Use a single 'untypedModuleSymbol' for all untyped modules instead of creating a new one for each module.

src/compiler/checker.ts

@@ -132,6 +132,8 @@ namespace ts {
         const evolvingArrayTypes: EvolvingArrayType[] = [];
 
         const unknownSymbol = createSymbol(SymbolFlags.Property | SymbolFlags.Transient, "unknown");
+        const untypedModuleSymbol = createSymbol(SymbolFlags.ValueModule, "<untyped>");
+        untypedModuleSymbol.exports = createMap<Symbol>();
         const resolvingSymbol = createSymbol(SymbolFlags.Transient, "__resolving__");
 
         const anyType = createIntrinsicType(TypeFlags.Any, "any");
@@ -1490,10 +1492,10 @@ namespace ts {
                         resolvedModule.resolvedFileName);
                     return undefined;
                 }
-                // Unlike a failed import, an untyped module produces a dummy symbol. This is checked for by `isUntypedOrShorthandAmbientModuleSymbol`.
-                const untypedSymbol = createSymbol(SymbolFlags.ValueModule, `"${moduleName}"`);
-                untypedSymbol.exports = createMap<Symbol>();
-                return untypedSymbol;
+                // Unlike a failed import, an untyped module produces a dummy symbol.
+                // This is checked for by `isUntypedOrShorthandAmbientModuleSymbol`.
+                // This must be different than `unknownSymbol` because `getBaseConstructorTypeOfClass` won't fail for `unknownSymbol`.
+                return untypedModuleSymbol;
             }
 
             if (moduleNotFoundError) {

tests/baselines/reference/extendsUntypedModule.js

@@ -12,16 +12,21 @@ class A extends Foo { }
 
 //// [a.js]
 "use strict";
-var __extends = (this && this.__extends) || function (d, b) {
-    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
-    function __() { this.constructor = d; }
-    d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
-};
+var __extends = (this && this.__extends) || (function () {
+    var extendStatics = Object.setPrototypeOf ||
+        ({ __proto__: [] } instanceof Array && function (d, b) { d.__proto__ = b; }) ||
+        function (d, b) { for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p]; };
+    return function (d, b) {
+        extendStatics(d, b);
+        function __() { this.constructor = d; }
+        d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
+    };
+})();
 var foo_1 = require("foo");
 var A = (function (_super) {
     __extends(A, _super);
     function A() {
-        return _super.apply(this, arguments) || this;
+        return _super !== null && _super.apply(this, arguments) || this;
     }
     return A;
 }(foo_1["default"]));

tests/cases/fourslash/untypedModuleImport.ts

@@ -12,7 +12,7 @@ verify.numberOfErrorsInCurrentFile(0);
 
 goTo.marker("fooModule");
 verify.goToDefinitionIs([]);
-verify.quickInfoIs('module "foo"');
+verify.quickInfoIs('module <untyped>');
 verify.referencesAre([])
 
 goTo.marker("foo");