Decompiling a ReadOnlySpan<T> built from an array literal references the undeclared <PrivateImplementationDetails> cache (does not recompile)

[sailro]

Summary

Decompiling a ReadOnlySpan<T> that is created from an array literal on a target framework without RuntimeHelpers.CreateSpan (e.g. .NET Framework, or netstandard2.0 + the System.Memory package) produces code that does not recompile: it references the compiler-synthesized <PrivateImplementationDetails> cache field, which is never declared and whose angle-bracket name is not expressible in C#.

Input code

Compiled in Release targeting .NET Framework 4.6.2 with the System.Memory package:

using System;

public static class SpanCache
{
    public static ReadOnlySpan<char> NewLine => new char[] { '\r', '\n' };
}

On such a framework Roslyn caches the backing char[] in a lazy <PrivateImplementationDetails> static field (there is no RuntimeHelpers.CreateSpan to blit the data at run time).

Erroneous output

public static ReadOnlySpan<char> NewLine
{
    get
    {
        object obj = <PrivateImplementationDetails>.B7F5...7075_A1;
        if (obj == null)
        {
            obj = new char[2] { '\r', '\n' };
            <PrivateImplementationDetails>.B7F5...7075_A1 = (char[])obj;
        }
        return new ReadOnlySpan<char>((char[])obj);
    }
}

The synthesized <PrivateImplementationDetails> type is referenced but never declared, and its name is not expressible in C#. In whole-project output the name is escaped to _003CPrivateImplementationDetails_003E, which then fails to compile with CS0400 (the type is undeclared).

Expected output

The array literal should be recovered, so the <PrivateImplementationDetails> reference disappears — the same result the RuntimeHelpers.CreateSpan path (net7+) already produces:

public static ReadOnlySpan<char> NewLine => new ReadOnlySpan<char>(new char[2] { '\r', '\n' });

Details

• Product in use: ICSharpCode.Decompiler
• Version in use: master fe50c00f (11.0.0.9086); also reproduces on 10.1 (10.1.0.8386).
• Real-world occurrence: Markdig CodeInlineParser (net462).
• Root cause: ILSpy recovers the modern RuntimeHelpers.CreateSpan form (TransformArrayInitializers.TransformRuntimeHelpersCreateSpanInitialization) but not the legacy lazy-char[]-cache form, so the raw <PrivateImplementationDetails> field reference is left behind. This lazy cache has the same shape as the anonymous-method delegate cache already collapsed by CachedDelegateInitialization.
• I'm happy to contribute a fix.