Fix formatter blank line and over-indent for union template argument (#11010)

Fixes #11009

## Problem

When a `union` expression is used directly as one of multiple template
arguments and the argument list is long enough to wrap, `tsp format`
inserts a blank line before the union and indents its leading-`|`
variants one level deeper than the sibling arguments:

```tsp
model Picked
  is PickProperties<
    Sample,

      | "alpha"
      | "bravo"
      ...
  >;
```

## Fix

`printTemplateParameters` already emits a `softline` + an `indent` level
before each argument in a multi-argument list. `printUnion` was *also*
emitting its own leading `line` + `indent` (+ `align(2)`), so the two
stacked, producing the blank line and the extra indentation level.

`printUnion` now detects when the union is a direct argument of a
multi-argument template reference (`isInMultiTemplateArgumentList`) and,
in that case, lets the surrounding argument list control the line break
and indentation. Result:

```tsp
model Picked
  is PickProperties<
    Sample,
    | "alpha"
    | "bravo"
    ...
  >;
```

The standalone/value-position cases (e.g. `alias X = "a" | "b" | ...`)
and the single-template-argument (`shouldHug`) case are unchanged, since
there the surrounding context does not supply the line break/indent.

## Tests

- Added a regression test for the multi-argument case (#11009) and a
test pinning the single-argument behavior in `formatter.test.ts`.
- Reformatted two committed `http-client-java` test fixtures that
contained the buggy output (whitespace only; no semantic change).
- `pnpm prettier --check "**/*.tsp"`, formatter test suite, tsc, and
eslint all pass.

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Timothee Guerin <tiguerin@microsoft.com>

Author: 3 people

.chronus/changes/fix-formatter-union-template-argument-2026-5-17-0-0-0.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+Fix formatter inserting a blank line and over-indenting a `union` expression used directly as one of multiple template arguments (e.g. `PickProperties<Source, "a" | "b">`)

.chronus/changes/fix-formatter-union-template-argument-java-2026-5-17-0-0-0.md

@@ -0,0 +1,7 @@
+---
+changeKind: internal
+packages:
+  - "@typespec/http-client-java"
+---
+
+Reformat union template arguments in test files to match updated formatter style.

packages/compiler/src/formatter/print/printer.ts

@@ -1427,14 +1427,30 @@ export function printUnion(
   options: TypeSpecPrettierOptions,
   print: PrettierChildPrint,
 ) {
-  const types = path.map((typePath) => {
-    const printedType: Doc = align(2, print(typePath));
-    return printedType;
-  }, "options");
-
-  const shouldAddStartLine = true;
+  // A union that is one of several template arguments must not add its own
+  // leading line + indent: the argument list already provides them, so stacking
+  // both yields a blank line and an extra indent level for the variants.
+  // The per-variant align(2) is always kept though (matching prettier's union
+  // printer): it accounts for the "| " prefix so a variant that breaks (e.g. a
+  // nested template) stays aligned under its content.
+  // https://github.com/microsoft/typespec/issues/11009
+  const inMultiTemplateArgumentList = isInMultiTemplateArgumentList(path);
+  const types = path.map((typePath) => align(2, print(typePath)), "options");
+
+  const shouldAddStartLine = !inMultiTemplateArgumentList;
   const code = [ifBreak([shouldAddStartLine ? line : "", "| "], ""), join([line, "| "], types)];
-  return group(indent(code));
+  return inMultiTemplateArgumentList ? group(code) : group(indent(code));
+}
+
+/** Whether the node is a direct argument of a template reference with more than one argument. */
+function isInMultiTemplateArgumentList(path: AstPath<Node>): boolean {
+  // A `TemplateArgument` only ever lives in `TypeReference.arguments`, so the
+  // owning `TypeReference` is always the next node ancestor.
+  if (path.getParentNode()?.kind !== SyntaxKind.TemplateArgument) {
+    return false;
+  }
+  const reference = path.getParentNode(1);
+  return reference?.kind === SyntaxKind.TypeReference && reference.arguments.length > 1;
 }
 
