Sema: Fix crash with invalid @_opaqueReturnTypeOf attribute

If we demangle the name but we can't resolve the opaque return
declaration, we would dereference a null pointer and crash.

Speculative fix for rdar://161067984.

Author: slavapestov

lib/Sema/TypeCheckType.cpp

@@ -4039,13 +4039,12 @@ TypeResolver::resolveOpaqueReturnType(TypeRepr *repr, StringRef mangledName,
     builder.getNodeFactory().createNode(Node::Kind::OpaqueReturnTypeOf);
   opaqueNode->addChild(definingDeclNode, builder.getNodeFactory());
   auto *opaqueDecl = builder.resolveOpaqueTypeDecl(opaqueNode);
-
-  auto *ownerDecl = opaqueDecl->getNamingDecl();
-  if (!ownerDecl) {
+  if (!opaqueDecl) {
     diagnose(repr->getLoc(), diag::no_opaque_return_type_of);
     return ErrorType::get(getASTContext());
   }
 
+  auto *ownerDecl = opaqueDecl->getNamingDecl();
   auto genericSig = ownerDecl->getInnermostDeclContext()
     ->getGenericSignatureOfContext();
 

test/ModuleInterface/invalid-opaque-result-types.swift

@@ -0,0 +1,29 @@
+// Test that we emit a diagnostic (and don't crash) when we cannot resolve
+// an opaque result type reference.
+//
+// RUN: %target-typecheck-verify-swift -module-name OpaqueTypes
+//
+
+public struct G<T> {
+  public func f() -> some Any
+}
+
+// Type doesn't demangle
+public typealias Bad1 = @_opaqueReturnTypeOf("nonsense", 0) __
+// expected-error@-1 {{unable to resolve type for _opaqueReturnTypeOf attribute}}
+
+// Type demangles but doesn't name an opaque return type
+public typealias Bad2 = @_opaqueReturnTypeOf("$sSi", 0) __
+// expected-error@-1 {{unable to resolve type for _opaqueReturnTypeOf attribute}}
+
+// Bad index
+public typealias Bad3 = @_opaqueReturnTypeOf("$s11OpaqueTypes1GV1fQryF", 5) __<Int>
+// expected-error@-1 {{unable to resolve type for _opaqueReturnTypeOf attribute}}
+
+// Bad generic argument
+public typealias Bad4 = @_opaqueReturnTypeOf("$s11OpaqueTypes1GV1fQryF", 0) __<InvalidType>
+// expected-error@-1 {{cannot find type 'InvalidType' in scope}}
+
+// Missing generic argument
+public typealias Bad5 = @_opaqueReturnTypeOf("$s11OpaqueTypes1GV1fQryF", 0) __
+// expected-error@-1 {{unable to resolve type for _opaqueReturnTypeOf attribute}}