module resolution: prefer locally defined ambient modules, reuse resolutions to ambient modules from the old program (#11999)

module resolution: prefer locally defined ambient modules, reuse resolutions to ambient modules from the old program

src/compiler/checker.ts

@@ -107,7 +107,12 @@ namespace ts {
 
             getJsxElementAttributesType,
             getJsxIntrinsicTagNames,
-            isOptionalParameter
+            isOptionalParameter,
+            tryFindAmbientModuleWithoutAugmentations: moduleName => {
+                // we deliberately exclude augmentations
+                // since we are only interested in declarations of the module itself
+                return tryFindAmbientModule(moduleName, /*withAugmentations*/ false);
+            }
         };
 
         const tupleTypes: GenericType[] = [];
@@ -1370,16 +1375,11 @@ namespace ts {
                 return;
             }
 
-            const isRelative = isExternalModuleNameRelative(moduleName);
-            const quotedName = '"' + moduleName + '"';
-            if (!isRelative) {
-                const symbol = getSymbol(globals, quotedName, SymbolFlags.ValueModule);
-                if (symbol) {
-                    // merged symbol is module declaration symbol combined with all augmentations
-                    return getMergedSymbol(symbol);
-                }
+            const ambientModule = tryFindAmbientModule(moduleName, /*withAugmentations*/ true);
+            if (ambientModule) {
+                return ambientModule;
             }
-
+            const isRelative = isExternalModuleNameRelative(moduleName);
             const resolvedModule = getResolvedModule(getSourceFileOfNode(location), moduleReference);
             const resolutionDiagnostic = resolvedModule && getResolutionDiagnostic(compilerOptions, resolvedModule);
             const sourceFile = resolvedModule && !resolutionDiagnostic && host.getSourceFile(resolvedModule.resolvedFileName);
@@ -4734,6 +4734,15 @@ namespace ts {
             }
         }
 
+        function tryFindAmbientModule(moduleName: string, withAugmentations: boolean) {
+            if (isExternalModuleNameRelative(moduleName)) {
+                return undefined;
+            }
+            const symbol = getSymbol(globals, `"${moduleName}"`, SymbolFlags.ValueModule);
+            // merged symbol is module declaration symbol combined with all augmentations
+            return symbol && withAugmentations ? getMergedSymbol(symbol) : symbol;
+        }
+
         function isOptionalParameter(node: ParameterDeclaration) {
             if (hasQuestionToken(node) || isJSDocOptionalParameter(node)) {
                 return true;

src/compiler/diagnosticMessages.json

@@ -2889,6 +2889,14 @@
         "category": "Error",
         "code": 6143
     },
+    "Module '{0}' was resolved as locally declared ambient module in file '{1}'.": {
+        "category": "Message",
+        "code": 6144
+    },
+    "Module '{0}' was resolved as ambient module declared in '{1}' since this file was not modified.": {
+        "category": "Message",
+        "code": 6145
+    },
     "Variable '{0}' implicitly has an '{1}' type.": {
         "category": "Error",
         "code": 7005

src/compiler/moduleNameResolver.ts

@@ -2,12 +2,15 @@
 /// <reference path="diagnosticInformationMap.generated.ts" />
 
 namespace ts {
-    function trace(host: ModuleResolutionHost, message: DiagnosticMessage, ...args: any[]): void;
-    function trace(host: ModuleResolutionHost): void {
+
+    /* @internal */
+    export function trace(host: ModuleResolutionHost, message: DiagnosticMessage, ...args: any[]): void;
+    export function trace(host: ModuleResolutionHost): void {
         host.trace(formatMessage.apply(undefined, arguments));
     }
 
-    function isTraceEnabled(compilerOptions: CompilerOptions, host: ModuleResolutionHost): boolean {
+    /* @internal */
+    export function isTraceEnabled(compilerOptions: CompilerOptions, host: ModuleResolutionHost): boolean {
         return compilerOptions.traceResolution && host.trace !== undefined;
     }
 

src/compiler/program.ts

@@ -462,6 +462,130 @@ namespace ts {
             return classifiableNames;
         }
 