 export function printTypeReference(

packages/compiler/test/formatter/formatter.test.ts

@@ -1972,6 +1972,68 @@ union Foo {
   @doc("third")
   c: C,
 }
+`,
+      });
+    });
+
+    // Regression test for https://github.com/microsoft/typespec/issues/11009
+    it("does not add a blank line or extra indent for a union used as a template argument", async () => {
+      await assertFormat({
+        code: `
+model Picked is PickProperties<Sample, "alpha" | "bravo" | "charlie" | "delta" | "echo" | "foxtrot" | "golf" | "hotel">;
+`,
+        expected: `
+model Picked is PickProperties<
+  Sample,
+  | "alpha"
+  | "bravo"
+  | "charlie"
+  | "delta"
+  | "echo"
+  | "foxtrot"
+  | "golf"
+  | "hotel"
+>;
+`,
+      });
+    });
+
+    it("keeps leading | and indent for a union used as the only template argument", async () => {
+      await assertFormat({
+        code: `
+model Picked is PickProperties<"alpha" | "bravo" | "charlie" | "delta" | "echo" | "foxtrot" | "golf" | "hotel">;
+`,
+        expected: `
+model Picked
+  is PickProperties<
+    | "alpha"
+    | "bravo"
+    | "charlie"
+    | "delta"
+    | "echo"
+    | "foxtrot"
+    | "golf"
+    | "hotel">;
+`,
+      });
+    });
+
+    // Regression test for https://github.com/microsoft/typespec/issues/11009
+    it("keeps the variant alignment so a nested template argument stays indented", async () => {
+      await assertFormat({
+        code: `
+model Created is Operation<Request, Response | CreatedResponse<ResponseBodyModel, LroHeaders = AsyncOperationHeader<FinalResult = ResponseBodyModel>>, Error>;
+`,
+        expected: `
+model Created is Operation<
+  Request,
+  | Response
+  | CreatedResponse<
+      ResponseBodyModel,
+      LroHeaders = AsyncOperationHeader<FinalResult = ResponseBodyModel>
+    >,
+  Error
+>;
 `,
       });
     });

packages/http-client-java/generator/http-client-generator-test/src/main/resources/META-INF/azure-resourcemanager-armresourceprovider-generated_metadata.json

@@ -389,12 +389,11 @@ interface ImmutableResourceModel {
       Azure.ResourceManager.Foundations.DefaultBaseParameters<NginxConfigurationResponse>
     >,
     NginxConfigurationRequest,
-
-      | NginxConfigurationResponse
-      | ArmResourceCreatedResponse<
-          NginxConfigurationResponse,
-          LroHeaders = ArmAsyncOperationHeader<FinalResult = NginxConfigurationResponse>
-        >,
+    | NginxConfigurationResponse
+    | ArmResourceCreatedResponse<
+        NginxConfigurationResponse,
+        LroHeaders = ArmAsyncOperationHeader<FinalResult = NginxConfigurationResponse>
+      >,
     ErrorResponse,
     OptionalRequestBody = true
   >;

packages/http-client-java/generator/http-client-generator-test/src/main/resources/META-INF/tsptest-response_metadata.json

@@ -26,9 +26,8 @@ op RpcOperationWithAdditionalResponse<
   TErrorResponse = Azure.Core.Foundations.ErrorResponse
 > is Foundations.Operation<
   TParams & Azure.Core.Traits.Private.TraitProperties<Traits, TraitLocation.Parameters>,
-
-    | (TResponse & Azure.Core.Traits.Private.TraitProperties<Traits, TraitLocation.Response>)
-    | TAdditionalResponse,
+  | (TResponse & Azure.Core.Traits.Private.TraitProperties<Traits, TraitLocation.Response>)
+  | TAdditionalResponse,
   Traits,
   TErrorResponse
 >;