For duplicate source files of the same package, make one redirect to the other (#16274)

* For duplicate source files of the same package, make one redirect to the other

* Add reuseProgramStructure tests

* Copy `sourceFileToPackageId` and `isSourceFileTargetOfRedirect` only if we completely reuse old structure

* Use fallthrough instead of early exit from loop

* Use a set to efficiently detect duplicate package names

* Move map setting outside of createRedirectSourceFile

* Correctly handle seenPackageNames set

* sourceFileToPackageId -> sourceFileToPackageName

* Renames

* Respond to PR comments

* Fix bug where `oldSourceFile !== newSourceFile` because oldSourceFile was a redirect

* Clean up redirectInfo

* Respond to PR comments

src/compiler/core.ts

@@ -2210,20 +2210,14 @@ namespace ts {
     /** Must have ".d.ts" first because if ".ts" goes first, that will be detected as the extension instead of ".d.ts". */
     export const supportedTypescriptExtensionsForExtractExtension: ReadonlyArray<Extension> = [Extension.Dts, Extension.Ts, Extension.Tsx];
     export const supportedJavascriptExtensions: ReadonlyArray<Extension> = [Extension.Js, Extension.Jsx];
-    const allSupportedExtensions = [...supportedTypeScriptExtensions, ...supportedJavascriptExtensions];
+    const allSupportedExtensions: ReadonlyArray<Extension> = [...supportedTypeScriptExtensions, ...supportedJavascriptExtensions];
 
     export function getSupportedExtensions(options?: CompilerOptions, extraFileExtensions?: ReadonlyArray<JsFileExtensionInfo>): ReadonlyArray<string> {
         const needAllExtensions = options && options.allowJs;
         if (!extraFileExtensions || extraFileExtensions.length === 0 || !needAllExtensions) {
             return needAllExtensions ? allSupportedExtensions : supportedTypeScriptExtensions;
         }
-        const extensions: string[] = allSupportedExtensions.slice(0);
-        for (const extInfo of extraFileExtensions) {
-            if (extensions.indexOf(extInfo.extension) === -1) {
-                extensions.push(extInfo.extension);
-            }
-        }
-        return extensions;
+        return deduplicate([...allSupportedExtensions, ...extraFileExtensions.map(e => e.extension)]);
     }
 
     export function hasJavaScriptFileExtension(fileName: string) {
@@ -2590,6 +2584,11 @@ namespace ts {
         }
         Debug.fail(`File ${path} has unknown extension.`);
     }
+
+    export function isAnySupportedFileExtension(path: string): boolean {
+        return tryGetExtensionFromPath(path) !== undefined;
+    }
+
     export function tryGetExtensionFromPath(path: string): Extension | undefined {
         return find<Extension>(supportedTypescriptExtensionsForExtractExtension, e => fileExtensionIs(path, e)) || find(supportedJavascriptExtensions, e => fileExtensionIs(path, e));
     }

src/compiler/moduleNameResolver.ts

@@ -13,13 +13,32 @@ namespace ts {
         return compilerOptions.traceResolution && host.trace !== undefined;
     }
 
-    /**
-     * Result of trying to resolve a module.
-     * At least one of `ts` and `js` should be defined, or the whole thing should be `undefined`.
-     */
+    /** Array that is only intended to be pushed to, never read. */
+    /* @internal */
+    export interface Push<T> {
+        push(value: T): void;
+    }
+
+    function withPackageId(packageId: PackageId | undefined, r: PathAndExtension | undefined): Resolved {
+        return r && { path: r.path, extension: r.ext, packageId };
+    }
+
+    function noPackageId(r: PathAndExtension | undefined): Resolved {
+        return withPackageId(/*packageId*/ undefined, r);
+    }
+
+    /** Result of trying to resolve a module. */
     interface Resolved {
         path: string;
         extension: Extension;
+        packageId: PackageId | undefined;
+    }
+
+    /** Result of trying to resolve a module at a file. Needs to have 'packageId' added later. */
+    interface PathAndExtension {
+        path: string;
+        // (Use a different name than `extension` to make sure Resolved isn't assignable to PathAndExtension.)
+        ext: Extension;
     }
 