+        interface OldProgramState {
+            program: Program;
+            file: SourceFile;
+            modifiedFilePaths: Path[];
+        }
+
+        function resolveModuleNamesReusingOldState(moduleNames: string[], containingFile: string, file: SourceFile, oldProgramState?: OldProgramState) {
+            if (!oldProgramState && !file.ambientModuleNames.length) {
+                // if old program state is not supplied and file does not contain locally defined ambient modules
+                // then the best we can do is fallback to the default logic
+                return resolveModuleNamesWorker(moduleNames, containingFile);
+            }
+
+            // at this point we know that either 
+            // - file has local declarations for ambient modules
+            // OR
+            // - old program state is available
+            // OR
+            // - both of items above
+            // With this it is possible that we can tell how some module names from the initial list will be resolved
+            // without doing actual resolution (in particular if some name was resolved to ambient module).
+            // Such names should be excluded from the list of module names that will be provided to `resolveModuleNamesWorker`
+            // since we don't want to resolve them again.
+
+            // this is a list of modules for which we cannot predict resolution so they should be actually resolved
+            let unknownModuleNames: string[];
+            // this is a list of combined results assembles from predicted and resolved results.
+            // Order in this list matches the order in the original list of module names `moduleNames` which is important
+            // so later we can split results to resolutions of modules and resolutions of module augmentations.
+            let result: ResolvedModuleFull[];
+            // a transient placeholder that is used to mark predicted resolution in the result list
+            const predictedToResolveToAmbientModuleMarker: ResolvedModuleFull = <any>{};
+
+            for (let i = 0; i < moduleNames.length; i++) {
+                const moduleName = moduleNames[i];
+                // module name is known to be resolved to ambient module if
+                // - module name is contained in the list of ambient modules that are locally declared in the file
+                // - in the old program module name was resolved to ambient module whose declaration is in non-modified file
+                //   (so the same module declaration will land in the new program)
+                let isKnownToResolveToAmbientModule = false;
+                if (contains(file.ambientModuleNames, moduleName)) {
+                    isKnownToResolveToAmbientModule = true;
+                    if (isTraceEnabled(options, host)) {
+                        trace(host, Diagnostics.Module_0_was_resolved_as_locally_declared_ambient_module_in_file_1, moduleName, containingFile);
+                    }
+                }
+                else {
+                    isKnownToResolveToAmbientModule = checkModuleNameResolvedToAmbientModuleInNonModifiedFile(moduleName, oldProgramState);
+                }
+
+                if (isKnownToResolveToAmbientModule) {
+                    if (!unknownModuleNames) {
+                        // found a first module name for which result can be prediced
+                        // this means that this module name should not be passed to `resolveModuleNamesWorker`.
+                        // We'll use a separate list for module names that are definitely unknown.
+                        result = new Array(moduleNames.length);
+                        // copy all module names that appear before the current one in the list
+                        // since they are known to be unknown
+                        unknownModuleNames = moduleNames.slice(0, i);
+                    }
+                    // mark prediced resolution in the result list
+                    result[i] = predictedToResolveToAmbientModuleMarker;
+                }
+                else if (unknownModuleNames) {
+                    // found unknown module name and we are already using separate list for those - add it to the list
+                    unknownModuleNames.push(moduleName);
+                }
+            }
+
+            if (!unknownModuleNames) {
+                // we've looked throught the list but have not seen any predicted resolution
+                // use default logic
+                return resolveModuleNamesWorker(moduleNames, containingFile);
+            }
+
+            const resolutions = unknownModuleNames.length
+                ? resolveModuleNamesWorker(unknownModuleNames, containingFile)
+                : emptyArray;
+
+            // combine results of resolutions and predicted results
+            let j = 0;
+            for (let i = 0; i < result.length; i++) {
+                if (result[i] == predictedToResolveToAmbientModuleMarker) {
+                    result[i] = undefined;
+                }
+                else {
+                    result[i] = resolutions[j];
+                    j++;
+                }
+            }
+            Debug.assert(j === resolutions.length);
+            return result;
+
+            function checkModuleNameResolvedToAmbientModuleInNonModifiedFile(moduleName: string, oldProgramState?: OldProgramState): boolean {
+                if (!oldProgramState) {
+                    return false;
+                }
+                const resolutionToFile = getResolvedModule(oldProgramState.file, moduleName);
+                if (resolutionToFile) {
+                    // module used to be resolved to file - ignore it
+                    return false;
+                }
+                const ambientModule = oldProgram.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;
+                });
+
+                if (!firstUnmodifiedFile) {
+                    return false;
+                }
+
+                if (isTraceEnabled(options, host)) {
+                    trace(host, Diagnostics.Module_0_was_resolved_as_ambient_module_declared_in_1_since_this_file_was_not_modified, moduleName, firstUnmodifiedFile.fileName);
+                }
+                return true;
+            }
+        }
+
         function tryReuseStructureFromOldProgram(): boolean {
             if (!oldProgram) {
                 return false;
@@ -489,7 +613,7 @@ namespace ts {
             // check if program source files has changed in the way that can affect structure of the program
             const newSourceFiles: SourceFile[] = [];
             const filePaths: Path[] = [];
-            const modifiedSourceFiles: SourceFile[] = [];
+            const modifiedSourceFiles: { oldFile: SourceFile, newFile: SourceFile }[] = [];
 
