Merge pull request #17922 from Microsoft/baseExpressionTypeParameters

Class type parameters not in scope in base class expression

src/compiler/checker.ts

@@ -1016,7 +1016,18 @@ namespace ts {
                             }
                         }
                         break;
-
+                    case SyntaxKind.ExpressionWithTypeArguments:
+                        // The type parameters of a class are not in scope in the base class expression.
+                        if (lastLocation === (<ExpressionWithTypeArguments>location).expression && (<HeritageClause>location.parent).token === SyntaxKind.ExtendsKeyword) {
+                            const container = location.parent.parent;
+                            if (isClassLike(container) && (result = lookup(getSymbolOfNode(container).members, name, meaning & SymbolFlags.Type))) {
+                                if (nameNotFoundMessage) {
+                                    error(errorLocation, Diagnostics.Base_class_expressions_cannot_reference_class_type_parameters);
+                                }
+                                return undefined;
+                            }
+                        }
+                        break;
                     // It is not legal to reference a class's own type parameters from a computed property name that
                     // belongs to the class. For example:
                     //

src/compiler/diagnosticMessages.json

@@ -1912,6 +1912,10 @@
         "category": "Error",
         "code": 2560
     },
+    "Base class expressions cannot reference class type parameters.": {
+        "category": "Error",
+        "code": 2561
+    },
     "JSX element attributes type '{0}' may not be a union type.": {
         "category": "Error",
         "code": 2600