "'foo\\\(bool, bool\\\)::<lambda\\\(\\\)>' is deprecated" }
+#if __cpp_generic_lambdas >= 201304
+  auto e = [] [[deprecated]] (auto x) {};	// { dg-bogus "is deprecated" }
+  e (0.0);				// { dg-warning "'foo\\\(bool, bool\\\)::<lambda\\\(auto:1\\\)>\[^\n\r]*' is deprecated" "" { target c++14 } }
+#endif
+#if __cpp_generic_lambdas >= 201707
+  auto f = [] <typename T> [[deprecated]] (T) {};	// { dg-bogus "is deprecated" }
+  f (1);				// { dg-warning "'foo\\\(bool, bool\\\)::<lambda\\\(T\\\)>\[^\n\r]*' is deprecated" "" { target c++20 } }
+#endif
+  auto g = [][[nodiscard]](int) { return 1; };
+  g (1);				// { dg-warning "ignoring return value of 'foo\\\(bool, bool\\\)::<lambda\\\(int\\\)>', declared with attribute 'nodiscard'" }
+  auto h = [] [[nodiscard]] { return 0; };
+  h ();					// { dg-warning "ignoring return value of 'foo\\\(bool, bool\\\)::<lambda\\\(\\\)>', declared with attribute 'nodiscard'" }
+  auto i = [] [[ gnu::unavailable ]] () {};
+  auto j = [][[gnu::unavailable]] {};
+#if __cpp_generic_lambdas >= 201304
+  auto k = [] [[gnu::unavailable]] (auto x) {};	// { dg-bogus "is unavailable" }
+#endif
+#if __cpp_generic_lambdas >= 201707
+  auto l = [] <typename T> [[gnu::unavailable]] (T) {};	// { dg-bogus "is unavailable" }
+#endif
+}

gcc/testsuite/g++.dg/cpp23/lambda-attr2.C

@@ -0,0 +1,19 @@
+// P2173R1 - Attributes on Lambda-Expressions
+// { dg-do compile { target c++11 } }
+
+void
+foo (bool x, bool y)
+{
+  auto i = [] [[ gnu::unavailable ]] () {};
+  i ();			// { dg-error "'foo\\\(bool, bool\\\)::<lambda\\\(\\\)>' is unavailable" }
+  auto j = [][[gnu::unavailable]] {};
+  j ();			// { dg-error "'foo\\\(bool, bool\\\)::<lambda\\\(\\\)>' is unavailable" }
+#if __cpp_generic_lambdas >= 201304
+  auto k = [] [[gnu::unavailable]] (auto x) {};	// { dg-bogus "is unavailable" }
+  k (0.0);		// { dg-error "'foo\\\(bool, bool\\\)::<lambda\\\(auto:1\\\)>\[^\n\r]*' is unavailable" "" { target c++14 } }
+#endif
+#if __cpp_generic_lambdas >= 201707
+  auto l = [] <typename T> [[gnu::unavailable]] (T) {};	// { dg-bogus "is unavailable" }
+  l (1);		// { dg-error "'foo\\\(bool, bool\\\)::<lambda\\\(T\\\)>\[^\n\r]*' is unavailable" "" { target c++20 } }
+#endif
+}