             for (const oldSourceFile of oldProgram.getSourceFiles()) {
                 let newSourceFile = host.getSourceFileByPath
@@ -532,29 +656,8 @@ namespace ts {
                         return false;
                     }
 
-                    const newSourceFilePath = getNormalizedAbsolutePath(newSourceFile.fileName, currentDirectory);
-                    if (resolveModuleNamesWorker) {
-                        const moduleNames = map(concatenate(newSourceFile.imports, newSourceFile.moduleAugmentations), getTextOfLiteral);
-                        const resolutions = resolveModuleNamesWorker(moduleNames, newSourceFilePath);
-                        // ensure that module resolution results are still correct
-                        const resolutionsChanged = hasChangesInResolutions(moduleNames, resolutions, oldSourceFile.resolvedModules, moduleResolutionIsEqualTo);
-                        if (resolutionsChanged) {
-                            return false;
-                        }
-                    }
-                    if (resolveTypeReferenceDirectiveNamesWorker) {
-                        const typesReferenceDirectives = map(newSourceFile.typeReferenceDirectives, x => x.fileName);
-                        const resolutions = resolveTypeReferenceDirectiveNamesWorker(typesReferenceDirectives, newSourceFilePath);
-                        // ensure that types resolutions are still correct
-                        const resolutionsChanged = hasChangesInResolutions(typesReferenceDirectives, resolutions, oldSourceFile.resolvedTypeReferenceDirectiveNames, typeDirectiveIsEqualTo);
-                        if (resolutionsChanged) {
-                            return false;
-                        }
-                    }
-                    // pass the cache of module/types resolutions from the old source file
-                    newSourceFile.resolvedModules = oldSourceFile.resolvedModules;
-                    newSourceFile.resolvedTypeReferenceDirectiveNames = oldSourceFile.resolvedTypeReferenceDirectiveNames;
-                    modifiedSourceFiles.push(newSourceFile);
+                    // tentatively approve the file
+                    modifiedSourceFiles.push({ oldFile: oldSourceFile, newFile: newSourceFile });
                 }
                 else {
                     // file has no changes - use it as is
@@ -565,6 +668,33 @@ namespace ts {
                 newSourceFiles.push(newSourceFile);
             }
 
+            const modifiedFilePaths = modifiedSourceFiles.map(f => f.newFile.path);
+            // try to verify results of module resolution 
+            for (const { oldFile: oldSourceFile, newFile: newSourceFile } of modifiedSourceFiles) {
+                const newSourceFilePath = getNormalizedAbsolutePath(newSourceFile.fileName, currentDirectory);
+                if (resolveModuleNamesWorker) {
+                    const moduleNames = map(concatenate(newSourceFile.imports, newSourceFile.moduleAugmentations), getTextOfLiteral);
+                    const resolutions = resolveModuleNamesReusingOldState(moduleNames, newSourceFilePath, newSourceFile, { file: oldSourceFile, program: oldProgram, modifiedFilePaths });
+                    // ensure that module resolution results are still correct
+                    const resolutionsChanged = hasChangesInResolutions(moduleNames, resolutions, oldSourceFile.resolvedModules, moduleResolutionIsEqualTo);
+                    if (resolutionsChanged) {
+                        return false;
+                    }
+                }
+                if (resolveTypeReferenceDirectiveNamesWorker) {
+                    const typesReferenceDirectives = map(newSourceFile.typeReferenceDirectives, x => x.fileName);
+                    const resolutions = resolveTypeReferenceDirectiveNamesWorker(typesReferenceDirectives, newSourceFilePath);
+                    // ensure that types resolutions are still correct
+                    const resolutionsChanged = hasChangesInResolutions(typesReferenceDirectives, resolutions, oldSourceFile.resolvedTypeReferenceDirectiveNames, typeDirectiveIsEqualTo);
+                    if (resolutionsChanged) {
+                        return false;
+                    }
+                }
+                // pass the cache of module/types resolutions from the old source file
+                newSourceFile.resolvedModules = oldSourceFile.resolvedModules;
+                newSourceFile.resolvedTypeReferenceDirectiveNames = oldSourceFile.resolvedTypeReferenceDirectiveNames;
+            }
+
             // update fileName -> file mapping
             for (let i = 0, len = newSourceFiles.length; i < len; i++) {
                 filesByName.set(filePaths[i], newSourceFiles[i]);
@@ -574,7 +704,7 @@ namespace ts {
             fileProcessingDiagnostics = oldProgram.getFileProcessingDiagnostics();
 
