[5.0] IRGen: Don't assert on keypaths that use a subclass as root

 class A {
   let x : Int = 0
 }

 class B : A {}

 _ = \B.x

* No ABI impact
* Correctness fix: We would emit wrong offsets for stored properties in
this case in an non-assert build.

rdar://46562046

Author: aschwaighofer

lib/IRGen/GenKeyPath.cpp

@@ -837,6 +837,15 @@ emitKeyPathComponent(IRGenModule &IGM,
     // ObjC-ness and resilience of the class hierarchy, there might be a few
     // different ways we need to go about this.
     if (loweredBaseTy.getClassOrBoundGenericClass()) {
+
+      // Use the property's class type to determine the field access.
+      auto propertyBaseDecl = property->getDeclContext()->getSelfClassDecl();
+      auto currentBaseTy =
+          loweredBaseTy.getASTType()->getSuperclassForDecl(propertyBaseDecl);
+      assert(currentBaseTy->getClassOrBoundGenericClass() == propertyBaseDecl);
+      loweredBaseTy =
+          IGM.getLoweredType(AbstractionPattern::getOpaque(), currentBaseTy);
+
       switch (getClassFieldAccess(IGM, loweredBaseTy, property)) {
       case FieldAccess::ConstantDirect: {
         // Known constant fixed offset.

test/IRGen/keypaths.sil

@@ -87,6 +87,17 @@ sil_vtable C2 {}
 // CHECK-32-SAME: <i32 0x0380_0008> }>
 // CHECK-64-SAME: <i32 0x0380_0010> }>
 
+// -- %d1: C1.x
+// CHECK: [[KP_D1:@keypath(\..*)?]] = private global <{ {{.*}} }> <{
+// CHECK-SAME: [[WORD]]* @keypath_once
+// CHECK-SAME: @"symbolic
+// CHECK-SAME: @"symbolic
+//               -- instantiable in-line, size 4
+// CHECK-SAME: <i32 0x8000_0004>,
+// -- 0x0300_0000 (class) + mutable + offset of C.x
+// CHECK-32-SAME: <i32 0x0380_0008> }>
+// CHECK-64-SAME: <i32 0x0380_0010> }>
+
 // -- %e: C.y
 // CHECK: [[KP_E:@keypath(\..*)?]] = private global <{ {{.*}} }> <{
 // CHECK-SAME: [[WORD]]* @keypath_once
@@ -232,9 +243,10 @@ entry:
   %b = keypath $KeyPath<S, String>, (root $S; stored_property #S.y : $String)
   // CHECK: call %swift.refcounted* @swift_getKeyPath(i8* bitcast ({{.*}} [[KP_C]] to i8*), i8* undef)
   %c = keypath $KeyPath<S, C>, (root $S; stored_property #S.z : $C)
-
   // CHECK: call %swift.refcounted* @swift_getKeyPath(i8* bitcast ({{.*}} [[KP_D]] to i8*), i8* undef)
   %d = keypath $KeyPath<C, Int>, (root $C; stored_property #C.x : $Int)
+  // CHECK: call %swift.refcounted* @swift_getKeyPath(i8* bitcast ({{.*}} [[KP_D1]] to i8*), i8* undef)
+  %d1 = keypath $KeyPath<C1, Int>, (root $C1; stored_property #C.x : $Int)
   // CHECK: call %swift.refcounted* @swift_getKeyPath(i8* bitcast ({{.*}} [[KP_E]] to i8*), i8* undef)
   %e = keypath $KeyPath<C, String>, (root $C; stored_property #C.y : $String)
   // CHECK: call %swift.refcounted* @swift_getKeyPath(i8* bitcast ({{.*}} [[KP_F]] to i8*), i8* undef)