Decompiler emits ?. / ?? over a ref-struct-returning member, producing CS8978 (cannot be made nullable)

[sailro]

[Bug] Decompiler emits ?. / ?? over a ref-struct-returning member, producing CS8978 ("cannot be made nullable")

Component: ICSharpCode.Decompiler
Version: 10.1.0.8386 (NuGet) — also reproduces in current master.

A conditional expression whose non-null branch yields a ref struct (e.g. Span<T> / ReadOnlySpan<T>) with a default fallback is decompiled using null-conditional access (?.) and the null-coalescing operator (??). A ref struct cannot be made nullable, so the decompiled code does not compile.

Steps to reproduce

Compile this to any TFM where Span<T> is available (e.g. net48 + the System.Memory package, or net8.0):

using System;

public class Holder
{
    public byte[] Data;
    public Span<byte> GetSpan() => Data;
}

public class C
{
    public static Span<byte> M(Holder h) => h != null ? h.GetSpan() : default;
}

Decompile the resulting assembly.

Actual output

public static Span<byte> M(Holder h)
{
    return h?.GetSpan() ?? default(Span<byte>);
}

Recompiling this output fails with:

error CS8978: 'Span<byte>' cannot be made nullable.

Expected output

Something a ref struct can express, e.g.:

public static Span<byte> M(Holder h)
{
    return (h != null) ? h.GetSpan() : default(Span<byte>);
}

Root cause

NullPropagationTransform.TryNullPropagation (ICSharpCode.Decompiler/IL/Transforms/NullPropagationTransform.cs) takes the NullableType.IsNonNullableValueType(returnType) branch for Span<byte> — a ref struct has IsReferenceType == false and is not Nullable<T>, so IsNonNullableValueType returns true — and emits the ?. ?? fallback shape. There is no guard for by-ref-like (ref struct) types.

Suggested fix

Guard the value-type branch against ref structs:

else if (!removedRewrapOrNullableCtor
         && NullableType.IsNonNullableValueType(returnType)
         && !returnType.IsByRefLike)   // ref structs cannot be made nullable
{
    ...
}