             for (const modifiedFile of modifiedSourceFiles) {
-                fileProcessingDiagnostics.reattachFileDiagnostics(modifiedFile);
+                fileProcessingDiagnostics.reattachFileDiagnostics(modifiedFile.newFile);
             }
             resolvedTypeReferenceDirectives = oldProgram.getResolvedTypeReferenceDirectives();
             oldProgram.structureIsReused = true;
@@ -994,9 +1124,11 @@ namespace ts {
 
             const isJavaScriptFile = isSourceFileJavaScript(file);
             const isExternalModuleFile = isExternalModule(file);
+            const isDtsFile = isDeclarationFile(file);
 
             let imports: LiteralExpression[];
             let moduleAugmentations: LiteralExpression[];
+            let ambientModules: string[];
 
             // If we are importing helpers, we need to add a synthetic reference to resolve the
             // helpers library.
@@ -1018,6 +1150,7 @@ namespace ts {
 
             file.imports = imports || emptyArray;
             file.moduleAugmentations = moduleAugmentations || emptyArray;
+            file.ambientModuleNames = ambientModules || emptyArray;
 
             return;
 
@@ -1053,6 +1186,10 @@ namespace ts {
                                 (moduleAugmentations || (moduleAugmentations = [])).push(moduleName);
                             }
                             else if (!inAmbientModule) {
+                                if (isDtsFile) {
+                                    // for global .d.ts files record name of ambient module
+                                    (ambientModules || (ambientModules = [])).push(moduleName.text);
+                                }
                                 // An AmbientExternalModuleDeclaration declares an external module.
                                 // This type of declaration is permitted only in the global module.
                                 // The StringLiteral must specify a top - level external module name.
@@ -1298,7 +1435,7 @@ namespace ts {
             if (file.imports.length || file.moduleAugmentations.length) {
                 file.resolvedModules = createMap<ResolvedModuleFull>();
                 const moduleNames = map(concatenate(file.imports, file.moduleAugmentations), getTextOfLiteral);
-                const resolutions = resolveModuleNamesWorker(moduleNames, getNormalizedAbsolutePath(file.fileName, currentDirectory));
+                const resolutions = resolveModuleNamesReusingOldState(moduleNames, getNormalizedAbsolutePath(file.fileName, currentDirectory), file);
                 Debug.assert(resolutions.length === moduleNames.length);
                 for (let i = 0; i < moduleNames.length; i++) {
                     const resolution = resolutions[i];

src/compiler/types.ts

@@ -2102,6 +2102,7 @@ namespace ts {
         /* @internal */ imports: LiteralExpression[];
         /* @internal */ moduleAugmentations: LiteralExpression[];
         /* @internal */ patternAmbientModules?: PatternAmbientModule[];
+        /* @internal */ ambientModuleNames: string[];
         // The synthesized identifier for an imported external helpers module.
         /* @internal */ externalHelpersModuleName?: Identifier;
     }
@@ -2296,6 +2297,8 @@ namespace ts {
         isOptionalParameter(node: ParameterDeclaration): boolean;
         getAmbientModules(): Symbol[];
 
