Document that public Microsoft.Build.Tasks.CallTarget is not the live implementation

[jankratochvilcz]

Per @rainersigwald's comment on #13616: the public Microsoft.Build.Tasks.CallTarget class is not the implementation that actually runs at build time — the MSBuild engine resolves <CallTarget> to the intrinsic implementation in Microsoft.Build.BackEnd.IntrinsicTasks.CallTarget (which is already enlightened for multithreaded mode). This type is retained only in case some third-party task derives from it.

This PR adds a <remarks> note to that effect on the public class so future readers/contributors don't try to enlighten or otherwise modify this dead-code path.

No behavioral change.

[Copilot]

Pull request overview

Adds documentation clarifying that the public Microsoft.Build.Tasks.CallTarget type is retained for compatibility and is not the intrinsic implementation used when MSBuild executes the <CallTarget> element.

Changes:

• Adds a <remarks> note to Microsoft.Build.Tasks.CallTarget explaining that MSBuild uses an intrinsic CallTarget implementation at build time.
• Documents that the public type exists primarily for backward compatibility (e.g., third-party derivation).

[Copilot]

The remarks reference Microsoft.Build.BackEnd.IntrinsicTasks.CallTarget, but the intrinsic implementation currently lives in src/Build/BackEnd/Components/RequestBuilder/IntrinsicTasks/CallTarget.cs under namespace Microsoft.Build.BackEnd (type Microsoft.Build.BackEnd.CallTarget). As written, the doc points readers to a non-existent type/namespace; please update the fully qualified type name (and consider using <c>...</c> or a <see cref> if it won’t break doc generation).

Suggested change

	/// Microsoft.Build.BackEnd.IntrinsicTasks.CallTarget. This type is retained only for backwards
	/// <c>Microsoft.Build.BackEnd.CallTarget</c>. This type is retained only for backwards

[github-actions]

Review Summary

Documentation-only change — no behavioral or breaking-change concerns. The intent is correct and valuable: future contributors should know that Microsoft.Build.Tasks.CallTarget is dead code retained only for third-party derivation.

Issue found

Incorrect namespace in the comment. The comment references Microsoft.Build.BackEnd.IntrinsicTasks.CallTarget, but the actual fully-qualified name of the intrinsic implementation is Microsoft.Build.BackEnd.CallTarget. "IntrinsicTasks" is the folder path, not the namespace. See inline comment for details and a suggested fix.

Verification

I confirmed the claim is otherwise accurate by inspecting:

• TaskExecutionHost.cs:971 — the engine explicitly redirects Microsoft.Build.Tasks.CallTarget → the intrinsic CallTarget type via IntrinsicTaskFactory.
• src/Build/BackEnd/Components/RequestBuilder/IntrinsicTasks/CallTarget.cs:13 — namespace is Microsoft.Build.BackEnd, and the file already has a comment noting it replaces the Tasks version.

Generated by Expert Code Review (on open) for issue #13670 · ● 1.4M

[jankratochvilcz]

Both reviewers correctly caught that the namespace I gave was wrong — the intrinsic implementation lives in Microsoft.Build.BackEnd (the IntrinsicTasks segment is the folder path, not the namespace). Fixed in 2ff1470 by referencing Microsoft.Build.BackEnd.CallTarget and including the source path for navigability.

I didn't go with <see cref="BackEnd.CallTarget"/> because the intrinsic type is internal to Microsoft.Build and the docs live in Microsoft.Build.Tasks, so the cref wouldn't resolve.