Allow extensionless mains for cjs mode packages even from an esm import

src/compiler/moduleNameResolver.ts

@@ -1819,7 +1819,17 @@ 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, candidate, onlyRecordFailures, state, /*considerPackageJson*/ false);
+
+            // Disable `EsmMode` for the resolution of the package path for cjs-mode packages (so the `main` field can omit extensions)
+            // (technically it only emits a deprecation warning in esm packages right now, but that's probably
+            // enough to mean we don't need to support it)
+            const features = state.features;
+            if (jsonContent?.type !== "module") {
+                state.features &= ~NodeResolutionFeatures.EsmMode;
+            }
+            const result = nodeLoadModuleByRelativeName(nextExtensions, candidate, onlyRecordFailures, state, /*considerPackageJson*/ false);
+            state.features = features;
+            return result;
         };
 
         const onlyRecordFailuresForPackageFile = packageFile ? !directoryProbablyExists(getDirectoryPath(packageFile), state.host) : undefined;

tests/baselines/reference/nodeNextEsmImportsOfPackagesWithExtensionlessMains.js

@@ -0,0 +1,42 @@
+//// [tests/cases/compiler/nodeNextEsmImportsOfPackagesWithExtensionlessMains.ts] ////
+
+//// [package.json]
+{
+    "name": "@types/ip",
+    "version": "1.1.0",
+    "main": "",
+    "types": "index"
+}
+//// [index.d.ts]
+export function address(): string;
+//// [package.json]
+{
+    "name": "nullthrows",
+    "version": "1.1.1",
+    "main": "nullthrows.js",
+    "types": "nullthrows.d.ts"
+}
+//// [nullthrows.d.ts]
+declare function nullthrows(x: any): any;
+declare namespace nullthrows {
+    export {nullthrows as default};
+}
+export = nullthrows;
+//// [package.json]
+{
+    "type": "module"
+}
+//// [index.ts]
+import * as ip from 'ip';
+import nullthrows from 'nullthrows'; // shouldn't be callable, `nullthrows` is a cjs package, so the `default` is the module itself
+
+export function getAddress(): string {
+  return nullthrows(ip.address());
+}
+
+//// [index.js]
+import * as ip from 'ip';
+import nullthrows from 'nullthrows'; // shouldn't be callable, `nullthrows` is a cjs package, so the `default` is the module itself
+export function getAddress() {
+    return nullthrows(ip.address());
+}

tests/baselines/reference/nodeNextEsmImportsOfPackagesWithExtensionlessMains.symbols

@@ -0,0 +1,35 @@
+=== tests/cases/compiler/index.ts ===
+import * as ip from 'ip';
+>ip : Symbol(ip, Decl(index.ts, 0, 6))
+
+import nullthrows from 'nullthrows'; // shouldn't be callable, `nullthrows` is a cjs package, so the `default` is the module itself
+>nullthrows : Symbol(nullthrows, Decl(index.ts, 1, 6))
+
+export function getAddress(): string {
+>getAddress : Symbol(getAddress, Decl(index.ts, 1, 36))
+
+  return nullthrows(ip.address());
+>nullthrows : Symbol(nullthrows, Decl(index.ts, 1, 6))
+>ip.address : Symbol(ip.address, Decl(index.d.ts, 0, 0))
+>ip : Symbol(ip, Decl(index.ts, 0, 6))
+>address : Symbol(ip.address, Decl(index.d.ts, 0, 0))
+}
+=== tests/cases/compiler/node_modules/@types/ip/index.d.ts ===
+export function address(): string;
+>address : Symbol(address, Decl(index.d.ts, 0, 0))
+
+=== tests/cases/compiler/node_modules/nullthrows/nullthrows.d.ts ===
+declare function nullthrows(x: any): any;
+>nullthrows : Symbol(nullthrows, Decl(nullthrows.d.ts, 0, 0), Decl(nullthrows.d.ts, 0, 41))
+>x : Symbol(x, Decl(nullthrows.d.ts, 0, 28))
+
+declare namespace nullthrows {
+>nullthrows : Symbol(nullthrows, Decl(nullthrows.d.ts, 0, 0), Decl(nullthrows.d.ts, 0, 41))
+
+    export {nullthrows as default};
+>nullthrows : Symbol(nullthrows, Decl(nullthrows.d.ts, 0, 0), Decl(nullthrows.d.ts, 0, 41))
+>default : Symbol(default, Decl(nullthrows.d.ts, 2, 12))
+}
+export = nullthrows;
+>nullthrows : Symbol(nullthrows, Decl(nullthrows.d.ts, 0, 0), Decl(nullthrows.d.ts, 0, 41))
+

tests/baselines/reference/nodeNextEsmImportsOfPackagesWithExtensionlessMains.types

@@ -0,0 +1,37 @@
+=== tests/cases/compiler/index.ts ===
+import * as ip from 'ip';
+>ip : typeof ip
+
+import nullthrows from 'nullthrows'; // shouldn't be callable, `nullthrows` is a cjs package, so the `default` is the module itself
+>nullthrows : typeof nullthrows
+
+export function getAddress(): string {
+>getAddress : () => string
+
+  return nullthrows(ip.address());
+>nullthrows(ip.address()) : any
+>nullthrows : typeof nullthrows
+>ip.address() : string
+>ip.address : () => string
+>ip : typeof ip
+>address : () => string
+}
+=== tests/cases/compiler/node_modules/@types/ip/index.d.ts ===
+export function address(): string;
+>address : () => string
+
+=== tests/cases/compiler/node_modules/nullthrows/nullthrows.d.ts ===
+declare function nullthrows(x: any): any;
+>nullthrows : typeof nullthrows
+>x : any
+
+declare namespace nullthrows {
+>nullthrows : typeof nullthrows
+
+    export {nullthrows as default};
+>nullthrows : typeof nullthrows
+>default : typeof nullthrows
+}
+export = nullthrows;
+>nullthrows : typeof nullthrows
+

tests/cases/compiler/nodeNextEsmImportsOfPackagesWithExtensionlessMains.ts

@@ -0,0 +1,36 @@
+// @noImplicitReferences: true
+// @module: nodenext
+// @outDir: esm
+// @filename: node_modules/@types/ip/package.json
+{
+    "name": "@types/ip",
+    "version": "1.1.0",
+    "main": "",
+    "types": "index"
+}
+// @filename: node_modules/@types/ip/index.d.ts
+export function address(): string;
+// @filename: node_modules/nullthrows/package.json
+{
+    "name": "nullthrows",
+    "version": "1.1.1",
+    "main": "nullthrows.js",
+    "types": "nullthrows.d.ts"
+}
+// @filename: node_modules/nullthrows/nullthrows.d.ts
+declare function nullthrows(x: any): any;
+declare namespace nullthrows {
+    export {nullthrows as default};
+}
+export = nullthrows;
+// @filename: package.json
+{
+    "type": "module"
+}
+// @filename: index.ts
+import * as ip from 'ip';
+import nullthrows from 'nullthrows'; // shouldn't be callable, `nullthrows` is a cjs package, so the `default` is the module itself
+
+export function getAddress(): string {
+  return nullthrows(ip.address());
+}