Fix SSG Named Export Transform

packages/next/build/babel/plugins/next-ssg-transform.ts

@@ -27,15 +27,11 @@ export default function nextTransformSsg({
                 return
               }
 
-              if (declaration.type === 'VariableDeclaration') {
+              if (declaration.type !== 'FunctionDeclaration') {
                 return
               }
 
-              const name =
-                declaration.type === 'FunctionDeclaration'
-                  ? declaration.id && declaration.id.name
-                  : null
-
+              const name = declaration.id && declaration.id.name
               if (name == null) {
                 throw new Error(`invariant: null function declaration`)
               }

test/unit/babel-plugin-next-ssg-transform.test.js

@@ -241,5 +241,25 @@ describe('babel plugin (next-ssg-transform)', () => {
         `"const __NEXT_COMP=function Test(){return __jsx(\\"div\\",null);};__NEXT_COMP.__NEXT_SPR=true export default __NEXT_COMP;"`
       )
     })
+
+    it('should not crash for class declarations', () => {
+      const output = babel(trim`
+        function unstable_getStaticPaths() {
+          return []
+        }
+
+        export { unstable_getStaticPaths }
+
+        export class MyClass {}
+
+        export default function Test() {
+          return <div />
+        }
+      `)
+
+      expect(output).toMatchInlineSnapshot(
+        `"export class MyClass{}const __NEXT_COMP=function Test(){return __jsx(\\"div\\",null);};__NEXT_COMP.__NEXT_SPR=true export default __NEXT_COMP;"`
+      )
+    })
   })
 })