Do not print trailing commas in arrow function type parameter lists in ts code blocks

changelog_unreleased/markdown/15286.md

@@ -0,0 +1,19 @@
+#### Do not print trailing commas in arrow function type parameter lists in `ts` code blocks (#15286 by @sosukesuzuki)
+
+<!-- prettier-ignore -->
+````md
+<!-- Input -->
+```ts
+const foo = <T>() => {}
+```
+
+<!-- Prettier stable -->
+```ts
+const foo = <T,>() => {}
+```
+
+<!-- Prettier main -->
+```ts
+const foo = <T>() => {}
+```
+````

src/language-markdown/embed.js

@@ -17,8 +17,13 @@ function embed(path, options) {
           Math.max(3, getMaxContinuousCount(node.value, styleUnit) + 1),
         );
         const newOptions = { parser };
-        if (node.lang === "tsx") {
-          newOptions.filepath = "dummy.tsx";
+
+        // Override the filepath option.
+        // This is because whether the trailing comma of type parameters
+        // should be printed depends on whether it is `*.ts` or `*.tsx`.
+        // https://github.com/prettier/prettier/issues/15282
+        if (node.lang === "ts" || node.lang === "tsx") {
+          newOptions.filepath = `dummy.${node.lang}`;
         }
 
         const doc = await textToDoc(

tests/format/markdown/code/__snapshots__/jsfmt.spec.js.snap

@@ -291,6 +291,25 @@ hello world
 ================================================================================
 `;
 
+exports[`ts-trailing-comma.md - {"proseWrap":"always"} format 1`] = `
+====================================options=====================================
+parsers: ["markdown"]
+printWidth: 80
+proseWrap: "always"
+                                                                                | printWidth
+=====================================input======================================
+\`\`\`ts
+const test = <T,>(value: T) => {};
+\`\`\`
+
+=====================================output=====================================
+\`\`\`ts
+const test = <T>(value: T) => {};
+\`\`\`
+
+================================================================================
+`;
+
 exports[`tsx-trailing-comma.md - {"proseWrap":"always"} format 1`] = `
 ====================================options=====================================
 parsers: ["markdown"]

tests/format/markdown/code/ts-trailing-comma.md

@@ -0,0 +1,3 @@
+```ts
+const test = <T,>(value: T) => {};
+```