c++: Improve checking of decls with trailing return type [PR95820]

c++: Improve checking of decls with trailing return type [PR95820]

This is an ICE-on-invalid but I've been seeing it when reducing
various testcases, so it's more important for me than usually.

splice_late_return_type now checks that if we've seen a late return
type, the function return type was auto.  That's a fair assumption
but grokdeclarator/cdk_function wasn't giving errors for function
pointers and similar.  So we want to perform various checks not only
when funcdecl_p || inner_declarator == NULL.  But only give the
!late_return_type errors when funcdecl_p, to accept e.g.

auto (*fp)() = f;

in C++11.  Here's a diff -w to ease the review:

--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -12102,14 +12102,9 @@ grokdeclarator (const cp_declarator *declarator,

 	    /* Handle a late-specified return type.  */
 	    tree late_return_type = declarator->u.function.late_return_type;
-	    if (funcdecl_p
-		/* This is the case e.g. for
-		   using T = auto () -> int.  */
-		|| inner_declarator == NULL)
-	      {
 	    if (tree auto_node = type_uses_auto (type))
 	      {
-		    if (!late_return_type)
+		if (!late_return_type && funcdecl_p)
 		  {
 		    if (current_class_type
 			&& LAMBDA_TYPE_P (current_class_type))
@@ -12201,7 +12196,6 @@ grokdeclarator (const cp_declarator *declarator,
 			    "type specifier", name);
 		return error_mark_node;
 	      }
-	      }
 	    type = splice_late_return_type (type, late_return_type);
 	    if (type == error_mark_node)
 	      return error_mark_node;

gcc/cp/ChangeLog:

	PR c++/95820
	* decl.c (grokdeclarator) <case cdk_function>: Check also
	pointers/references/... to functions.

gcc/testsuite/ChangeLog:

	PR c++/95820
	* g++.dg/cpp1y/auto-fn58.C: New test.