String concatenation drops Span<char>.ToString() and does not recompile (CS0019)

[sailro]

Summary

ILSpy removes an explicit ToString() call from a Span<char> operand when reconstructing
string concatenation. Unlike ordinary value types, a byref-like type cannot be converted to
object for the built-in string concatenation operator, so the resulting C# does not compile.

Input code

Compiled in Release targeting .NET 8.0 with C# 12:

using System;

public static class C
{
    public static string M(Span<char> span) =>
        "prefix" + span.ToString();
}

Erroneous output

using System;

public static class C
{
    public static string M(Span<char> span)
    {
        return "prefix" + span;
    }
}

Recompiling this output fails:

error CS0019: Operator '+' cannot be applied to operands of type 'string' and 'Span<char>'

Expected output

The explicit call must be retained:

return "prefix" + span.ToString();

Details

• Product in use: ICSharpCode.Decompiler
• Version in use: master 7554c1b0 (11.0.0.9223)
• The repro was verified in an isolated compile, decompile, recompile round trip.
• With string-concatenation reconstruction disabled, ILSpy correctly emits string.Concat("prefix", span.ToString()).
• RemoveRedundantToStringInConcat assumes that the built-in operator can reproduce the removed call, but that is not true for byref-like values.
• I am happy to contribute a fix.