packages/compiler-sfc/src/compileScript.ts (1)

177-183: Normalize “no lang” to JS to avoid surprising breakages

As written, <script lang="js"> + <script setup> (no lang) will now be considered mismatched, even though “no lang” effectively means JS. That could be a subtle breaking change for users who explicitly set lang="js" on only one block.

Consider normalizing missing lang to 'js' for the comparison. This preserves the newly added behavior for real mismatches like ts vs tsx (and js vs jsx) while avoiding a strict mismatch for js vs implicit js.

Proposed minimal change:

-  if (script && scriptSetup && scriptLang !== scriptSetupLang) {
-    throw new Error(
-      `[@vue/compiler-sfc] <script> and <script setup> must have the same ` +
-        `language type.`,
-    )
-  }
+  const normalizeLang = (l?: string) => (l == null ? 'js' : l)
+  if (
+    script &&
+    scriptSetup &&
+    normalizeLang(scriptLang) !== normalizeLang(scriptSetupLang)
+  ) {
+    throw new Error(
+      `[@vue/compiler-sfc] <script> and <script setup> must have the same ` +
+        `language type.`,
+    )
+  }

Optional: enhance diagnostics by including the detected types:

-    throw new Error(
-      `[@vue/compiler-sfc] <script> and <script setup> must have the same ` +
-        `language type.`,
-    )
+    const a = normalizeLang(scriptLang)
+    const b = normalizeLang(scriptSetupLang)
+    throw new Error(
+      `[@vue/compiler-sfc] <script> and <script setup> must have the same language type. ` +
+      `Found "${a}" and "${b}".`
+    )

packages/compiler-sfc/__tests__/compileScript.spec.ts (1)

916-923: Add a couple more mismatch assertions to harden coverage

The new test covering ts vs tsx is great and directly proves the regression is fixed before Babel. Consider adding:

• js vs jsx mismatch
• Explicit lang="js" vs implicit JS (no lang) — only if you keep strict inequality. If you adopt normalization (treat missing as js), assert that this passes instead.

Here are two illustrative cases you can add under the same “errors” suite:

+  expect(() =>
+    compile(
+      `<script lang="js">foo()</script><script setup lang="jsx">bar()</script>`,
+    ),
+  ).toThrow(`<script> and <script setup> must have the same language type`)

If you adopt normalization (missing => js), then also assert no-throw for explicit js vs implicit js:

+  expect(() =>
+    compile(
+      `<script lang="js">foo()</script><script setup>bar()</script>`,
+    ),
+  ).not.toThrow()

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