c++: dependent bases and 'this' availability [PR103831]

Here during satisfaction of B's constraints we're failing to reject the
object-less call to the non-static member function A::size ultimately
because satisfaction is performed in the (access) context of the class
template B, which has a dependent base, and so the any_dependent_bases_p
check within build_new_method_call causes us to not reject the call.
(Subsequent constexpr evaluation of the call succeeds since the function
is effectively static.)

This patch fixes this by refining the any_dependent_bases_p check within
build_new_method_call: if we're in a context where 'this' is unavailable,
then we cannot resolve the implicit object regardless of the presence of
a dependent base.  So let's also check current_class_ptr alongside a_d_b_p.

	PR c++/103831

gcc/cp/ChangeLog:

	* call.c (build_new_method_call): Consider dependent bases only
	if 'this' is available.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-class3.C: New test.
	* g++.dg/template/non-dependent18.C: New test.

gcc/cp/call.c

@@ -11125,7 +11125,7 @@ build_new_method_call (tree instance, tree fns, vec<tree, va_gc> **args,
 		     we know we really need it.  */
 		  cand->first_arg = instance;
 		}
-	      else if (any_dependent_bases_p ())
+	      else if (current_class_ptr && any_dependent_bases_p ())
 		/* We can't tell until instantiation time whether we can use
 		   *this as the implicit object argument.  */;
 	      else

gcc/testsuite/g++.dg/cpp2a/concepts-class3.C

@@ -0,0 +1,12 @@
+// PR c++/103831
+// { dg-do compile { target c++20 } }
+
+struct A {
+  constexpr int size() { return 42; } // non-static
+};
+
+template<class T>
+  requires (T::size() == 42) // { dg-error "without object" }
+struct B : T { };
+
+template struct B<A>; // { dg-error "constraint" }

gcc/testsuite/g++.dg/template/non-dependent18.C

@@ -0,0 +1,19 @@
+// PR c++/103831
+// { dg-do compile { target c++11 } }
+
+struct A {
+  constexpr int size() { return 42; } // non-static
+};
+
+template<class T>
+struct B : T {
+  static_assert(A::size() == 42, ""); // { dg-error "without object" }
+
+  static int f() {
+    static_assert(A::size() == 42, ""); // { dg-error "without object" }
+    return A::size(); // { dg-error "without object" }
+  }
+
+  int n = A::size();
+  static const int m = A::size(); // { dg-error "without object" }
+};