Formatter adds a blank line and over-indents a `union` used as a template argument

[oha-4]

Describe the bug

When a union is passed directly as a template argument (e.g. PickProperties<T, "a" | "b" | ...>) and the template argument list is long enough to wrap to multiple lines, tsp format inserts a blank line after the preceding argument and over-indents the union variants. The result looks unbalanced/broken compared to the surrounding code.

Note: a short union that fits on one line does not reproduce this — the template arguments must exceed the print width and wrap.

Reproduction

model Sample {
  alpha: string;
  bravo: string;
  charlie: string;
  delta: string;
  echo: string;
  foxtrot: string;
  golf: string;
  hotel: string;
}
model Picked is PickProperties<Sample, "alpha" | "bravo" | "charlie" | "delta" | "echo" | "foxtrot" | "golf" | "hotel">;

Run tsp format on the above.

Actual

model Picked
  is PickProperties<
    Sample,

      | "alpha"
      | "bravo"
      | "charlie"
      | "delta"
      | "echo"
      | "foxtrot"
      | "golf"
      | "hotel"
  >;

Note the empty line after Sample, and the extra indentation applied to the leading-| union variants (they are indented one level deeper than the Sample argument).

Expected

No blank line, and the union variants aligned to the same indentation as the other template arguments, e.g.

model Picked is PickProperties<
  Sample,
  | "alpha"
  | "bravo"
  | "charlie"
  | "delta"
  | "echo"
  | "foxtrot"
  | "golf"
  | "hotel"
>;

(exact wrapping is up to the formatter, but there should be no inserted blank line and no extra indent level for the union)

Checklist

• Reproduced with @typespec/compiler 1.13.0
• The issue appears to come from the combination of template-argument line breaking and the union's leading-| printing in packages/compiler/src/formatter/print/printer.ts.

Workaround

Extracting the union into an alias avoids the broken formatting, since the template argument becomes a single reference:

alias PickedKeys =
  | "alpha"
  | "bravo"
  | "charlie"
  | "delta"
  | "echo"
  | "foxtrot"
  | "golf"
  | "hotel";
model Picked is PickProperties<Sample, PickedKeys>;

[github-actions]

🎯 Issue Triage

tsp format inserts a spurious blank line and applies an extra indent level when a union expression (e.g. "id" | "name" | "status") appears as a template argument. The workaround is extracting the union into an alias.

Category: Bug
Area: compiler:core

🔗 [Playground Reproduction]((typespec.io/redacted)

📋 Reproduction code

model Item {
  id: string;
  name: string;
  status: string;
}
model ItemSummary is PickProperties<Item, "id" | "name" | "status">;

🔍 Analysis

• Affected component: packages/compiler/src/formatter/print/printer.ts — the logic that formats template argument lists combined with the leading-| union printing.
• Root cause (suspected): When a template argument list is broken across lines, the printer appears to introduce a blank line before union-type arguments and applies an extra indent level to the |-prefixed union variants (treating them as if they need additional nesting beyond the template argument level).
• Confirmed version: @typespec/compiler 1.13.0
• Workaround: Extract the union into a named alias; the template argument then becomes a single identifier reference, bypassing the broken formatting path.

📝 Suggested next steps

• Investigate printer.ts logic for how typeUnion / unionVariant nodes are indented when inside a template argument list that has been line-broken.
• Add a formatter snapshot test covering PickProperties<Item, "a" | "b" | "c"> to prevent regressions.

Generated by Agentic Triage

[oha-4]

Playground repro: https://typespec.io/playground/?e=%40typespec%2Fopenapi3&v=file-output&options=%7B%7D&vs=%7B%7D&c=bW9kZWwgU2FtcGxlIHsKICBhbHBoYTogc3RyaW5nOwogIGJyYXZvzBFjaGFybGllzBNkZWx0zTVlY2jNNGZveHRyb3TME2dvbGbMEGhvdGVsyhF9CuYAm1BpY2tlZAogIGlzxQxQcm9wZXJ0aWVzPAogICDnALssCsUNICB8ICLlAMIiyhDlAMHLEOcAwMsS5QC%2FyxDkAL7LD%2BcAvcsS5AC8yw%2FlALvEED47Cg%3D%3D