move-only types can't witness associatedtype requirements

Author: kavon

lib/Sema/TypeCheckProtocol.cpp

@@ -4596,6 +4596,14 @@ swift::checkTypeWitness(Type type, AssociatedTypeDecl *assocType,
       proto->isComputingRequirementSignature())
     return ErrorType::get(ctx);
 
+  // No move-only type can witness an associatedtype requirement.
+  if (type->isPureMoveOnly()) {
+    // describe the failure reason as it not conforming to Copyable
+    auto *copyable = ctx.getProtocol(KnownProtocolKind::Copyable);
+    assert(copyable && "missing _Copyable from stdlib!");
+    return CheckTypeWitnessResult(copyable->getDeclaredInterfaceType());
+  }
+
   const auto depTy = DependentMemberType::get(proto->getSelfInterfaceType(),
                                               assocType);
 

test/Constraints/moveonly_constraints.swift

@@ -162,4 +162,27 @@ func checkStdlibTypes(_ mo: MO) {
   _ = [MO]() // expected-error {{move-only type 'MO' cannot be used with generics yet}}
 
   let s: String = "hello \(mo)" // expected-error {{move-only type 'MO' cannot be used with generics yet}}
+}
+
+// ensure that associated types can't be witnessed by move-only types
+
+protocol HasType<Ty> {
+  associatedtype Ty // expected-note 3{{protocol requires nested type 'Ty'; do you want to add it?}}
+}
+
+class SomeGuy: HasType { // expected-error {{type 'SomeGuy' does not conform to protocol 'HasType'}}
+  typealias Ty = MO // expected-note {{possibly intended match 'SomeGuy.Ty' (aka 'MO') does not conform to '_Copyable'}}
+}
+
+struct AnotherGuy: HasType { // expected-error {{type 'AnotherGuy' does not conform to protocol 'HasType'}}
+  @_moveOnly struct Ty {} // expected-note {{possibly intended match 'AnotherGuy.Ty' does not conform to '_Copyable'}}
+}
+
+protocol Gives: HasType {
+  func give() -> Ty
+}
+
+struct GenerousGuy: Gives { // expected-error {{type 'GenerousGuy' does not conform to protocol 'HasType'}}
+  typealias Ty = MO // expected-note {{possibly intended match 'GenerousGuy.Ty' (aka 'MO') does not conform to '_Copyable'}}
+  func give() -> Ty {}
 }