Sema: Fix checkTypeWitness() for Objective-C protocol compositions

I forgot that a protocol composition (C & P) can satisfy a superclass
requirement [T : D] in one narrow case implemented in
TypeBase::isExactSuperclassOf():

- D is a superclass of C
- P is an @objc protocol
- C is declared in Objective-C

This case was ruled out here because the code assumed the type witness
had to be a concrete class or archetype to satisfy a superclass
requirement.

Fixes rdar://problem/123543200.

Author: slavapestov

lib/Sema/AssociatedTypeInference.cpp

@@ -190,12 +190,19 @@ checkTypeWitness(Type type, AssociatedTypeDecl *assocType,
     assert(superclassDecl);
 
     // Fish a class declaration out of the type witness.
-    auto classDecl = type->getClassOrBoundGenericClass();
-    if (!classDecl) {
-      if (auto archetype = type->getAs<ArchetypeType>()) {
-        if (auto superclassType = archetype->getSuperclass())
+    ClassDecl *classDecl = nullptr;
+
+    if (auto archetype = type->getAs<ArchetypeType>()) {
+      if (auto superclassType = archetype->getSuperclass())
           classDecl = superclassType->getClassOrBoundGenericClass();
-      }
+    } else if (type->isObjCExistentialType()) {
+      // For self-conforming Objective-C existentials, the exact check is
+      // implemented in TypeBase::isExactSuperclassOf(). Here, we just always
+      // look through into a superclass of a composition.
+      if (auto superclassType = type->getSuperclass())
+        classDecl = superclassType->getClassOrBoundGenericClass();
+    } else {
+      classDecl = type->getClassOrBoundGenericClass();
     }
 
     if (!classDecl || !superclassDecl->isSuperclassOf(classDecl))

test/decl/protocol/req/associated_type_inference_objc_existential.swift

@@ -0,0 +1,17 @@
+// RUN: %target-typecheck-verify-swift
+
+// REQUIRES: objc_interop
+
+import Foundation
+
+protocol P1 {
+  associatedtype A: NSObject
+}
+
+@objc protocol P2 {}
+
+struct S: P1 {
+  typealias A = NSObject & P2
+}
+
+let x: (any NSObject & P2).Type = S.A.self