[CSSimplify] Resolve key path expression

by creating a BoundGenericType for the keypath expr
with the keypath base and replacing the value with
the value typeVar that will be resolved when the value
typeVar is directly bound.

Author: Amritpan Kaur

include/swift/Sema/ConstraintSystem.h

@@ -3982,6 +3982,20 @@ class ConstraintSystem {
   bool resolveClosure(TypeVariableType *typeVar, Type contextualType,
                       ConstraintLocatorBuilder locator);
 
+  /// Given the fact a contextual type is now available for the type
+  /// variable representing one of the key path expressions, let's set a
+  /// pre-determined key path expression type.
+  ///
+  /// \param typeVar The type variable representing a key path expression.
+  /// \param contextualType The contextual type this key path would be
+  /// converted to.
+  /// \param locator The locator associated with contextual type.
+  ///
+  /// \returns `true` if it was possible to generate constraints for
+  /// the keyPath expression, `false` otherwise.
+  bool resolveKeyPath(TypeVariableType *typeVar, Type contextualType,
+                      ConstraintLocatorBuilder locator);
+
   /// Given the fact that contextual type is now available for the type
   /// variable representing a pack expansion type, let's resolve the expansion.
   ///

lib/Sema/CSSimplify.cpp

@@ -11517,6 +11517,28 @@ bool ConstraintSystem::resolveClosure(TypeVariableType *typeVar,
   return !generateConstraints(AnyFunctionRef{closure}, closure->getBody());
 }
 
+bool ConstraintSystem::resolveKeyPath(TypeVariableType *typeVar,
+                                      Type contextualType,
+                                      ConstraintLocatorBuilder locator) {
+
+  if (auto *BGT = contextualType->getAs<BoundGenericType>()) {
+    auto args = BGT->getGenericArgs();
+    if (isKnownKeyPathType(contextualType) && args.size() >= 1) {
+      auto root = BGT->getGenericArgs()[0];
+
+      auto *keyPathLocator = typeVar->getImpl().getLocator();
+      auto *keyPath = castToExpr<KeyPathExpr>(keyPathLocator->getAnchor());
+      auto *keyPathValueTV = getKeyPathValueType(keyPath);
+      contextualType = BoundGenericType::get(
+          args.size() == 1 ? getASTContext().getKeyPathDecl() : BGT->getDecl(),
+          /*parent=*/Type(), {root, keyPathValueTV});
+    }
+  }
+
+  assignFixedType(typeVar, contextualType);
+  return true;
+}
+
 bool ConstraintSystem::resolvePackExpansion(TypeVariableType *typeVar,
                                             Type contextualType) {
   auto *locator = typeVar->getImpl().getLocator();