     /**
@@ -43,7 +62,7 @@ namespace ts {
 
     function createResolvedModuleWithFailedLookupLocations(resolved: Resolved | undefined, isExternalLibraryImport: boolean, failedLookupLocations: string[]): ResolvedModuleWithFailedLookupLocations {
         return {
-            resolvedModule: resolved && { resolvedFileName: resolved.path, extension: resolved.extension, isExternalLibraryImport },
+            resolvedModule: resolved && { resolvedFileName: resolved.path, extension: resolved.extension, isExternalLibraryImport, packageId: resolved.packageId },
             failedLookupLocations
         };
     }
@@ -54,9 +73,16 @@ namespace ts {
         traceEnabled: boolean;
     }
 
+    interface PackageJson {
+        name?: string;
+        version?: string;
+        typings?: string;
+        types?: string;
+        main?: string;
+    }
+
     /** Reads from "main" or "types"/"typings" depending on `extensions`. */
-    function tryReadPackageJsonFields(readTypes: boolean, packageJsonPath: string, baseDirectory: string, state: ModuleResolutionState): string | undefined {
-        const jsonContent = readJson(packageJsonPath, state.host);
+    function tryReadPackageJsonFields(readTypes: boolean, jsonContent: PackageJson, baseDirectory: string, state: ModuleResolutionState): string | undefined {
         return readTypes ? tryReadFromField("typings") || tryReadFromField("types") : tryReadFromField("main");
 
         function tryReadFromField(fieldName: "typings" | "types" | "main"): string | undefined {
@@ -83,7 +109,7 @@ namespace ts {
         }
     }
 
-    function readJson(path: string, host: ModuleResolutionHost): { typings?: string, types?: string, main?: string } {
+    function readJson(path: string, host: ModuleResolutionHost): PackageJson {
         try {
             const jsonText = host.readFile(path);
             return jsonText ? JSON.parse(jsonText) : {};
@@ -646,7 +672,7 @@ namespace ts {
                 if (extension !== undefined) {
                     const path = tryFile(candidate, failedLookupLocations, /*onlyRecordFailures*/ false, state);
                     if (path !== undefined) {
-                        return { path, extension };
+                        return { path, extension, packageId: undefined };
                     }
                 }
 
@@ -709,7 +735,7 @@ namespace ts {
                 }
                 const resolved = loadModuleFromNodeModules(extensions, moduleName, containingDirectory, failedLookupLocations, state, cache);
                 // For node_modules lookups, get the real path so that multiple accesses to an `npm link`-ed module do not create duplicate files.
-                return resolved && { value: resolved.value && { resolved: { path: realpath(resolved.value.path, host, traceEnabled), extension: resolved.value.extension }, isExternalLibraryImport: true } };
+                return resolved && { value: resolved.value && { resolved: { ...resolved.value, path: realpath(resolved.value.path, host, traceEnabled) }, isExternalLibraryImport: true } };
             }
             else {
                 const candidate = normalizePath(combinePaths(containingDirectory, moduleName));
@@ -747,7 +773,7 @@ namespace ts {
             }
             const resolvedFromFile = loadModuleFromFile(extensions, candidate, failedLookupLocations, onlyRecordFailures, state);
             if (resolvedFromFile) {
-                return resolvedFromFile;
+                return noPackageId(resolvedFromFile);
             }
         }
         if (!onlyRecordFailures) {
@@ -768,11 +794,15 @@ namespace ts {
         return !host.directoryExists || host.directoryExists(directoryName);
     }
 
+    function loadModuleFromFileNoPackageId(extensions: Extensions, candidate: string, failedLookupLocations: Push<string>, onlyRecordFailures: boolean, state: ModuleResolutionState): Resolved {
+        return noPackageId(loadModuleFromFile(extensions, candidate, failedLookupLocations, onlyRecordFailures, state));
+    }
+
     /**
      * @param {boolean} onlyRecordFailures - if true then function won't try to actually load files but instead record all attempts as failures. This flag is necessary
      * in cases when we know upfront that all load attempts will fail (because containing folder does not exists) however we still need to record all failed lookup locations.
      */
-    function loadModuleFromFile(extensions: Extensions, candidate: string, failedLookupLocations: Push<string>, onlyRecordFailures: boolean, state: ModuleResolutionState): Resolved | undefined {
+    function loadModuleFromFile(extensions: Extensions, candidate: string, failedLookupLocations: Push<string>, onlyRecordFailures: boolean, state: ModuleResolutionState): PathAndExtension | undefined {
         // First, try adding an extension. An import of "foo" could be matched by a file "foo.ts", or "foo.js" by "foo.js.ts"
         const resolvedByAddingExtension = tryAddingExtensions(candidate, extensions, failedLookupLocations, onlyRecordFailures, state);
         if (resolvedByAddingExtension) {
@@ -792,7 +822,7 @@ namespace ts {
     }
 
