c++: constinit on pointer to function [PR104066]

[dcl.constinit]: "The constinit specifier shall be applied only to
a declaration of a variable with static or thread storage duration."

Thus, this ought to be OK:

  constinit void (*p)() = nullptr;

but the error message I introduced when implementing constinit was
not looking at funcdecl_p, so the code above was rejected.

Fixed thus.  I'm checking constinit_p first because I think that's
far more likely to be false than funcdecl_p.

	PR c++/104066

gcc/cp/ChangeLog:

	* decl.cc (grokdeclarator): Check funcdecl_p before complaining
	about constinit.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/constinit18.C: New test.

gcc/cp/decl.cc

@@ -13071,7 +13071,7 @@ grokdeclarator (const cp_declarator *declarator,
 			  "an array", name);
 		return error_mark_node;
 	      }
-	    if (constinit_p)
+	    if (constinit_p && funcdecl_p)
 	      {
 		error_at (declspecs->locations[ds_constinit],
 			  "%<constinit%> on function return type is not "

gcc/testsuite/g++.dg/cpp2a/constinit18.C

@@ -0,0 +1,12 @@
+// PR c++/104066
+// { dg-do compile { target c++20 } }
+
+constinit void (*p)() = nullptr;
+constinit void (*pp)() = nullptr;
+void fn();
+constinit void (&r)() = fn;
+
+extern constinit long (* const syscall_reexported) (long, ...);
+
+constinit void bad (); // { dg-error ".constinit. on function return type is not allowed" }
+constinit void bad () { } // { dg-error ".constinit. on function return type is not allowed" }