Fix formatting of empty type parameters

changelog_unreleased/flow/14073.md

@@ -0,0 +1,13 @@
+#### Fix formatting of empty type parameters (#14073 by @fisker)
+
+<!-- prettier-ignore -->
+```jsx
+// Input
+const foo: bar</* comment */> = () => baz;
+
+// Prettier stable
+Error: Comment "comment" was not printed. Please report this error!
+
+// Prettier main
+const foo: bar</* comment */> = () => baz;
+```

src/language-js/print/type-parameters.js

@@ -44,12 +44,12 @@ function printTypeParameters(path, options, print, paramsKey) {
   );
 
   const shouldInline =
-    !isArrowFunctionVariable &&
-    (isParameterInTestCall ||
-      node[paramsKey].length === 0 ||
-      (node[paramsKey].length === 1 &&
-        (node[paramsKey][0].type === "NullableTypeAnnotation" ||
-          shouldHugType(node[paramsKey][0]))));
+    node[paramsKey].length === 0 ||
+    (!isArrowFunctionVariable &&
+      (isParameterInTestCall ||
+        (node[paramsKey].length === 1 &&
+          (node[paramsKey][0].type === "NullableTypeAnnotation" ||
+            shouldHugType(node[paramsKey][0])))));
 
   if (shouldInline) {
     return [

tests/format/typescript/typeparams/empty-parameters-with-arrow-function/__snapshots__/jsfmt.spec.js.snap

@@ -0,0 +1,35 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`issue-13817.ts - {"arrowParens":"avoid","trailingComma":"all"} format 1`] = `
+====================================options=====================================
+arrowParens: "avoid"
+parsers: ["typescript", "flow", "babel-flow"]
+printWidth: 80
+trailingComma: "all"
+                                                                                | printWidth
+=====================================input======================================
+const xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx: xxxxxxxxxxxxxxxxxxxxxx<> =
+  arg => null;
+
+const xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx: xxxxxxxxxxxxxxxxxxxxxx</* comment */> =
+  arg => null;
+
+
+const xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx: xxxxxxxxxxxxxxxxxxxxxx<
+  // comment
+> =
+  arg => null;
+
+=====================================output=====================================
+const xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx: xxxxxxxxxxxxxxxxxxxxxx<> =
+  arg => null;
+
+const xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx: xxxxxxxxxxxxxxxxxxxxxx</* comment */> =
+  arg => null;
+
+const xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx: xxxxxxxxxxxxxxxxxxxxxx<
+  // comment
+> = arg => null;
+
+================================================================================
+`;

tests/format/typescript/typeparams/empty-parameters-with-arrow-function/issue-13817.ts

@@ -0,0 +1,11 @@
+const xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx: xxxxxxxxxxxxxxxxxxxxxx<> =
+  arg => null;
+
+const xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx: xxxxxxxxxxxxxxxxxxxxxx</* comment */> =
+  arg => null;
+
+
+const xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx: xxxxxxxxxxxxxxxxxxxxxx<
+  // comment
+> =
+  arg => null;

tests/format/typescript/typeparams/empty-parameters-with-arrow-function/jsfmt.spec.js

@@ -0,0 +1,6 @@
+run_spec(
+  __dirname,
+  ["typescript", "flow", "babel-flow"],
+  // #13817 require those options to reproduce
+  { arrowParens: "avoid", trailingComma: "all" }
+);