Do not erase signatures in getSignatureOfTypeTag

src/compiler/checker.ts

@@ -12182,8 +12182,7 @@ namespace ts {
             // should be attached to a function declaration or expression
             if (!(isInJSFile(node) && isFunctionLikeDeclaration(node))) return undefined;
             const typeTag = getJSDocTypeTag(node);
-            const signature = typeTag && typeTag.typeExpression && getSingleCallSignature(getTypeFromTypeNode(typeTag.typeExpression));
-            return signature && getErasedSignature(signature);
+            return typeTag?.typeExpression && getSingleCallSignature(getTypeFromTypeNode(typeTag.typeExpression));
         }
 
         function getReturnTypeOfTypeTag(node: SignatureDeclaration | JSDocSignature) {

tests/baselines/reference/typeTagNoErasure.errors.txt

@@ -0,0 +1,14 @@
+tests/cases/conformance/jsdoc/typeTagNoErasure.js(7,6): error TS2345: Argument of type 'string' is not assignable to parameter of type 'number'.
+
+
+==== tests/cases/conformance/jsdoc/typeTagNoErasure.js (1 errors) ====
+    /** @template T @typedef {<T1 extends T>(data: T1) => T1} Test */
+
+    /** @type {Test<number>} */
+    const test = dibbity => dibbity
+
+    test(1) // ok, T=1
+    test('hi') // error, T=number
+         ~~~~
+!!! error TS2345: Argument of type 'string' is not assignable to parameter of type 'number'.
+

tests/baselines/reference/typeTagNoErasure.js

@@ -0,0 +1,23 @@
+//// [typeTagNoErasure.js]
+/** @template T @typedef {<T1 extends T>(data: T1) => T1} Test */
+
+/** @type {Test<number>} */
+const test = dibbity => dibbity
+
+test(1) // ok, T=1
+test('hi') // error, T=number
+
+
+//// [typeTagNoErasure.js]
+/** @template T @typedef {<T1 extends T>(data: T1) => T1} Test */
+/** @type {Test<number>} */
+var test = function (dibbity) { return dibbity; };
+test(1); // ok, T=1
+test('hi'); // error, T=number
+
+
+//// [typeTagNoErasure.d.ts]
+/** @template T @typedef {<T1 extends T>(data: T1) => T1} Test */
+/** @type {Test<number>} */
+declare const test: Test<number>;
+type Test<T> = <T1 extends T>(data: T1) => T1;

tests/baselines/reference/typeTagNoErasure.symbols

@@ -0,0 +1,15 @@
+=== tests/cases/conformance/jsdoc/typeTagNoErasure.js ===
+/** @template T @typedef {<T1 extends T>(data: T1) => T1} Test */
+
+/** @type {Test<number>} */
+const test = dibbity => dibbity
+>test : Symbol(test, Decl(typeTagNoErasure.js, 3, 5))
+>dibbity : Symbol(dibbity, Decl(typeTagNoErasure.js, 3, 12))
+>dibbity : Symbol(dibbity, Decl(typeTagNoErasure.js, 3, 12))
+
+test(1) // ok, T=1
+>test : Symbol(test, Decl(typeTagNoErasure.js, 3, 5))
+
+test('hi') // error, T=number
+>test : Symbol(test, Decl(typeTagNoErasure.js, 3, 5))
+

tests/baselines/reference/typeTagNoErasure.types

@@ -0,0 +1,20 @@
+=== tests/cases/conformance/jsdoc/typeTagNoErasure.js ===
+/** @template T @typedef {<T1 extends T>(data: T1) => T1} Test */
+
+/** @type {Test<number>} */
+const test = dibbity => dibbity
+>test : Test<number>
+>dibbity => dibbity : <T1 extends number>(dibbity: T1) => T1
+>dibbity : T1
+>dibbity : T1
+
+test(1) // ok, T=1
+>test(1) : 1
+>test : Test<number>
+>1 : 1
+
+test('hi') // error, T=number
+>test('hi') : number
+>test : Test<number>
+>'hi' : "hi"
+

tests/cases/conformance/jsdoc/typeTagNoErasure.ts

@@ -0,0 +1,11 @@
+// @checkJs:true
+// @declaration: true
+// @outdir: out/
+// @filename: typeTagNoErasure.js
+/** @template T @typedef {<T1 extends T>(data: T1) => T1} Test */
+
+/** @type {Test<number>} */
+const test = dibbity => dibbity
+
+test(1) // ok, T=1
+test('hi') // error, T=number