     /** Try to return an existing file that adds one of the `extensions` to `candidate`. */
-    function tryAddingExtensions(candidate: string, extensions: Extensions, failedLookupLocations: Push<string>, onlyRecordFailures: boolean, state: ModuleResolutionState): Resolved | undefined {
+    function tryAddingExtensions(candidate: string, extensions: Extensions, failedLookupLocations: Push<string>, onlyRecordFailures: boolean, state: ModuleResolutionState): PathAndExtension | undefined {
         if (!onlyRecordFailures) {
             // check if containing folder exists - if it doesn't then just record failures for all supported extensions without disk probing
             const directory = getDirectoryPath(candidate);
@@ -810,9 +840,9 @@ namespace ts {
                 return tryExtension(Extension.Js) || tryExtension(Extension.Jsx);
         }
 
-        function tryExtension(extension: Extension): Resolved | undefined {
-            const path = tryFile(candidate + extension, failedLookupLocations, onlyRecordFailures, state);
-            return path && { path, extension };
+        function tryExtension(ext: Extension): PathAndExtension | undefined {
+            const path = tryFile(candidate + ext, failedLookupLocations, onlyRecordFailures, state);
+            return path && { path, ext };
         }
     }
 
@@ -838,12 +868,23 @@ namespace ts {
     function loadNodeModuleFromDirectory(extensions: Extensions, candidate: string, failedLookupLocations: Push<string>, onlyRecordFailures: boolean, state: ModuleResolutionState, considerPackageJson = true): Resolved | undefined {
         const directoryExists = !onlyRecordFailures && directoryProbablyExists(candidate, state.host);
 
+        let packageId: PackageId | undefined;
+
         if (considerPackageJson) {
             const packageJsonPath = pathToPackageJson(candidate);
             if (directoryExists && state.host.fileExists(packageJsonPath)) {
-                const fromPackageJson = loadModuleFromPackageJson(packageJsonPath, extensions, candidate, failedLookupLocations, state);
+                if (state.traceEnabled) {
+                    trace(state.host, Diagnostics.Found_package_json_at_0, packageJsonPath);
+                }
+                const jsonContent = readJson(packageJsonPath, state.host);
+
+                if (typeof jsonContent.name === "string" && typeof jsonContent.version === "string") {
+                    packageId = { name: jsonContent.name, version: jsonContent.version };
+                }
+
+                const fromPackageJson = loadModuleFromPackageJson(jsonContent, extensions, candidate, failedLookupLocations, state);
                 if (fromPackageJson) {
-                    return fromPackageJson;
+                    return withPackageId(packageId, fromPackageJson);
                 }
             }
             else {
@@ -855,15 +896,11 @@ namespace ts {
             }
         }
 
-        return loadModuleFromFile(extensions, combinePaths(candidate, "index"), failedLookupLocations, !directoryExists, state);
+        return withPackageId(packageId, loadModuleFromFile(extensions, combinePaths(candidate, "index"), failedLookupLocations, !directoryExists, state));
     }
 
-    function loadModuleFromPackageJson(packageJsonPath: string, extensions: Extensions, candidate: string, failedLookupLocations: Push<string>, state: ModuleResolutionState): Resolved | undefined {
-        if (state.traceEnabled) {
-            trace(state.host, Diagnostics.Found_package_json_at_0, packageJsonPath);
-        }
-
-        const file = tryReadPackageJsonFields(extensions !== Extensions.JavaScript, packageJsonPath, candidate, state);
+    function loadModuleFromPackageJson(jsonContent: PackageJson, extensions: Extensions, candidate: string, failedLookupLocations: Push<string>, state: ModuleResolutionState): PathAndExtension | undefined {
+        const file = tryReadPackageJsonFields(extensions !== Extensions.JavaScript, jsonContent, candidate, state);
         if (!file) {
             return undefined;
         }
@@ -883,13 +920,18 @@ namespace ts {
         // Even if extensions is DtsOnly, we can still look up a .ts file as a result of package.json "types"
         const nextExtensions = extensions === Extensions.DtsOnly ? Extensions.TypeScript : extensions;
         // Don't do package.json lookup recursively, because Node.js' package lookup doesn't.
-        return nodeLoadModuleByRelativeName(nextExtensions, file, failedLookupLocations, onlyRecordFailures, state, /*considerPackageJson*/ false);
+        const result = nodeLoadModuleByRelativeName(nextExtensions, file, failedLookupLocations, onlyRecordFailures, state, /*considerPackageJson*/ false);
+        if (result) {
+            // It won't have a `packageId` set, because we disabled `considerPackageJson`.
+            Debug.assert(result.packageId === undefined);
+            return { path: result.path, ext: result.extension };
+        }
     }
 
