Add support for const type parameters

Summary: Add support for `const` type parameters.

Reviewed By: panagosg7

Differential Revision: D67406406

fbshipit-source-id: ee1b0d9238ab1b7cfa3f2d862c3b2115a591e10b

tools/hermes-parser/js/prettier-plugin-hermes-parser/__tests__/const-type-param-test.js

@@ -0,0 +1,103 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict-local
+ * @format
+ */
+
+// $FlowExpectedError[cannot-resolve-module]
+import prettierConfig from '../../.prettierrc.json';
+
+import * as prettier from 'prettier';
+
+function format(code: string) {
+  const options = {
+    ...prettierConfig,
+    parser: 'hermes',
+    requirePragma: false,
+    plugins: [require('../src/index.js')],
+  };
+  return prettier.format(code, options);
+}
+
+describe(`'const' type parameters`, () => {
+  test('type', async () => {
+    expect(
+      format(`
+        type T<const X> = X;
+      `),
+    ).toMatchInlineSnapshot(`
+      "type T<const X> = X;
+      "
+    `);
+  });
+
+  test('function: basic', async () => {
+    expect(
+      format(`
+        function f<const T>(): void {}
+      `),
+    ).toMatchInlineSnapshot(`
+      "function f<const T>(): void {}
+      "
+    `);
+  });
+
+  test('function: with variance', async () => {
+    expect(
+      format(`
+        function f<const +T>(): void {}
+      `),
+    ).toMatchInlineSnapshot(`
+      "function f<const +T>(): void {}
+      "
+    `);
+  });
+
+  test('arrow: basic', async () => {
+    expect(
+      format(`
+        <const T>(x: T) => {}
+      `),
+    ).toMatchInlineSnapshot(`
+      "<const T>(x: T) => {};
+      "
+    `);
+  });
+
+  test('arrow: with variance', async () => {
+    expect(
+      format(`
+        <const +T>(x: T) => {}
+      `),
+    ).toMatchInlineSnapshot(`
+      "<const +T>(x: T) => {};
+      "
+    `);
+  });
+
+  test('class: basic', async () => {
+    expect(
+      format(`
+        class C<const T>{}
+      `),
+    ).toMatchInlineSnapshot(`
+      "class C<const T> {}
+      "
+    `);
+  });
+
+  test('class: with variance', async () => {
+    expect(
+      format(`
+        class C<const T>{}
+      `),
+    ).toMatchInlineSnapshot(`
+      "class C<const T> {}
+      "
+    `);
+  });
+});

tools/hermes-parser/js/prettier-plugin-hermes-parser/src/third-party/internal-prettier-v3/plugins/estree.js

@@ -9033,7 +9033,7 @@ Expected it to be ${EXPECTED_TYPE_VALUES}.`;
   }
   function printTypeParameter(path, options2, print3) {
     const { node, parent } = path;
-    const parts = [node.type === "TSTypeParameter" && node.const ? "const " : ""];
+    const parts = [node.const ? "const " : ""];
     const name = node.type === "TSTypeParameter" ? print3("name") : node.name;
     if (parent.type === "TSMappedType") {
       if (parent.readonly) {