+        /* @internal */ tryFindAmbientModuleWithoutAugmentations(moduleName: string): Symbol;
+
         // Should not be called directly.  Should only be accessed through the Program instance.
         /* @internal */ getDiagnostics(sourceFile?: SourceFile, cancellationToken?: CancellationToken): Diagnostic[];
         /* @internal */ getGlobalDiagnostics(): Diagnostic[];

src/harness/unittests/moduleResolution.ts

@@ -1065,5 +1065,69 @@ import b = require("./moduleB");
             assert.equal(diagnostics2.length, 1, "expected one diagnostic");
             assert.equal(diagnostics1[0].messageText, diagnostics2[0].messageText, "expected one diagnostic");
         });
+
+        it ("Modules in the same .d.ts file are preferred to external files", () => {
+            const f = {
+                name: "/a/b/c/c/app.d.ts",
+                content: `
+                declare module "fs" {
+                    export interface Stat { id: number }
+                }
+                declare module "fs-client" {
+                    import { Stat } from "fs";
+                    export function foo(): Stat;
+                }`
+            };
+            const file = createSourceFile(f.name, f.content, ScriptTarget.ES2015);
+            const compilerHost: CompilerHost = {
+                fileExists : fileName => fileName === file.fileName,
+                getSourceFile: fileName => fileName === file.fileName ? file : undefined,
+                getDefaultLibFileName: () => "lib.d.ts",
+                writeFile: notImplemented,
+                getCurrentDirectory: () => "/",
+                getDirectories: () => [],
+                getCanonicalFileName: f => f.toLowerCase(),
+                getNewLine: () => "\r\n",
+                useCaseSensitiveFileNames: () => false,
+                readFile: fileName => fileName === file.fileName ? file.text : undefined,
+                resolveModuleNames() {
+                    assert(false, "resolveModuleNames should not be called");
+                    return undefined;
+                }
+            };
+            createProgram([f.name], {}, compilerHost);
+        });
+
+        it ("Modules in .ts file are not checked in the same file", () => {
+            const f = {
+                name: "/a/b/c/c/app.ts",
+                content: `
+                declare module "fs" {
+                    export interface Stat { id: number }
+                }
+                declare module "fs-client" {
+                    import { Stat } from "fs";
+                    export function foo(): Stat;
+                }`
+            };
+            const file = createSourceFile(f.name, f.content, ScriptTarget.ES2015);
+            const compilerHost: CompilerHost = {
+                fileExists : fileName => fileName === file.fileName,
+                getSourceFile: fileName => fileName === file.fileName ? file : undefined,
+                getDefaultLibFileName: () => "lib.d.ts",
+                writeFile: notImplemented,
+                getCurrentDirectory: () => "/",
+                getDirectories: () => [],
+                getCanonicalFileName: f => f.toLowerCase(),
+                getNewLine: () => "\r\n",
+                useCaseSensitiveFileNames: () => false,
+                readFile: fileName => fileName === file.fileName ? file.text : undefined,
+                resolveModuleNames(moduleNames: string[], _containingFile: string) {
+                    assert.deepEqual(moduleNames, ["fs"]);
+                    return [undefined];
+                }
+            };
+            createProgram([f.name], {}, compilerHost);
+        });
     });
 }

src/harness/unittests/reuseProgramStructure.ts

@@ -22,6 +22,11 @@ namespace ts {
 
     interface ProgramWithSourceTexts extends Program {
         sourceTexts?: NamedSourceText[];
+        host: TestCompilerHost;
+    }
+
+    interface TestCompilerHost extends CompilerHost {
+        getTrace(): string[];
     }
 
     class SourceText implements IScriptSnapshot {
@@ -101,10 +106,21 @@ namespace ts {
         return file;
     }
 
-    function createTestCompilerHost(texts: NamedSourceText[], target: ScriptTarget): CompilerHost {
-        const files = arrayToMap(texts, t => t.name, t => createSourceFileWithText(t.name, t.text, target));
+    function createTestCompilerHost(texts: NamedSourceText[], target: ScriptTarget, oldProgram?: ProgramWithSourceTexts): TestCompilerHost {
+        const files = arrayToMap(texts, t => t.name, t => {
+            if (oldProgram) {
+                const oldFile = <SourceFileWithText>oldProgram.getSourceFile(t.name);
+                if (oldFile && oldFile.sourceText.getVersion() === t.text.getVersion()) {
+                    return oldFile;
+                }
+            }
+            return createSourceFileWithText(t.name, t.text, target);
+        });
+        const trace: string[] = [];
 
         return {
+            trace: s => trace.push(s),
+            getTrace: () => trace,
             getSourceFile(fileName): SourceFile {
                 return files[fileName];
             },
@@ -130,23 +146,25 @@ namespace ts {
             fileExists: fileName => fileName in files,
             readFile: fileName => {
                 return fileName in files ? files[fileName].text : undefined;
-            }
+            },
         };
     }
 
