c++: Fix return type deduction during satisfaction

During satisfaction that's entered through the three-parameter version
of satisfy_declaration_constraints, current_function_decl gets set to
the dependent DECL_TEMPLATE_RESULT for sake of access checking.  This
makes the predicate in_template_function return true during satisfaction
from this entrypoint, which in turn makes calls to mark_used exit early
before it does its full processing.  This leads to us accepting the
invalid testcase below due to mark_used never attempting to deduce the
return type of A::foo() and detecting failure thereof.

It seems wrong for in_template_function to be true during instantiation
or during satisfaction, so this patch makes in_template_function inspect
the less volatile cfun->decl instead of current_function_decl.

gcc/cp/ChangeLog:

	* pt.c (in_template_function): Inspect cfun->decl instead of
	current_function_decl.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-requires23.C: New test.

gcc/cp/pt.c

@@ -10805,14 +10805,16 @@ uses_template_parms (tree t)
   return dependent_p;
 }
 
-/* Returns true iff current_function_decl is an incompletely instantiated
+/* Returns true iff we're processing an incompletely instantiated function
    template.  Useful instead of processing_template_decl because the latter
    is set to 0 during instantiate_non_dependent_expr.  */
 
 bool
 in_template_function (void)
 {
-  tree fn = current_function_decl;
+  /* Inspect the less volatile cfun->decl instead of current_function_decl;
+     the latter might get set for e.g. access checking during satisfaction.  */
+  tree fn = cfun ? cfun->decl : NULL_TREE;
   bool ret;
   ++processing_template_decl;
   ret = (fn && DECL_LANG_SPECIFIC (fn)

gcc/testsuite/g++.dg/cpp2a/concepts-requires23.C

@@ -0,0 +1,9 @@
+// { dg-do compile { target c++20 } }
+
+// Verify f<A>'s associated constraints evaluate to false due
+// to return type deduction failure for A::foo().
+
+template <class T> concept fooable = requires { T::foo(0); };
+template <fooable T> int f ();
+struct A { static auto *foo(auto); };
+int a = f<A>(); // { dg-error "no match" }