[CSDiagnostics] Add a diagnostic for an invalid key path subscript index argument

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -704,6 +704,10 @@ ERROR(expr_swift_keypath_anyobject_root,none,
       "the root type of a Swift key path cannot be 'AnyObject'", ())
 ERROR(expr_keypath_multiparam_func_conversion, none,
       "cannot convert key path into a multi-argument function type %0", (Type))
+ERROR(cannot_convert_type_to_keypath_subscript_index,none,
+      "cannot use value of type %0 as a key path subscript index; argument must be"
+      " a key path",
+      (Type))
 WARNING(expr_deprecated_writable_keypath,Deprecation,
         "forming a writable keypath to property %0 that is read-only in this context "
         "is deprecated and will be removed in a future release",

lib/Sema/CSDiagnostics.cpp

@@ -9357,3 +9357,8 @@ bool InvalidTypeSpecializationArity::diagnoseAsError() {
                                   /*generic=*/nullptr);
   return true;
 }
+
+bool InvalidTypeAsKeyPathSubscriptIndex::diagnoseAsError() {
+  emitDiagnostic(diag::cannot_convert_type_to_keypath_subscript_index, ArgType);
+  return true;
+}

lib/Sema/CSDiagnostics.h

@@ -3128,6 +3128,24 @@ class InvalidTypeSpecializationArity final : public FailureDiagnostic {
   bool diagnoseAsError() override;
 };
 
+/// Diagnose attempts to pass non-keypath as an argument to key path subscript:
+///
+/// \code
+/// func test(data: Int) {
+///   data[keyPath: 42] // error `42` is not a key path
+/// }
+/// \endcode
+class InvalidTypeAsKeyPathSubscriptIndex final : public FailureDiagnostic {
+  Type ArgType;
+
+public:
+  InvalidTypeAsKeyPathSubscriptIndex(const Solution &solution, Type argType,
+                                     ConstraintLocator *locator)
+      : FailureDiagnostic(solution, locator), ArgType(resolveType(argType)) {}
+
+  bool diagnoseAsError() override;
+};
+
 } // end namespace constraints
 } // end namespace swift