[CSDiag] Diagnose KeyPath tuple access as unsupported even when sugared

Follow-up for [SR-9213](https://bugs.swift.org/browse/SR-9213)

Author: xedin

lib/Sema/CSDiag.cpp

@@ -6697,16 +6697,16 @@ static bool diagnoseKeyPathComponents(ConstraintSystem &CS, KeyPathExpr *KPE,
                                            : defaultUnqualifiedLookupOptions),
                                corrections);
 
-      if (currentType)
-        if (isa<TupleType>(currentType.getPointer())) {
+      if (currentType) {
+        if (currentType->is<TupleType>()) {
           TC.diagnose(KPE->getLoc(), diag::expr_keypath_unimplemented_tuple);
           isInvalid = true;
           break;
         }
         else
           TC.diagnose(componentNameLoc, diag::could_not_find_type_member,
                       currentType, componentName);
-      else
+      } else
         TC.diagnose(componentNameLoc, diag::use_unresolved_identifier,
                     componentName, false);
 

test/expr/unary/keypath/keypath-unimplemented.swift

@@ -22,3 +22,14 @@ let _: KeyPath<TupleKeypath, Int> = \TupleKeypath.labeled.foo // expected-error
 let _: KeyPath<TupleKeypath, String> = \TupleKeypath.labeled.bar // expected-error {{key path support for tuples is not implemented}}
 let _: KeyPath<TupleKeypath, Int> = \TupleKeypath.unlabeled.0 // expected-error {{key path support for tuples is not implemented}}
 let _: KeyPath<TupleKeypath, String> = \TupleKeypath.unlabeled.1 // expected-error {{key path support for tuples is not implemented}}
+
+struct S {
+  typealias X = (lhs: Int, rhs: Int)
+  typealias Y = (Int, Int)
+
+  let x: X
+  let y: Y
+}
+
+let _: KeyPath<S, Int> = \S.x.lhs // expected-error {{key path support for tuples is not implemented}}
+let _: KeyPath<S, Int> = \S.y.0   // expected-error {{key path support for tuples is not implemented}}