Fix debug assert in Uri.CreateThisFromUri when combining UNC base with "file:" relative URI

[Copilot]

Uri(Uri, Uri) triggers a Debug.Assert failure in CreateThisFromUri when combining a UNC file URI base with a "file:" relative URI:

var baseUri = new Uri(@"\\aa", UriKind.Absolute); // file://aa/
var relUri  = new Uri("file:", UriKind.Relative);
_ = new Uri(baseUri, relUri);                     // Debug.Assert failure

Description

Root cause: CreateThisFromUri copies all flags from otherUri verbatim, including Debug_LeftConstructor. In Uri(Uri, Uri), CreateThisFromUri is called twice — first to initialize from relUri, and again after ResolveHelper returns a resolved URI. On the second call, this._flags already carries Debug_LeftConstructor (copied from the first call), so DebugAssertInCtor() fires.

Fix: Strip Debug_LeftConstructor from the copied flags in CreateThisFromUri:

// Before
_flags = otherUri._flags;

// After
_flags = otherUri._flags & ~Flags.Debug_LeftConstructor;

Debug_LeftConstructor is per-instance state — it must only be set via DebugSetLeftCtor() when the current constructor finishes, never inherited from another URI.

A regression test covering the UNC + "file:" relative combination is added to UncTest.cs.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Harmless debug assert hit in Uri's DebugAssertInCtor</issue_title>
<issue_description>```c#
// Uri.CreateThisFromUri fires Debug.Assert: (_flags & Flags.Debug_LeftConstructor) == 0
// when combining a UNC file URI base with a "file:" relative URI.
//
// Source: System.Uri.DebugAssertInCtor() at Uri.cs:205
// System.Uri.CreateThisFromUri() at UriExt.cs:891

var baseUri = new Uri(@"\aa", UriKind.Absolute); // parses as file://aa/
var relUri = new Uri("file:", UriKind.Relative);
_ = new Uri(baseUri, relUri); // Debug.Assert failure

The assert violation is in CreateThisFromUri — it expects the constructor flag Debug_LeftConstructor to not be set, but the re-entrant URI construction from a UNC base + file: relative violates this invariant.

This is from the extra `DebugAssertInCtor` I added to `DebugSetLeftCtor` in dotnet/runtime#123932. We should either remove the assert or avoid copying the Debug_LeftConstructor flag in CreateThisFromUri.</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Harmless debug assert hit in Uri's DebugAssertInCtor #124659

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[Copilot]

Pull request overview

This PR fixes a debug assertion failure in Uri.CreateThisFromUri when combining a UNC file URI base with a "file:" relative URI. The root cause was that CreateThisFromUri was copying all flags verbatim from the source URI, including the Debug_LeftConstructor flag, which is per-instance state that should only be set when the current constructor finishes.

Changes:

• Strip Debug_LeftConstructor flag when copying flags in CreateThisFromUri to prevent re-entrant constructor calls from triggering false assertions
• Add regression test covering the UNC + "file:" relative URI combination scenario

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Private.Uri/src/System/UriExt.cs	Strip Debug_LeftConstructor flag when copying flags from source URI in CreateThisFromUri method
src/libraries/System.Private.Uri/tests/FunctionalTests/UncTest.cs	Add regression test verifying that combining UNC base URI with "file:" relative URI does not trigger debug assertion