-    function newProgram(texts: NamedSourceText[], rootNames: string[], options: CompilerOptions): Program {
+    function newProgram(texts: NamedSourceText[], rootNames: string[], options: CompilerOptions): ProgramWithSourceTexts {
         const host = createTestCompilerHost(texts, options.target);
         const program = <ProgramWithSourceTexts>createProgram(rootNames, options, host);
         program.sourceTexts = texts;
+        program.host = host;
         return program;
     }
 
-    function updateProgram(oldProgram: Program, rootNames: string[], options: CompilerOptions, updater: (files: NamedSourceText[]) => void) {
+    function updateProgram(oldProgram: ProgramWithSourceTexts, rootNames: string[], options: CompilerOptions, updater: (files: NamedSourceText[]) => void) {
         const texts: NamedSourceText[] = (<ProgramWithSourceTexts>oldProgram).sourceTexts.slice(0);
         updater(texts);
-        const host = createTestCompilerHost(texts, options.target);
+        const host = createTestCompilerHost(texts, options.target, oldProgram);
         const program = <ProgramWithSourceTexts>createProgram(rootNames, options, host, oldProgram);
         program.sourceTexts = texts;
+        program.host = host;
         return program;
     }
 
@@ -355,6 +373,112 @@ namespace ts {
             assert.isTrue(!program_3.structureIsReused);
             checkResolvedTypeDirectivesCache(program_1, "/a.ts", createMap({ "typedefs": { resolvedFileName: "/types/typedefs/index.d.ts", primary: true } }));
         });
+
+        it("can reuse ambient module declarations from non-modified files", () => {
+            const files = [
+                { name: "/a/b/app.ts", text: SourceText.New("", "import * as fs from 'fs'", "") },
+                { name: "/a/b/node.d.ts", text: SourceText.New("", "", "declare module 'fs' {}") }
+            ];
+            const options = { target: ScriptTarget.ES2015, traceResolution: true };
+            const program = newProgram(files, files.map(f => f.name), options);
+            assert.deepEqual(program.host.getTrace(),
+                [
+                    "======== Resolving module 'fs' from '/a/b/app.ts'. ========",
+                    "Module resolution kind is not specified, using 'Classic'.",
+                    "File '/a/b/fs.ts' does not exist.",
+                    "File '/a/b/fs.tsx' does not exist.",
+                    "File '/a/b/fs.d.ts' does not exist.",
+                    "File '/a/fs.ts' does not exist.",
+                    "File '/a/fs.tsx' does not exist.",
+                    "File '/a/fs.d.ts' does not exist.",
+                    "File '/fs.ts' does not exist.",
+                    "File '/fs.tsx' does not exist.",
+                    "File '/fs.d.ts' does not exist.",
+                    "File '/a/b/node_modules/@types/fs.ts' does not exist.",
+                    "File '/a/b/node_modules/@types/fs.tsx' does not exist.",
+                    "File '/a/b/node_modules/@types/fs.d.ts' does not exist.",
+                    "File '/a/b/node_modules/@types/fs/package.json' does not exist.",
+                    "File '/a/b/node_modules/@types/fs/index.ts' does not exist.",
+                    "File '/a/b/node_modules/@types/fs/index.tsx' does not exist.",
+                    "File '/a/b/node_modules/@types/fs/index.d.ts' does not exist.",
+                    "File '/a/node_modules/@types/fs.ts' does not exist.",
+                    "File '/a/node_modules/@types/fs.tsx' does not exist.",
+                    "File '/a/node_modules/@types/fs.d.ts' does not exist.",
+                    "File '/a/node_modules/@types/fs/package.json' does not exist.",
+                    "File '/a/node_modules/@types/fs/index.ts' does not exist.",
+                    "File '/a/node_modules/@types/fs/index.tsx' does not exist.",
+                    "File '/a/node_modules/@types/fs/index.d.ts' does not exist.",
+                    "File '/node_modules/@types/fs.ts' does not exist.",
+                    "File '/node_modules/@types/fs.tsx' does not exist.",
+                    "File '/node_modules/@types/fs.d.ts' does not exist.",
+                    "File '/node_modules/@types/fs/package.json' does not exist.",
+                    "File '/node_modules/@types/fs/index.ts' does not exist.",
+                    "File '/node_modules/@types/fs/index.tsx' does not exist.",
+                    "File '/node_modules/@types/fs/index.d.ts' does not exist.",
+                    "File '/a/b/fs.js' does not exist.",
