prevent MO types from matching with existentials in the solver

Author: kavon

lib/Sema/CSSimplify.cpp

@@ -3823,6 +3823,18 @@ ConstraintSystem::matchExistentialTypes(Type type1, Type type2,
     return getTypeMatchAmbiguous();
   }
 
+  // move-only types cannot match with any existential types.
+  if (type1->isPureMoveOnly()) {
+    // tailor error message
+    if (shouldAttemptFixes()) {
+      auto *fix = MustBeCopyable::create(*this, type1,
+                                        getConstraintLocator(locator));
+      if (!recordFix(fix))
+        return getTypeMatchSuccess();
+    }
+    return getTypeMatchFailure(locator);
+  }
+
   // FIXME: Feels like a hack.
   if (type1->is<InOutType>())
     return getTypeMatchFailure(locator);

test/Constraints/moveonly_constraints.swift

@@ -137,14 +137,24 @@ func checkMethodCalls() {
   takeMaybe(true ? .none : .just(MO())) // expected-error 3{{move-only type 'MO' cannot be used with generics yet}}
 }
 
-func checkCasting(_ b: any Box) {
+func checkCasting(_ b: any Box, _ mo: MO) {
   // casting dynamically is allowed, but should always fail since you can't
   // construct such a type.
   let box = b as! ValBox<MO> // expected-error {{move-only type 'MO' cannot be used with generics yet}}
   let dup = box
 
   let _: MO = dup.get()
   let _: MO = dup.val
+
+  let _: Sendable = (MO(), MO()) // expected-error {{move-only type '(MO, MO)' cannot be used with generics yet}}
+  let _: Sendable = MO() // expected-error {{move-only type 'MO' cannot be used with generics yet}}
+  let _: _Copyable = mo // expected-error {{move-only type 'MO' cannot be used with generics yet}}
+  let _: AnyObject = MO() // expected-error {{move-only type 'MO' cannot be used with generics yet}}
+  let _: Any = mo // expected-error {{move-only type 'MO' cannot be used with generics yet}}
+
+  // FIXME: this shouldn't be allowed
+  let _: AnyHashable = MO() as! AnyHashable
+
 }
 
 func checkStdlibTypes(_ mo: MO) {