Fix nullable analysis for reassignment to a union value

[AlekseyTs]

Fixes #85852

Microsoft Reviewers: Open in CodeFlow

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[Copilot]

Copilot review overview

🔵 Needs a closer look

The change affects core nullable flow analysis, and CI is still running.

Review effort: Balanced
Findings: None

What changed in this PR

Fixes union nullability state restoration after reassignment.

Changes:

• Applies union-specific Value defaults during member-state inheritance.
• Adds regression coverage for standard and member-provider unions.

File	Description
NullableWalker.cs	Corrects reassignment flow state.
UnionsTests.cs	Covers reported and related scenarios.

[AlekseyTs]

@RikkiGibson, @jjonescz, @dotnet/roslyn-compiler Please review

[333fred]

Can we add some versions of this where null is an explicit case in the switch, even though it's still Result<string, int> without string??

[AlekseyTs]

I guess we can.

[RikkiGibson]

I am wondering if this adjustment should be moved even "deeper" into the stack. For example, within GetTypeOrReturnTypeWithAnnotations(), or ApplyUnconditionalAnnotations(), or a new helper which is called from similar places as those methods.

The reason I bring this up is, it looks like there are still places where the flow state of the Value doesn't match what we expect. For example, in the following test, a warning is reported in M1().Value.ToString(), but not on u.Value.ToString().

[Fact]
public void M()
{
    var source = """
        #nullable enable
        class Program
        {
            U M1() => 42;

            void M2()
            {
                M1().Value.ToString();
            }

            void M3()
            {
                U u = M1();
                u.Value.ToString();
            }
        }

        union U(string, int);
        """;

    CreateCompilation([source, UnionAttributeSource, IUnionSource]).VerifyEmitDiagnostics();
}

[RikkiGibson]

Done review pass