     /** Resolve from an arbitrarily specified file. Return `undefined` if it has an unsupported extension. */
-    function resolvedIfExtensionMatches(extensions: Extensions, path: string): Resolved | undefined {
-        const extension = tryGetExtensionFromPath(path);
-        return extension !== undefined && extensionIsOk(extensions, extension) ? { path, extension } : undefined;
+    function resolvedIfExtensionMatches(extensions: Extensions, path: string): PathAndExtension | undefined {
+        const ext = tryGetExtensionFromPath(path);
+        return ext !== undefined && extensionIsOk(extensions, ext) ? { path, ext } : undefined;
     }
 
     /** True if `extension` is one of the supported `extensions`. */
@@ -911,7 +953,7 @@ namespace ts {
     function loadModuleFromNodeModulesFolder(extensions: Extensions, moduleName: string, nodeModulesFolder: string, nodeModulesFolderExists: boolean, failedLookupLocations: Push<string>, state: ModuleResolutionState): Resolved | undefined {
         const candidate = normalizePath(combinePaths(nodeModulesFolder, moduleName));
 
-        return loadModuleFromFile(extensions, candidate, failedLookupLocations, !nodeModulesFolderExists, state) ||
+        return loadModuleFromFileNoPackageId(extensions, candidate, failedLookupLocations, !nodeModulesFolderExists, state) ||
             loadNodeModuleFromDirectory(extensions, candidate, failedLookupLocations, !nodeModulesFolderExists, state);
     }
 
@@ -996,7 +1038,7 @@ namespace ts {
             if (traceEnabled) {
                 trace(host, Diagnostics.Resolution_for_module_0_was_found_in_cache, moduleName);
             }
-            return { value: result.resolvedModule && { path: result.resolvedModule.resolvedFileName, extension: result.resolvedModule.extension } };
+            return { value: result.resolvedModule && { path: result.resolvedModule.resolvedFileName, extension: result.resolvedModule.extension, packageId: result.resolvedModule.packageId } };
         }
     }
 
@@ -1010,7 +1052,7 @@ namespace ts {
         return createResolvedModuleWithFailedLookupLocations(resolved && resolved.value, /*isExternalLibraryImport*/ false, failedLookupLocations);
 
         function tryResolve(extensions: Extensions): SearchResult<Resolved> {
-            const resolvedUsingSettings = tryLoadModuleUsingOptionalResolutionSettings(extensions, moduleName, containingDirectory, loadModuleFromFile, failedLookupLocations, state);
+            const resolvedUsingSettings = tryLoadModuleUsingOptionalResolutionSettings(extensions, moduleName, containingDirectory, loadModuleFromFileNoPackageId, failedLookupLocations, state);
             if (resolvedUsingSettings) {
                 return { value: resolvedUsingSettings };
             }
@@ -1024,7 +1066,7 @@ namespace ts {
                         return resolutionFromCache;
                     }
                     const searchName = normalizePath(combinePaths(directory, moduleName));
-                    return toSearchResult(loadModuleFromFile(extensions, searchName, failedLookupLocations, /*onlyRecordFailures*/ false, state));
+                    return toSearchResult(loadModuleFromFileNoPackageId(extensions, searchName, failedLookupLocations, /*onlyRecordFailures*/ false, state));
                 });
                 if (resolved) {
                     return resolved;
@@ -1036,7 +1078,7 @@ namespace ts {
             }
             else {
                 const candidate = normalizePath(combinePaths(containingDirectory, moduleName));
-                return toSearchResult(loadModuleFromFile(extensions, candidate, failedLookupLocations, /*onlyRecordFailures*/ false, state));
+                return toSearchResult(loadModuleFromFileNoPackageId(extensions, candidate, failedLookupLocations, /*onlyRecordFailures*/ false, state));
             }
         }
     }

src/compiler/program.ts

@@ -472,6 +472,15 @@ namespace ts {
             resolveTypeReferenceDirectiveNamesWorker = (typeReferenceDirectiveNames, containingFile) => loadWithLocalCache(checkAllDefined(typeReferenceDirectiveNames), containingFile, loader);
         }
 
