Copy type parameters to function with @type tag

src/compiler/checker.ts

@@ -12743,12 +12743,14 @@ namespace ts {
 
         // Return list of type parameters with duplicates removed (duplicate identifier errors are generated in the actual
         // type checking functions).
-        function getTypeParametersFromDeclaration(declaration: DeclarationWithTypeParameters): TypeParameter[] | undefined {
+        function getTypeParametersFromDeclaration(declaration: DeclarationWithTypeParameters): readonly TypeParameter[] | undefined {
             let result: TypeParameter[] | undefined;
             for (const node of getEffectiveTypeParameterDeclarations(declaration)) {
                 result = appendIfUnique(result, getDeclaredTypeOfTypeParameter(node.symbol));
             }
-            return result;
+            return result?.length ? result
+                : isFunctionDeclaration(declaration) ? getSignatureOfTypeTag(declaration)?.typeParameters
+                : undefined;
         }
 
         function symbolsToArray(symbols: SymbolTable): Symbol[] {

src/compiler/utilitiesPublic.ts

@@ -932,6 +932,12 @@ namespace ts {
     /**
      * Gets the effective type parameters. If the node was parsed in a
      * JavaScript file, gets the type parameters from the `@template` tag from JSDoc.
+     *
+     * This does *not* return type parameters from a jsdoc reference to a generic type, eg
+     *
+     * type Id = <T>(x: T) => T
+     * /** @type {Id} /
+     * function id(x) { return x }
      */
     export function getEffectiveTypeParameterDeclarations(node: DeclarationWithTypeParameters): readonly TypeParameterDeclaration[] {
         if (isJSDocSignature(node)) {

tests/baselines/reference/api/tsserverlibrary.d.ts

@@ -4464,6 +4464,12 @@ declare namespace ts {
     /**
      * Gets the effective type parameters. If the node was parsed in a
      * JavaScript file, gets the type parameters from the `@template` tag from JSDoc.
+     *
+     * This does *not* return type parameters from a jsdoc reference to a generic type, eg
+     *
+     * type Id = <T>(x: T) => T
+     * /** @type {Id} /
+     * function id(x) { return x }
      */
     function getEffectiveTypeParameterDeclarations(node: DeclarationWithTypeParameters): readonly TypeParameterDeclaration[];
     function getEffectiveConstraintOfTypeParameter(node: TypeParameterDeclaration): TypeNode | undefined;

tests/baselines/reference/api/typescript.d.ts

@@ -4464,6 +4464,12 @@ declare namespace ts {
     /**
      * Gets the effective type parameters. If the node was parsed in a
      * JavaScript file, gets the type parameters from the `@template` tag from JSDoc.
+     *
+     * This does *not* return type parameters from a jsdoc reference to a generic type, eg
+     *
+     * type Id = <T>(x: T) => T
+     * /** @type {Id} /
+     * function id(x) { return x }
      */
     function getEffectiveTypeParameterDeclarations(node: DeclarationWithTypeParameters): readonly TypeParameterDeclaration[];
     function getEffectiveConstraintOfTypeParameter(node: TypeParameterDeclaration): TypeNode | undefined;

tests/baselines/reference/typeTagOnFunctionReferencesGeneric.js

@@ -0,0 +1,45 @@
+//// [typeTagOnFunctionReferencesGeneric.js]
+/**
+ * @typedef {<T>(m : T) => T} IFn
+ */
+
+/**@type {IFn}*/
+export function inJs(l) {
+    return l;
+}
+inJs(1); // lints error. Why?
+
+/**@type {IFn}*/
+const inJsArrow = (j) => {
+    return j;
+}
+inJsArrow(2); // no error gets linted as expected
+
+
+//// [typeTagOnFunctionReferencesGeneric.js]
+"use strict";
+/**
+ * @typedef {<T>(m : T) => T} IFn
+ */
+exports.__esModule = true;
+exports.inJs = void 0;
+/**@type {IFn}*/
+function inJs(l) {
+    return l;
+}
+exports.inJs = inJs;
+inJs(1); // lints error. Why?
+/**@type {IFn}*/
+var inJsArrow = function (j) {
+    return j;
+};
+inJsArrow(2); // no error gets linted as expected
+
+
+//// [typeTagOnFunctionReferencesGeneric.d.ts]
+/**
+ * @typedef {<T>(m : T) => T} IFn
+ */
+/**@type {IFn}*/
+export function inJs<T>(l: T): T;
+export type IFn = <T>(m: T) => T;

tests/baselines/reference/typeTagOnFunctionReferencesGeneric.symbols

@@ -0,0 +1,27 @@
+=== tests/cases/conformance/salsa/typeTagOnFunctionReferencesGeneric.js ===
+/**
+ * @typedef {<T>(m : T) => T} IFn
+ */
+
+/**@type {IFn}*/
+export function inJs(l) {
+>inJs : Symbol(inJs, Decl(typeTagOnFunctionReferencesGeneric.js, 0, 0))
+>l : Symbol(l, Decl(typeTagOnFunctionReferencesGeneric.js, 5, 21))
+
+    return l;
+>l : Symbol(l, Decl(typeTagOnFunctionReferencesGeneric.js, 5, 21))
+}
+inJs(1); // lints error. Why?
+>inJs : Symbol(inJs, Decl(typeTagOnFunctionReferencesGeneric.js, 0, 0))
+
+/**@type {IFn}*/
+const inJsArrow = (j) => {
+>inJsArrow : Symbol(inJsArrow, Decl(typeTagOnFunctionReferencesGeneric.js, 11, 5))
+>j : Symbol(j, Decl(typeTagOnFunctionReferencesGeneric.js, 11, 19))
+
+    return j;
+>j : Symbol(j, Decl(typeTagOnFunctionReferencesGeneric.js, 11, 19))
+}
+inJsArrow(2); // no error gets linted as expected
+>inJsArrow : Symbol(inJsArrow, Decl(typeTagOnFunctionReferencesGeneric.js, 11, 5))
+

tests/baselines/reference/typeTagOnFunctionReferencesGeneric.types

@@ -0,0 +1,32 @@
+=== tests/cases/conformance/salsa/typeTagOnFunctionReferencesGeneric.js ===
+/**
+ * @typedef {<T>(m : T) => T} IFn
+ */
+
+/**@type {IFn}*/
+export function inJs(l) {
+>inJs : <T>(l: T) => T
+>l : T
+
+    return l;
+>l : T
+}
+inJs(1); // lints error. Why?
+>inJs(1) : 1
+>inJs : <T>(l: T) => T
+>1 : 1
+
+/**@type {IFn}*/
+const inJsArrow = (j) => {
+>inJsArrow : IFn
+>(j) => {    return j;} : <T>(j: T) => T
+>j : T
+
+    return j;
+>j : T
+}
+inJsArrow(2); // no error gets linted as expected
+>inJsArrow(2) : 2
+>inJsArrow : IFn
+>2 : 2
+

tests/cases/conformance/salsa/typeTagOnFunctionReferencesGeneric.ts

@@ -0,0 +1,19 @@
+// @checkJs: true
+// @declaration: true
+// @outDir: out/
+// @filename: typeTagOnFunctionReferencesGeneric.js
+/**
+ * @typedef {<T>(m : T) => T} IFn
+ */
+
+/**@type {IFn}*/
+export function inJs(l) {
+    return l;
+}
+inJs(1); // lints error. Why?
+
+/**@type {IFn}*/
+const inJsArrow = (j) => {
+    return j;
+}
+inJsArrow(2); // no error gets linted as expected