TypeScript: Fixed to break line and add a semi in one execution on one line long mapped types (#6420)

CHANGELOG.unreleased.md

@@ -453,6 +453,26 @@ const foo = [abc, def, ghi, jkl, mno, pqr, stu, vwx, yz] as (
 )[];
 ```
 
+#### TypeScript: Fixed to break line and add a semicolon in one execution on one line long mapped types ([#6420] by [@sosukesuzuki])
+
+Previously, when Prettier formatted long, one-line mapped types, it would break the line but didn’t add a semicolon – until you ran Prettier again (which broke Prettier’s idempotency rule). Now, Prettier adds the semicolon in the first run, fixing the issue.
+
+<!-- prettier-ignore -->
+```ts
+// Input
+type FooBar<T> = { [P in keyof T]: T[P] extends Something ? Something<T[P]> : T[P] }
+
+// Prettier (stable)
+type FooBar<T> = {
+  [P in keyof T]: T[P] extends Something ? Something<T[P]> : T[P]
+};
+
+// Prettier (master)
+type FooBar<T> = {
+  [P in keyof T]: T[P] extends Something ? Something<T[P]> : T[P];
+};
+```
+
 [#5910]: https://github.com/prettier/prettier/pull/5910
 [#6186]: https://github.com/prettier/prettier/pull/6186
 [#6206]: https://github.com/prettier/prettier/pull/6206
@@ -468,6 +488,7 @@ const foo = [abc, def, ghi, jkl, mno, pqr, stu, vwx, yz] as (
 [#6307]: https://github.com/prettier/prettier/pull/6307
 [#6340]: https://github.com/prettier/prettier/pull/6340
 [#6412]: https://github.com/prettier/prettier/pull/6412
+[#6420]: https://github.com/prettier/prettier/pull/6420
 [@duailibe]: https://github.com/duailibe
 [@gavinjoyce]: https://github.com/gavinjoyce
 [@sosukesuzuki]: https://github.com/sosukesuzuki

src/language-js/printer-estree.js

@@ -3260,7 +3260,7 @@ function printPathNoParens(path, options, print, args) {
                 : "",
               ": ",
               path.call(print, "typeAnnotation"),
-              shouldBreak && options.semi ? ";" : ""
+              ifBreak(semi, "")
             ])
           ),
           comments.printDanglingComments(path, options, /* sameIndent */ true),

tests/typescript_conditional_types/__snapshots__/jsfmt.spec.js.snap

@@ -32,7 +32,7 @@ export type DeepReadonly<T> = T extends any[]
   : T;
 
 type NonFunctionPropertyNames<T> = {
-  [K in keyof T]: T[K] extends Function ? never : K
+  [K in keyof T]: T[K] extends Function ? never : K;
 }[keyof T];
 
 interface DeepReadonlyArray<T> extends ReadonlyArray<DeepReadonly<T>> {}