[ConstraintSystem] Support existential opening on key path subscript index

Author: xedin

lib/Sema/CSBindings.cpp

@@ -1426,6 +1426,25 @@ PotentialBindings::inferFromRelational(Constraint *constraint) {
       return llvm::None;
   }
 
+  if (TypeVar->getImpl().isKeyPathSubscriptIndex()) {
+    // Key path subscript index can only be a r-value non-optional
+    // type that is a subtype of a known KeyPath type.
+    type = type->getRValueType()->lookThroughAllOptionalTypes();
+
+    // If argument to a key path subscript is an existential,
+    // we can erase it to superclass (if any) here and solver
+    // will perform the opening if supertype turns out to be
+    // a valid key path type of its subtype.
+    if (kind == AllowedBindingKind::Supertypes) {
+      if (type->isExistentialType()) {
+        type = type->getExistentialLayout().explicitSuperclass;
+      }
+    }
+
+    if (!type)
+      return llvm::None;
+  }
+
   if (auto *locator = TypeVar->getImpl().getLocator()) {
     // Don't allow a protocol type to get propagated from the base to the result
     // type of a chain, Result should always be a concrete type which conforms

test/expr/unary/keypath/keypath.swift

@@ -1233,3 +1233,7 @@ func test_keypath_coercion_to_function() {
   let fn = \User.email as (User) -> String // Ok
   _ = users.map(fn) // Ok
 }
+
+func test_keypath_application_with_composition(v: String, kp: any KeyPath<String, Int> & PP) {
+  _ = v[keyPath: kp] // Ok
+}