[TypeChecker] Limit @dynamicMemberLookup parameter to compose only with Sendable

Author: xedin

lib/Sema/CSApply.cpp

@@ -2372,8 +2372,10 @@ namespace {
       };
 
       Type keyPathTy = argType;
-      if (auto *existential = keyPathTy->getAs<ExistentialType>())
-        keyPathTy = existential->getExistentialLayout().explicitSuperclass;
+      if (auto *existential = keyPathTy->getAs<ExistentialType>()) {
+        keyPathTy = existential->getSuperclass();
+        assert(isKnownKeyPathType(keyPathTy));
+      }
 
       SmallVector<Component, 2> components;
       auto *componentLoc = cs.getConstraintLocator(

lib/Sema/ConstraintSystem.cpp

@@ -3487,8 +3487,10 @@ void ConstraintSystem::bindOverloadType(
     // and U is a leaf type (aka member type).
     auto paramTy = fnType->getParams()[0].getPlainType();
 
-    if (auto *existential = paramTy->getAs<ExistentialType>())
-      paramTy = existential->getExistentialLayout().explicitSuperclass;
+    if (auto *existential = paramTy->getAs<ExistentialType>()) {
+      paramTy = existential->getSuperclass();
+      assert(isKnownKeyPathType(paramTy));
+    }
 
     auto keyPathTy = paramTy->castTo<BoundGenericType>();
 

lib/Sema/TypeCheckAttr.cpp

@@ -1828,10 +1828,17 @@ bool swift::isValidKeyPathDynamicMemberLookup(SubscriptDecl *decl,
 
   auto paramTy = decl->getIndices()->get(0)->getInterfaceType();
 
-  // Allow to compose key path type with a protocol as a way to express
-  // additional requirements on the parameter.
+  // Allow to compose key path type with a `Sendable` protocol as
+  // a way to express sendability requirement.
   if (auto *existential = paramTy->getAs<ExistentialType>()) {
-    paramTy = existential->getExistentialLayout().explicitSuperclass;
+    auto layout = existential->getExistentialLayout();
+
+    auto protocols = layout.getProtocols();
+    if (!(protocols.size() == 1 &&
+          protocols[0] == ctx.getProtocol(KnownProtocolKind::Sendable)))
+      return false;
+
+    paramTy = layout.getSuperclass();
     if (!paramTy)
       return false;
   }