Weird pattern matching behavior

[CorrM]

Version Used:
Microsoft (R) Build Engine version 17.2.0+41abc5629 for .NET
C:\Program Files\dotnet\sdk\6.0.302\Roslyn\bincore\csc.dll

Steps to Reproduce:
Why this code compiles

public static void MakePropertyValue<T>(T val)
{
    if (val is Span<byte> || val is ReadOnlySpan<byte>)
    {
    }
}

but this code can't compiles!

public static void MakePropertyValue<T>(T val)
{
	if (val is Span<byte> or ReadOnlySpan<byte>)
    {
    }
}

Expected Behavior:
Compile !

Actual Behavior:
Error CS8121 : An expression of type 'T' cannot be handled by a pattern of type 'Span'.
Error CS8121 : An expression of type 'T' cannot be handled by a pattern of type 'ReadOnlySpan'.

[svick]

Your first example compiles, but doesn't work the way you want, and you get warnings about that:

warning CS0184: The given expression is never of the provided ('Span<byte>') type

This is because the is operator doesn't work for ref structs like Span<T>, see #63085.

As for why one is a warning and the other an error, I suspect this is because the is type test is an old feature that works the way it always did, but pattern matching is a new feature, so it can be stricter. Though per #63085 (comment), this might change in the future.

In effect, I think this issue can be closed as a duplicate of #63085.