[CSBindings] Key path cannot be bound to typ erased type even if its existential

Extend existing check to handle type erased versions when they
are wrapped in an existential.

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -6389,6 +6389,11 @@ class TypeVarRefCollector : public ASTWalker {
 /// for a key path `{Any, Partial, Writable, ReferenceWritable}KeyPath`.
 bool isKnownKeyPathType(Type type);
 
+/// Determine whether the given type is a PartialKeyPath and
+/// AnyKeyPath or existential type thererof, for example,
+/// `PartialKeyPath<...> & Sendable`.
+bool isTypeErasedKeyPathType(Type type);
+
 /// Determine whether given declaration is one for a key path
 /// `{Writable, ReferenceWritable}KeyPath`.
 bool isKnownKeyPathDecl(ASTContext &ctx, ValueDecl *decl);

lib/Sema/CSBindings.cpp

@@ -2359,7 +2359,7 @@ bool TypeVarBindingProducer::computeNext() {
           auto supertype = *simplifiedSuper;
           // A key path type cannot be bound to type-erased key path variants.
           if (TypeVar->getImpl().isKeyPathType() &&
-              (supertype->isPartialKeyPath() || supertype->isAnyKeyPath()))
+              isTypeErasedKeyPathType(supertype))
             continue;
 
           addNewBinding(binding.withType(supertype));

lib/Sema/ConstraintSystem.cpp

@@ -6676,6 +6676,19 @@ bool constraints::isKnownKeyPathType(Type type) {
          type->isAnyKeyPath();
 }
 
+bool constraints::isTypeErasedKeyPathType(Type type) {
+  assert(type);
+
+  if (type->isPartialKeyPath() || type->isAnyKeyPath())
+    return true;
+
+  if (!type->isExistentialType())
+    return false;
+
+  auto superclass = type->getSuperclass();
+  return superclass ? isTypeErasedKeyPathType(superclass) : false;
+}
+
 bool constraints::hasExplicitResult(ClosureExpr *closure) {
   auto &ctx = closure->getASTContext();
   return evaluateOrDefault(ctx.evaluator,