Fix merge-patch deletion for record values

[Vivek JM (vivekjm)]

Summary

• make transformed Record value types nullable for JSON Merge Patch requests
• preserve the existing recursive visibility transform for model-valued records before adding the null deletion branch
• add regression coverage for scalar and model-valued records in both update modes

Fixes #11533.

Root cause

MergePatchUpdate made the record property itself nullable, allowing an entire map to be cleared, but only transformed the record's value when it was a complex merge-patch subject. A scalar value such as Record<string> therefore remained string, so { "tags": { "a": null } } could not represent deletion of one key.

Testing

• pnpm --filter '@typespec/http...' run build
• pnpm --filter @typespec/http exec vitest run test/merge-patch.test.ts — 38 tests passed
• pnpm --filter @typespec/http run build
• pnpm exec prettier --check packages/http/src/merge-patch.ts packages/http/test/merge-patch.test.ts .chronus/changes/merge-patch-null-record-values-2026-08-16.md
• pnpm --filter @typespec/http run lint
• git diff --check

Local setup note: the first focused test invocation ran before the HTTP package and its optional Streams dependency were built, so package resolution failed before any assertions executed. Building the HTTP dependency closure resolved that bootstrap issue; the focused suite and all checks above then passed.

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[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Enables MergePatchUpdate-transformed Record properties to accept null values so clients can delete individual record keys via merge patch semantics.

Changes:

• Update merge-patch transformation so record indexer values become nullable (T | null).
• Add/adjust tests to assert record element/indexer value types are nullable unions.
• Add Chronus change entry documenting the fix.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
packages/http/src/merge-patch.ts	Makes record indexer values nullable to represent per-key deletion in merge patches.
packages/http/test/merge-patch.test.ts	Adds and updates tests validating nullable record values and related visibility transforms.
.chronus/changes/merge-patch-null-record-values-2026-08-16.md	Documents the behavioral fix for @typespec/http.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

Suppressed comments (2)

packages/http/src/merge-patch.ts:506

• Deduping via types.includes(type) makes nullableRecordValue O(n²) in the number of variants when flattening larger unions. Consider tracking a Set<Type> for membership checks while still building an array for union.create(...) to keep variant collection O(n).

  function nullableRecordValue(realm: Realm, type: Type): Union {
    const types: Type[] = [];
    addType(type);
    addType($(realm).intrinsic.null);
    return $(realm).union.create(types);

    function addType(type: Type): void {
      if ($(realm).union.is(type)) {
        for (const variant of type.variants.values()) {
          addType(variant.type);
        }
      } else if (!types.includes(type)) {
        types.push(type);
      }
    }
  }

packages/http/test/merge-patch.test.ts:1026

• This test checks null-ness via direct reference equality to $(runner.program).intrinsic.null, while other helpers (e.g., checkNullableUnion) detect null intrinsics by kind/name. To avoid brittle failures if null intrinsics aren’t guaranteed to be referentially identical, consider using the same intrinsic check pattern here (e.g., $(program).intrinsic.is(...) and name comparison).

      expect(
        [...recordValue.variants.values()].filter(
          (variant) => variant.type === $(runner.program).intrinsic.null,
        ),
      ).toHaveLength(1);