+        // Map from a stringified PackageId to the source file with that id.
+        // Only one source file may have a given packageId. Others become redirects (see createRedirectSourceFile).
+        // `packageIdToSourceFile` is only used while building the program, while `sourceFileToPackageName` and `isSourceFileTargetOfRedirect` are kept around.
+        const packageIdToSourceFile = createMap<SourceFile>();
+        // Maps from a SourceFile's `.path` to the name of the package it was imported with.
+        let sourceFileToPackageName = createMap<string>();
+        // See `sourceFileIsRedirectedTo`.
+        let redirectTargetsSet = createMap<true>();
+
         const filesByName = createMap<SourceFile | undefined>();
         // stores 'filename -> file association' ignoring case
         // used to track cases when two file names differ only in casing
@@ -548,6 +557,8 @@ namespace ts {
             isSourceFileFromExternalLibrary,
             dropDiagnosticsProducingTypeChecker,
             getSourceFileFromReference,
+            sourceFileToPackageName,
+            redirectTargetsSet,
         };
 
         verifyCompilerOptions();
@@ -773,8 +784,12 @@ namespace ts {
             const modifiedSourceFiles: { oldFile: SourceFile, newFile: SourceFile }[] = [];
             oldProgram.structureIsReused = StructureIsReused.Completely;
 
-            for (const oldSourceFile of oldProgram.getSourceFiles()) {
-                const newSourceFile = host.getSourceFileByPath
+            const oldSourceFiles = oldProgram.getSourceFiles();
+            const enum SeenPackageName { Exists, Modified }
+            const seenPackageNames = createMap<SeenPackageName>();
+
+            for (const oldSourceFile of oldSourceFiles) {
+                let newSourceFile = host.getSourceFileByPath
                     ? host.getSourceFileByPath(oldSourceFile.fileName, oldSourceFile.path, options.target)
                     : host.getSourceFile(oldSourceFile.fileName, options.target);
 
@@ -782,10 +797,46 @@ namespace ts {
                     return oldProgram.structureIsReused = StructureIsReused.Not;
                 }
 
+                Debug.assert(!newSourceFile.redirectInfo, "Host should not return a redirect source file from `getSourceFile`");
+
+                let fileChanged: boolean;
+                if (oldSourceFile.redirectInfo) {
+                    // We got `newSourceFile` by path, so it is actually for the unredirected file.
+                    // This lets us know if the unredirected file has changed. If it has we should break the redirect.
+                    if (newSourceFile !== oldSourceFile.redirectInfo.unredirected) {
+                        // Underlying file has changed. Might not redirect anymore. Must rebuild program.
+                        return oldProgram.structureIsReused = StructureIsReused.Not;
+                    }
+                    fileChanged = false;
+                    newSourceFile = oldSourceFile; // Use the redirect.
+                }
+                else if (oldProgram.redirectTargetsSet.has(oldSourceFile.path)) {
+                    // If a redirected-to source file changes, the redirect may be broken.
+                    if (newSourceFile !== oldSourceFile) {
+                        return oldProgram.structureIsReused = StructureIsReused.Not;
+                    }
+                    fileChanged = false;
+                }
+                else {
+                    fileChanged = newSourceFile !== oldSourceFile;
+                }
+
                 newSourceFile.path = oldSourceFile.path;
                 filePaths.push(newSourceFile.path);
 
-                if (oldSourceFile !== newSourceFile) {
+                const packageName = oldProgram.sourceFileToPackageName.get(oldSourceFile.path);
+                if (packageName !== undefined) {
+                    // If there are 2 different source files for the same package name and at least one of them changes,
+                    // they might become redirects. So we must rebuild the program.
+                    const prevKind = seenPackageNames.get(packageName);
+                    const newKind = fileChanged ? SeenPackageName.Modified : SeenPackageName.Exists;
+                    if ((prevKind !== undefined && newKind === SeenPackageName.Modified) || prevKind === SeenPackageName.Modified) {
+                        return oldProgram.structureIsReused = StructureIsReused.Not;
+                    }
+                    seenPackageNames.set(packageName, newKind);
+                }
+
+                if (fileChanged) {
                     // The `newSourceFile` object was created for the new program.
 
