ESM mode nonrelative imports should assume index.js entrypoints even …

src/compiler/moduleNameResolver.ts

@@ -1394,7 +1394,13 @@ namespace ts {
                 onlyRecordFailures = true;
             }
         }
-        return loadNodeModuleFromDirectory(extensions, candidate, onlyRecordFailures, state, considerPackageJson);
+        // esm mode relative imports shouldn't do any directory lookups (either inside `package.json`
+        // files or implicit `index.js`es). This is a notable depature from cjs norms, where `./foo/pkg`
+        // could have been redirected by `./foo/pkg/package.json` to an arbitrary location!
+        if (!(state.features & NodeResolutionFeatures.EsmMode)) {
+            return loadNodeModuleFromDirectory(extensions, candidate, onlyRecordFailures, state, considerPackageJson);
+        }
+        return undefined;
     }
 
     /*@internal*/
@@ -2178,7 +2184,7 @@ namespace ts {
             if (packageInfo && packageInfo.packageJsonContent.exports && state.features & NodeResolutionFeatures.Exports) {
                 return loadModuleFromExports(packageInfo, extensions, combinePaths(".", rest), state, cache, redirectedReference)?.value;
             }
-            const pathAndExtension =
+            let pathAndExtension =
                 loadModuleFromFile(extensions, candidate, onlyRecordFailures, state) ||
                 loadNodeModuleFromDirectoryWorker(
                     extensions,
@@ -2188,6 +2194,16 @@ namespace ts {
                     packageInfo && packageInfo.packageJsonContent,
                     packageInfo && packageInfo.versionPaths
                 );
+            if (
+                !pathAndExtension && packageInfo
+                && packageInfo.packageJsonContent.exports === undefined
+                && packageInfo.packageJsonContent.main === undefined
+                && state.features & NodeResolutionFeatures.EsmMode
+            ) {
+                // EsmMode disables index lookup in `loadNodeModuleFromDirectoryWorker` generally, however non-relative package resolutions still assume
+                // a default `index.js` entrypoint if no `main` or `exports` are present
+                pathAndExtension = loadModuleFromFile(extensions, combinePaths(candidate, "index.js"), onlyRecordFailures, state);
+            }
             return withPackageId(packageInfo, pathAndExtension);
         };
 

tests/baselines/reference/nodeNextImportModeImplicitIndexResolution.errors.txt

@@ -0,0 +1,31 @@
+tests/cases/compiler/index.ts(2,31): error TS2834: Relative import paths need explicit file extensions in EcmaScript imports when '--moduleResolution' is 'node12' or 'nodenext'. Consider adding an extension to the import path.
+tests/cases/compiler/index.ts(3,31): error TS2834: Relative import paths need explicit file extensions in EcmaScript imports when '--moduleResolution' is 'node12' or 'nodenext'. Consider adding an extension to the import path.
+
+
+==== tests/cases/compiler/node_modules/pkg/package.json (0 errors) ====
+    {
+        "name": "pkg",
+        "version": "0.0.1"
+    }
+==== tests/cases/compiler/node_modules/pkg/index.d.ts (0 errors) ====
+    export const item = 4;
+==== tests/cases/compiler/pkg/package.json (0 errors) ====
+    {
+        "private": true
+    }
+==== tests/cases/compiler/pkg/index.d.ts (0 errors) ====
+    export const item = 4;
+==== tests/cases/compiler/package.json (0 errors) ====
+    {
+        "type": "module",
+        "private": true
+    }
+==== tests/cases/compiler/index.ts (2 errors) ====
+    import { item } from "pkg"; // should work (`index.js` is assumed to be the entrypoint for packages found via nonrelative import)
+    import { item as item2 } from "./pkg";  // shouldn't work (`index.js` is _not_ assumed to be the entrypoint for packages found via relative import)
+                                  ~~~~~~~
+!!! error TS2834: Relative import paths need explicit file extensions in EcmaScript imports when '--moduleResolution' is 'node12' or 'nodenext'. Consider adding an extension to the import path.
+    import { item as item3 } from "./node_modules/pkg" // _even if they're in a node_modules folder_
+                                  ~~~~~~~~~~~~~~~~~~~~
+!!! error TS2834: Relative import paths need explicit file extensions in EcmaScript imports when '--moduleResolution' is 'node12' or 'nodenext'. Consider adding an extension to the import path.
+

