Sema: Fix crash when enum case satisfies requirement in @rethrows protocol

Author: slavapestov

lib/Sema/TypeCheckEffects.cpp

@@ -135,7 +135,18 @@ static bool classifyWitness(ModuleDecl *module,
                             ProtocolConformance *conformance, 
                             AbstractFunctionDecl *req) {
   auto declRef = conformance->getWitnessDeclRef(req);
-  auto witnessDecl = cast<AbstractFunctionDecl>(declRef.getDecl());
+  if (!declRef) {
+    // Invalid conformance.
+    return true;
+  }
+
+  auto witnessDecl = dyn_cast<AbstractFunctionDecl>(declRef.getDecl());
+  if (!witnessDecl) {
+    // Enum element constructors never throw.
+    assert(isa<EnumElementDecl>(declRef.getDecl()));
+    return false;
+  }
+
   switch (witnessDecl->getRethrowingKind()) {
     case FunctionRethrowingKind::None:
       // Witness doesn't throw at all, so it contributes nothing.

test/attr/attr_rethrows_protocol.swift

@@ -191,4 +191,18 @@ struct ThirdWitness : Third {
 
 func takesFirst<T : First>(_: T) rethrows {}
 
-takesFirst(FirstWitness())
+takesFirst(FirstWitness())
+
+// Crash with enum case
+@rethrows protocol WitnessedByEnumCase {
+  static func foo(_: Int) throws -> Self
+}
+
+enum MyEnum : WitnessedByEnumCase {
+  case foo(Int)
+  case bar
+}
+
+func takesWitnessedByEnumCase<T : WitnessedByEnumCase>(_: T) rethrows {}
+
+takesWitnessedByEnumCase(MyEnum.bar)