                     if (oldSourceFile.hasNoDefaultLib !== newSourceFile.hasNoDefaultLib) {
@@ -897,6 +948,9 @@ namespace ts {
             }
             resolvedTypeReferenceDirectives = oldProgram.getResolvedTypeReferenceDirectives();
 
+            sourceFileToPackageName = oldProgram.sourceFileToPackageName;
+            redirectTargetsSet = oldProgram.redirectTargetsSet;
+
             return oldProgram.structureIsReused = StructureIsReused.Completely;
         }
 
@@ -1537,7 +1591,7 @@ namespace ts {
         /** This has side effects through `findSourceFile`. */
         function processSourceFile(fileName: string, isDefaultLib: boolean, refFile?: SourceFile, refPos?: number, refEnd?: number): void {
             getSourceFileFromReferenceWorker(fileName,
-                fileName => findSourceFile(fileName, toPath(fileName), isDefaultLib, refFile, refPos, refEnd),
+                fileName => findSourceFile(fileName, toPath(fileName), isDefaultLib, refFile, refPos, refEnd, /*packageId*/ undefined),
                 (diagnostic, ...args) => {
                     fileProcessingDiagnostics.add(refFile !== undefined && refEnd !== undefined && refPos !== undefined
                         ? createFileDiagnostic(refFile, refPos, refEnd - refPos, diagnostic, ...args)
@@ -1556,8 +1610,26 @@ namespace ts {
             }
         }
 
+        function createRedirectSourceFile(redirectTarget: SourceFile, unredirected: SourceFile, fileName: string, path: Path): SourceFile {
+            const redirect: SourceFile = Object.create(redirectTarget);
+            redirect.fileName = fileName;
+            redirect.path = path;
+            redirect.redirectInfo = { redirectTarget, unredirected };
+            Object.defineProperties(redirect, {
+                id: {
+                    get(this: SourceFile) { return this.redirectInfo.redirectTarget.id; },
+                    set(this: SourceFile, value: SourceFile["id"]) { this.redirectInfo.redirectTarget.id = value; },
+                },
+                symbol: {
+                    get(this: SourceFile) { return this.redirectInfo.redirectTarget.symbol; },
+                    set(this: SourceFile, value: SourceFile["symbol"]) { this.redirectInfo.redirectTarget.symbol = value; },
+                },
+            });
+            return redirect;
+        }
+
         // Get source file from normalized fileName
-        function findSourceFile(fileName: string, path: Path, isDefaultLib: boolean, refFile?: SourceFile, refPos?: number, refEnd?: number): SourceFile {
+        function findSourceFile(fileName: string, path: Path, isDefaultLib: boolean, refFile: SourceFile, refPos: number, refEnd: number, packageId: PackageId | undefined): SourceFile | undefined {
             if (filesByName.has(path)) {
                 const file = filesByName.get(path);
                 // try to check if we've already seen this file but with a different casing in path
@@ -1600,6 +1672,26 @@ namespace ts {
                 }
             });
 
+            if (packageId) {
+                const packageIdKey = `${packageId.name}@${packageId.version}`;
+                const fileFromPackageId = packageIdToSourceFile.get(packageIdKey);
+                if (fileFromPackageId) {
+                    // Some other SourceFile already exists with this package name and version.
+                    // Instead of creating a duplicate, just redirect to the existing one.
+                    const dupFile = createRedirectSourceFile(fileFromPackageId, file, fileName, path);
+                    redirectTargetsSet.set(fileFromPackageId.path, true);
+                    filesByName.set(path, dupFile);
+                    sourceFileToPackageName.set(path, packageId.name);
+                    files.push(dupFile);
+                    return dupFile;
+                }
+                else if (file) {
+                    // This is the first source file to have this packageId.
+                    packageIdToSourceFile.set(packageIdKey, file);
+                    sourceFileToPackageName.set(path, packageId.name);
+                }
+            }
+
             filesByName.set(path, file);
             if (file) {
                 sourceFilesFoundSearchingNodeModules.set(path, currentNodeModulesDepth > 0);
@@ -1762,7 +1854,7 @@ namespace ts {
                     else if (shouldAddFile) {
                         const path = toPath(resolvedFileName);
                         const pos = skipTrivia(file.text, file.imports[i].pos);
-                        findSourceFile(resolvedFileName, path, /*isDefaultLib*/ false, file, pos, file.imports[i].end);
+                        findSourceFile(resolvedFileName, path, /*isDefaultLib*/ false, file, pos, file.imports[i].end, resolution.packageId);
                     }
 
