[CSDiagnostics] Create KeyPathRootTypeMismatchFailure to diagnose key path root fail on key path application

Author: LucianoPAlmeida

lib/Sema/CSDiagnostics.cpp

@@ -6224,3 +6224,23 @@ bool CoercionAsForceCastFailure::diagnoseAsError() {
       .highlight(getSourceRange());
   return true;
 }
+
+bool KeyPathRootTypeMismatchFailure::diagnoseAsError() {
+  auto &cs = getConstraintSystem();
+  auto locator = getLocator();
+  auto anchor = locator->getAnchor();
+
+  assert(locator->isKeyPathRoot() && "Expect a key path root");
+
+  if (auto *subscriptExpr = dyn_cast_or_null<SubscriptExpr>(anchor)) {
+    auto *tupleExpr = cast<TupleExpr>(subscriptExpr->getIndex());
+    auto keyPathType = cs.getType(tupleExpr->getElement(0));
+    auto baseType = cs.getType(subscriptExpr->getBase());
+
+    emitDiagnosticAt(anchor->getLoc(),
+                     diag::expr_smart_keypath_application_type_mismatch,
+                     keyPathType, baseType->getRValueType());
+    return true;
+  }
+  return false;
+}

lib/Sema/CSDiagnostics.h

@@ -2022,6 +2022,24 @@ class CoercionAsForceCastFailure final : public ContextualFailure {
 
   bool diagnoseAsError() override;
 };
+
+/// Diagnose an key path root type that cannot be applied to an instance
+/// base that has another type.
+///
+/// \code
+/// func f (_ bar: Bar , keyPath: KeyPath<Foo, Int> ) {
+///   bar[keyPath: keyPath]
+/// }
+/// \endcode
+class KeyPathRootTypeMismatchFailure final : public ContextualFailure {
+public:
+  KeyPathRootTypeMismatchFailure(const Solution &solution, Type lhs, Type rhs,
+                                 ConstraintLocator *locator)
+      : ContextualFailure(solution, lhs, rhs, locator) {}
+
+  bool diagnoseAsError() override;
+};
+
 } // end namespace constraints
 } // end namespace swift