+                    "File '/a/b/fs.jsx' does not exist.",
+                    "File '/a/fs.js' does not exist.",
+                    "File '/a/fs.jsx' does not exist.",
+                    "File '/fs.js' does not exist.",
+                    "File '/fs.jsx' does not exist.",
+                    "======== Module name 'fs' was not resolved. ========",
+                ], "should look for 'fs'");
+
+            const program_2 = updateProgram(program, program.getRootFileNames(), options, f => {
+                f[0].text = f[0].text.updateProgram("var x = 1;");
+            });
+            assert.deepEqual(program_2.host.getTrace(), [
+                "Module 'fs' was resolved as ambient module declared in '/a/b/node.d.ts' since this file was not modified."
+            ], "should reuse 'fs' since node.d.ts was not changed");
+
+            const program_3 = updateProgram(program_2, program_2.getRootFileNames(), options, f => {
+                f[0].text = f[0].text.updateProgram("var y = 1;");
+                f[1].text = f[1].text.updateProgram("declare var process: any");
+            });
+            assert.deepEqual(program_3.host.getTrace(),
+                [
+                    "======== Resolving module 'fs' from '/a/b/app.ts'. ========",
+                    "Module resolution kind is not specified, using 'Classic'.",
+                    "File '/a/b/fs.ts' does not exist.",
+                    "File '/a/b/fs.tsx' does not exist.",
+                    "File '/a/b/fs.d.ts' does not exist.",
+                    "File '/a/fs.ts' does not exist.",
+                    "File '/a/fs.tsx' does not exist.",
+                    "File '/a/fs.d.ts' does not exist.",
+                    "File '/fs.ts' does not exist.",
+                    "File '/fs.tsx' does not exist.",
+                    "File '/fs.d.ts' does not exist.",
+                    "File '/a/b/node_modules/@types/fs.ts' does not exist.",
+                    "File '/a/b/node_modules/@types/fs.tsx' does not exist.",
+                    "File '/a/b/node_modules/@types/fs.d.ts' does not exist.",
+                    "File '/a/b/node_modules/@types/fs/package.json' does not exist.",
+                    "File '/a/b/node_modules/@types/fs/index.ts' does not exist.",
+                    "File '/a/b/node_modules/@types/fs/index.tsx' does not exist.",
+                    "File '/a/b/node_modules/@types/fs/index.d.ts' does not exist.",
+                    "File '/a/node_modules/@types/fs.ts' does not exist.",
+                    "File '/a/node_modules/@types/fs.tsx' does not exist.",
+                    "File '/a/node_modules/@types/fs.d.ts' does not exist.",
+                    "File '/a/node_modules/@types/fs/package.json' does not exist.",
+                    "File '/a/node_modules/@types/fs/index.ts' does not exist.",
+                    "File '/a/node_modules/@types/fs/index.tsx' does not exist.",
+                    "File '/a/node_modules/@types/fs/index.d.ts' does not exist.",
+                    "File '/node_modules/@types/fs.ts' does not exist.",
+                    "File '/node_modules/@types/fs.tsx' does not exist.",
+                    "File '/node_modules/@types/fs.d.ts' does not exist.",
+                    "File '/node_modules/@types/fs/package.json' does not exist.",
+                    "File '/node_modules/@types/fs/index.ts' does not exist.",
+                    "File '/node_modules/@types/fs/index.tsx' does not exist.",
+                    "File '/node_modules/@types/fs/index.d.ts' does not exist.",
+                    "File '/a/b/fs.js' does not exist.",
+                    "File '/a/b/fs.jsx' does not exist.",
+                    "File '/a/fs.js' does not exist.",
+                    "File '/a/fs.jsx' does not exist.",
+                    "File '/fs.js' does not exist.",
+                    "File '/fs.jsx' does not exist.",
+                    "======== Module name 'fs' was not resolved. ========",
+                ], "should look for 'fs' again since node.d.ts was changed");
+
+        });
     });
 
     describe("host is optional", () => {

src/services/services.ts

@@ -472,6 +472,7 @@ namespace ts {
         public imports: LiteralExpression[];
         public moduleAugmentations: LiteralExpression[];
         private namedDeclarations: Map<Declaration[]>;
+        public ambientModuleNames: string[];
 
         constructor(kind: SyntaxKind, pos: number, end: number) {
             super(kind, pos, end);