Proposal: Always allow type-only imports to reference .ts extensions

src/compiler/checker.ts

@@ -4905,8 +4905,13 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 }
             }
             else if (resolvedModule.resolvedUsingTsExtension && !shouldAllowImportingTsExtension(compilerOptions, currentSourceFile.fileName)) {
-                const tsExtension = Debug.checkDefined(tryExtractTSExtension(moduleReference));
-                error(errorNode, Diagnostics.An_import_path_can_only_end_with_a_0_extension_when_allowImportingTsExtensions_is_enabled, tsExtension);
+                const importOrExport =
+                    findAncestor(location, isImportDeclaration)?.importClause ||
+                    findAncestor(location, or(isImportEqualsDeclaration, isExportDeclaration));
+                if (!(importOrExport?.isTypeOnly || findAncestor(location, isImportTypeNode))) {
+                    const tsExtension = Debug.checkDefined(tryExtractTSExtension(moduleReference));
+                    error(errorNode, Diagnostics.An_import_path_can_only_end_with_a_0_extension_when_allowImportingTsExtensions_is_enabled, tsExtension);
+                }
             }
 
             if (sourceFile.symbol) {

tests/baselines/reference/allowsImportingTsExtension.errors.txt

@@ -0,0 +1,21 @@
+b.ts(2,16): error TS5097: An import path can only end with a '.ts' extension when 'allowImportingTsExtensions' is enabled.
+b.ts(3,30): error TS5097: An import path can only end with a '.ts' extension when 'allowImportingTsExtensions' is enabled.
+b.ts(5,25): error TS5097: An import path can only end with a '.ts' extension when 'allowImportingTsExtensions' is enabled.
+
+
+==== a.ts (0 errors) ====
+    export class A {}
+
+==== b.ts (3 errors) ====
+    import type { A } from "./a.ts"; // ok
+    import {} from "./a.ts"; // error
+                   ~~~~~~~~
+!!! error TS5097: An import path can only end with a '.ts' extension when 'allowImportingTsExtensions' is enabled.
+    import { type A as _A } from "./a.ts"; // error
+                                 ~~~~~~~~
+!!! error TS5097: An import path can only end with a '.ts' extension when 'allowImportingTsExtensions' is enabled.
+    type __A = import("./a.ts").A; // ok
+    const aPromise = import("./a.ts"); // error
+                            ~~~~~~~~
+!!! error TS5097: An import path can only end with a '.ts' extension when 'allowImportingTsExtensions' is enabled.
+

tests/baselines/reference/allowsImportingTsExtension.js

@@ -0,0 +1,19 @@
+//// [tests/cases/conformance/externalModules/typeOnly/allowsImportingTsExtension.ts] ////
+
+//// [a.ts]
+export class A {}
+
+//// [b.ts]
+import type { A } from "./a.ts"; // ok
+import {} from "./a.ts"; // error
+import { type A as _A } from "./a.ts"; // error
+type __A = import("./a.ts").A; // ok
+const aPromise = import("./a.ts"); // error
+
+
+//// [a.js]
+export class A {
+}
+//// [b.js]
+const aPromise = import("./a.ts"); // error
+export {};

tests/baselines/reference/allowsImportingTsExtension.symbols

@@ -0,0 +1,23 @@
+//// [tests/cases/conformance/externalModules/typeOnly/allowsImportingTsExtension.ts] ////
+
+=== a.ts ===
+export class A {}
+>A : Symbol(A, Decl(a.ts, 0, 0))
+
+=== b.ts ===
+import type { A } from "./a.ts"; // ok
+>A : Symbol(A, Decl(b.ts, 0, 13))
+
+import {} from "./a.ts"; // error
+import { type A as _A } from "./a.ts"; // error
+>A : Symbol(A, Decl(a.ts, 0, 0))
+>_A : Symbol(_A, Decl(b.ts, 2, 8))
+
+type __A = import("./a.ts").A; // ok
+>__A : Symbol(__A, Decl(b.ts, 2, 38))
+>A : Symbol(A, Decl(a.ts, 0, 0))
+
+const aPromise = import("./a.ts"); // error
+>aPromise : Symbol(aPromise, Decl(b.ts, 4, 5))
+>"./a.ts" : Symbol("a", Decl(a.ts, 0, 0))
+

tests/baselines/reference/allowsImportingTsExtension.types

@@ -0,0 +1,23 @@
+//// [tests/cases/conformance/externalModules/typeOnly/allowsImportingTsExtension.ts] ////
+
+=== a.ts ===
+export class A {}
+>A : A
+
+=== b.ts ===
+import type { A } from "./a.ts"; // ok
+>A : A
+
+import {} from "./a.ts"; // error
+import { type A as _A } from "./a.ts"; // error
+>A : typeof A
+>_A : typeof A
+
+type __A = import("./a.ts").A; // ok
+>__A : A
+
+const aPromise = import("./a.ts"); // error
+>aPromise : Promise<typeof import("a")>
+>import("./a.ts") : Promise<typeof import("a")>
+>"./a.ts" : "./a.ts"
+

tests/cases/conformance/externalModules/typeOnly/allowsImportingTsExtension.ts

@@ -0,0 +1,13 @@
+// @allowImportingTsExtensions: false
+// @target: esnext
+// @module: esnext
+
+// @Filename: a.ts
+export class A {}
+
+// @Filename: b.ts
+import type { A } from "./a.ts"; // ok
+import {} from "./a.ts"; // error
+import { type A as _A } from "./a.ts"; // error
+type __A = import("./a.ts").A; // ok
+const aPromise = import("./a.ts"); // error