createProgram: don't use TypeChecker

src/compiler/program.ts

@@ -951,25 +951,23 @@ namespace ts {
             // If we change our policy of rechecking failed lookups on each program create,
             // we should adjust the value returned here.
             function moduleNameResolvesToAmbientModuleInNonModifiedFile(moduleName: string, oldProgramState: OldProgramState): boolean {
+                if (!oldProgramState.program) {
+                    return false;
+                }
                 const resolutionToFile = getResolvedModule(oldProgramState.oldSourceFile!, moduleName); // TODO: GH#18217
-                const resolvedFile = resolutionToFile && oldProgramState.program && oldProgramState.program.getSourceFile(resolutionToFile.resolvedFileName);
+                const resolvedFile = resolutionToFile && oldProgramState.program.getSourceFile(resolutionToFile.resolvedFileName);
                 if (resolutionToFile && resolvedFile && !resolvedFile.externalModuleIndicator) {
                     // In the old program, we resolved to an ambient module that was in the same
                     //   place as we expected to find an actual module file.
                     // We actually need to return 'false' here even though this seems like a 'true' case
                     //   because the normal module resolution algorithm will find this anyway.
                     return false;
                 }
-                const ambientModule = oldProgramState.program && oldProgramState.program.getTypeChecker().tryFindAmbientModuleWithoutAugmentations(moduleName);
-                if (!(ambientModule && ambientModule.declarations)) {
-                    return false;
-                }
 
                 // at least one of declarations should come from non-modified source file
-                const firstUnmodifiedFile = forEach(ambientModule.declarations, d => {
-                    const f = getSourceFileOfNode(d);
-                    return !contains(oldProgramState.modifiedFilePaths, f.path) && f;
-                });
+                const firstUnmodifiedFile = oldProgramState.program.getSourceFiles().find(
+                    f => !contains(oldProgramState.modifiedFilePaths, f.path) && contains(f.ambientModuleNames, moduleName)
+                );
 
                 if (!firstUnmodifiedFile) {
                     return false;

src/testRunner/unittests/reuseProgramStructure.ts

@@ -399,6 +399,30 @@ namespace ts {
             assert.isDefined(program2.getSourceFile("/a.ts")!.resolvedModules!.get("a"), "'a' is not an unresolved module after re-use");
         });
 
+        it("works with updated SourceFiles", () => {
+            // adapted repro from https://github.com/Microsoft/TypeScript/issues/26166
+            const files = [
+                { name: "/a.ts", text: SourceText.New("", "", 'import * as a from "a";a;') },
+                { name: "/types/zzz/index.d.ts", text: SourceText.New("", "", 'declare module "a" { }') },
+            ];
+            const host = createTestCompilerHost(files, target);
+            const options: CompilerOptions = { target, typeRoots: ["/types"] };
+            const program1 = createProgram(["/a.ts"], options, host);
+            let sourceFile = program1.getSourceFile("/a.ts")!;
+            assert.isDefined(sourceFile, "'/a.ts' is included in the program");
+            sourceFile = updateSourceFile(sourceFile, "'use strict';" + sourceFile.text, { newLength: "'use strict';".length, span: { start: 0, length: 0 } });
+            assert.strictEqual(sourceFile.statements[2].getSourceFile(), sourceFile, "parent pointers are updated");
+            const updateHost: TestCompilerHost = {
+                ...host,
+                getSourceFile(fileName) {
+                    return fileName === sourceFile.fileName ? sourceFile : program1.getSourceFile(fileName);
+                }
+            };
+            const program2 = createProgram(["/a.ts"], options, updateHost, program1);
+            assert.isDefined(program2.getSourceFile("/a.ts")!.resolvedModules!.get("a"), "'a' is not an unresolved module after re-use");
+            assert.strictEqual(sourceFile.statements[2].getSourceFile(), sourceFile, "parent pointers are not altered");
+        });
+
         it("resolved type directives cache follows type directives", () => {
             const files = [
                 { name: "/a.ts", text: SourceText.New("/// <reference types='typedefs'/>", "", "var x = $") },