CS9361 not emitted for stackalloc nested in ternary/parens/cast under [SkipLocalsInit]

[EgorBo]

unsafe-v2

CS9361 (stackalloc without initializer inside SkipLocalsInit requires unsafe) only fires when the stackalloc is the direct RHS of the local initializer. Wrapping it in parentheses, a conditional, or a cast silences the diagnostic, even though the underlying behavior is identical.

Repro

[module: System.Runtime.CompilerServices.SkipLocalsInit]

public class C
{
    public void Direct()         { System.Span<char> s = stackalloc char[128]; }                        // CS9361 ✅
    public void Parens()         { System.Span<char> s = (stackalloc char[128]); }                      // no diagnostic ❌
    public void Ternary(bool b)  { System.Span<char> s = b ? stackalloc char[128] : default; }          // no diagnostic ❌
    public void Cast()           { System.Span<char> s = (System.Span<char>)stackalloc char[128]; }     // no diagnostic ❌
    public void Call()           { Consume(stackalloc char[128]); }    // no diagnostic ❌
    void Consume(System.Span<char> span){}
}

Expected

CS9361 should fire on every StackAllocArrayCreationExpressionSyntax whose result is consumed as Span<T> / ReadOnlySpan<T> under [SkipLocalsInit], regardless of intermediate conversions.

Version

Reproduced on Microsoft.CodeAnalysis.CSharp 5.8.0-1.26266.103 (dotnet/runtime live build).

Note

This issue was drafted with the help of GitHub Copilot.

[EgorBo]

lab.razor.fyi