tests/baselines/reference/nodeNextImportModeImplicitIndexResolution.js

@@ -0,0 +1,28 @@
+//// [tests/cases/compiler/nodeNextImportModeImplicitIndexResolution.ts] ////
+
+//// [package.json]
+{
+    "name": "pkg",
+    "version": "0.0.1"
+}
+//// [index.d.ts]
+export const item = 4;
+//// [package.json]
+{
+    "private": true
+}
+//// [index.d.ts]
+export const item = 4;
+//// [package.json]
+{
+    "type": "module",
+    "private": true
+}
+//// [index.ts]
+import { item } from "pkg"; // should work (`index.js` is assumed to be the entrypoint for packages found via nonrelative import)
+import { item as item2 } from "./pkg";  // shouldn't work (`index.js` is _not_ assumed to be the entrypoint for packages found via relative import)
+import { item as item3 } from "./node_modules/pkg" // _even if they're in a node_modules folder_
+
+
+//// [index.js]
+export {};

tests/baselines/reference/nodeNextImportModeImplicitIndexResolution.symbols

@@ -0,0 +1,18 @@
+=== tests/cases/compiler/node_modules/pkg/index.d.ts ===
+export const item = 4;
+>item : Symbol(item, Decl(index.d.ts, 0, 12))
+
+=== tests/cases/compiler/pkg/index.d.ts ===
+export const item = 4;
+>item : Symbol(item, Decl(index.d.ts, 0, 12))
+
+=== tests/cases/compiler/index.ts ===
+import { item } from "pkg"; // should work (`index.js` is assumed to be the entrypoint for packages found via nonrelative import)
+>item : Symbol(item, Decl(index.ts, 0, 8))
+
+import { item as item2 } from "./pkg";  // shouldn't work (`index.js` is _not_ assumed to be the entrypoint for packages found via relative import)
+>item2 : Symbol(item2, Decl(index.ts, 1, 8))
+
+import { item as item3 } from "./node_modules/pkg" // _even if they're in a node_modules folder_
+>item3 : Symbol(item3, Decl(index.ts, 2, 8))
+

tests/baselines/reference/nodeNextImportModeImplicitIndexResolution.types

@@ -0,0 +1,22 @@
+=== tests/cases/compiler/node_modules/pkg/index.d.ts ===
+export const item = 4;
+>item : 4
+>4 : 4
+
+=== tests/cases/compiler/pkg/index.d.ts ===
+export const item = 4;
+>item : 4
+>4 : 4
+
+=== tests/cases/compiler/index.ts ===
+import { item } from "pkg"; // should work (`index.js` is assumed to be the entrypoint for packages found via nonrelative import)
+>item : 4
+
+import { item as item2 } from "./pkg";  // shouldn't work (`index.js` is _not_ assumed to be the entrypoint for packages found via relative import)
+>item : any
+>item2 : any
+
+import { item as item3 } from "./node_modules/pkg" // _even if they're in a node_modules folder_
+>item : any
+>item3 : any
+

tests/cases/compiler/nodeNextImportModeImplicitIndexResolution.ts

@@ -0,0 +1,23 @@
+// @module: nodenext
+// @filename: node_modules/pkg/package.json
+{
+    "name": "pkg",
+    "version": "0.0.1"
+}
+// @filename: node_modules/pkg/index.d.ts
+export const item = 4;
+// @filename: pkg/package.json
+{
+    "private": true
+}
+// @filename: pkg/index.d.ts
+export const item = 4;
+// @filename: package.json
+{
+    "type": "module",
+    "private": true
+}
+// @filename: index.ts
+import { item } from "pkg"; // should work (`index.js` is assumed to be the entrypoint for packages found via nonrelative import)
+import { item as item2 } from "./pkg";  // shouldn't work (`index.js` is _not_ assumed to be the entrypoint for packages found via relative import)
+import { item as item3 } from "./node_modules/pkg" // _even if they're in a node_modules folder_