Merge pull request swiftlang#21655 from DougGregor/implements-attr-crash-rdar46678653

Author: swift-ci

lib/Sema/TypeCheckProtocol.cpp

@@ -967,9 +967,12 @@ witnessHasImplementsAttrForExactRequirement(ValueDecl *witness,
   assert(requirement->isProtocolRequirement());
   auto *PD = cast<ProtocolDecl>(requirement->getDeclContext());
   if (auto A = witness->getAttrs().getAttribute<ImplementsAttr>()) {
-    Type T = A->getProtocolType().getType();
-    if (T->castTo<ProtocolType>()->getDecl() == PD) {
-      return A->getMemberName() == requirement->getFullName();
+    if (Type T = A->getProtocolType().getType()) {
+      if (auto ProtoTy = T->getAs<ProtocolType>()) {
+        if (ProtoTy->getDecl() == PD) {
+          return A->getMemberName() == requirement->getFullName();
+        }
+      }
     }
   }
   return false;

validation-test/compiler_crashers_2_fixed/0187-rdar46678653.swift

@@ -0,0 +1,15 @@
+// RUN: not %target-swift-frontend -typecheck %s
+
+protocol P: class { }
+
+protocol Q {
+  func g()
+}
+
+protocol P { }
+
+struct S : Q {
+  @_implements(P, g())
+  func h() {}
+}
+