                     if (isFromNodeModulesSearch) {

src/compiler/types.ts

@@ -2255,6 +2255,17 @@ namespace ts {
     }
 
 
+    /* @internal */
+    export interface RedirectInfo {
+        /** Source file this redirects to. */
+        readonly redirectTarget: SourceFile;
+        /**
+         * Source file for the duplicate package. This will not be used by the Program,
+         * but we need to keep this around so we can watch for changes in underlying.
+         */
+        readonly unredirected: SourceFile;
+    }
+
     // Source files are declarations when they are external modules.
     export interface SourceFile extends Declaration {
         kind: SyntaxKind.SourceFile;
@@ -2265,6 +2276,13 @@ namespace ts {
         /* @internal */ path: Path;
         text: string;
 
+        /**
+         * If two source files are for the same version of the same package, one will redirect to the other.
+         * (See `createRedirectSourceFile` in program.ts.)
+         * The redirect will have this set. The other will not have anything set, but see Program#sourceFileIsRedirectedTo.
+         */
+        /* @internal */ redirectInfo?: RedirectInfo | undefined;
+
         amdDependencies: AmdDependency[];
         moduleName: string;
         referencedFiles: FileReference[];
@@ -2435,6 +2453,11 @@ namespace ts {
         /* @internal */ structureIsReused?: StructureIsReused;
 
         /* @internal */ getSourceFileFromReference(referencingFile: SourceFile, ref: FileReference): SourceFile | undefined;
+
+        /** Given a source file, get the name of the package it was imported from. */
+        /* @internal */ sourceFileToPackageName: Map<string>;
+        /** Set of all source files that some other source file redirects to. */
+        /* @internal */ redirectTargetsSet: Map<true>;
     }
 
     /* @internal */
@@ -3925,6 +3948,7 @@ namespace ts {
     /**
      * ResolvedModule with an explicitly provided `extension` property.
      * Prefer this over `ResolvedModule`.
+     * If changing this, remember to change `moduleResolutionIsEqualTo`.
      */
     export interface ResolvedModuleFull extends ResolvedModule {
         /**
@@ -3932,6 +3956,22 @@ namespace ts {
          * This is optional for backwards-compatibility, but will be added if not provided.
          */
         extension: Extension;
+        packageId?: PackageId;
+    }
+
+    /**
+     * Unique identifier with a package name and version.
+     * If changing this, remember to change `packageIdIsEqual`.
+     */
+    export interface PackageId {
+        /**
+         * Name of the package.
+         * Should not include `@types`.
+         * If accessing a non-index file, this should include its name e.g. "foo/bar".
+         */
+        name: string;
+        /** Version of the package, e.g. "1.2.3" */
+        version: string;
     }
 
     export const enum Extension {

src/compiler/utilities.ts

@@ -101,7 +101,12 @@ namespace ts {
     export function moduleResolutionIsEqualTo(oldResolution: ResolvedModuleFull, newResolution: ResolvedModuleFull): boolean {
         return oldResolution.isExternalLibraryImport === newResolution.isExternalLibraryImport &&
             oldResolution.extension === newResolution.extension &&
-            oldResolution.resolvedFileName === newResolution.resolvedFileName;
+            oldResolution.resolvedFileName === newResolution.resolvedFileName &&
+            packageIdIsEqual(oldResolution.packageId, newResolution.packageId);
+    }
+
+    function packageIdIsEqual(a: PackageId | undefined, b: PackageId | undefined): boolean {
+        return a === b || a && b && a.name === b.name && a.version === b.version;
     }
 
     export function typeDirectiveIsEqualTo(oldResolution: ResolvedTypeReferenceDirective, newResolution: ResolvedTypeReferenceDirective): boolean {