[TypeChecker] Allow @dynamicMemberLookup subscript index to compose key path with protocols

Allow to compose key path type with a protocol as a way to express
additional requirements on the parameter. For example:

```
struct Test<R> {
  subscript<V>(dynamicMember member: KeyPath<R, V> & Sendable) -> V {
    ...
  }
}
```

Author: xedin

lib/Sema/TypeCheckAttr.cpp

@@ -28,6 +28,7 @@
 #include "swift/AST/DiagnosticsParse.h"
 #include "swift/AST/DiagnosticsSema.h"
 #include "swift/AST/Effects.h"
+#include "swift/AST/ExistentialLayout.h"
 #include "swift/AST/GenericEnvironment.h"
 #include "swift/AST/ImportCache.h"
 #include "swift/AST/ModuleNameLookup.h"
@@ -1826,6 +1827,15 @@ bool swift::isValidKeyPathDynamicMemberLookup(SubscriptDecl *decl,
     return false;
 
   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.
+  if (auto *existential = paramTy->getAs<ExistentialType>()) {
+    paramTy = existential->getExistentialLayout().explicitSuperclass;
+    if (!paramTy)
+      return false;
+  }
+
   return paramTy->isKeyPath() ||
          paramTy->isWritableKeyPath() ||
          paramTy->isReferenceWritableKeyPath();

test/Concurrency/sendable_keypaths.swift

@@ -124,3 +124,17 @@ do {
   // TODO: This should be diagnosed by the isolation checker because implicitly synthesized closures captures a non-Sendable value.
   testSendableFn(v: v, \.[42, CondSendable(nonSendable)])
 }
+
+// @dynamicMemberLookup with Sendable requirement
+do {
+  @dynamicMemberLookup
+  struct Test<T> {
+    var obj: T
+
+    subscript<U>(dynamicMember member: KeyPath<T, U> & Sendable) -> U {
+      get { obj[keyPath: member] }
+    }
+  }
+
+  _ = Test(obj: "Hello").utf8.count // Ok
+}

test/Interpreter/keypath.swift

@@ -134,3 +134,17 @@ do {
   // CHECK: 42
   test(v: S(i: 42), \.i)
 }
+
+do {
+  @dynamicMemberLookup
+  struct Test<T> {
+    var obj: T
+
+    subscript<U>(dynamicMember member: KeyPath<T, U> & Sendable) -> U {
+      get { obj[keyPath: member] }
+    }
+  }
+
+  // CHECK: 5
+  print(Test(obj: "Hello").utf8.count)
+}