GetAlternateLookup<ReadOnlySpan<char>>() does not work on FrozenSet<string> in .NET 9 RC 1

[mfeemster]

Description

I have installed .NET 9 RC 1 and have found that calling GetAlternateLookup<ReadOnlySpan<char>>() on FrozenSet<string> appears to not be supported. It gives an error of:

error CS9244: The type 'ReadOnlySpan' may not be a ref struct or a type parameter allowing ref structs in order to use it as parameter 'TAlternate' in the generic type or method 'FrozenSet.TryGetAlternateLookup(out FrozenSet.AlternateLookup)'

Reproduction Steps

var hs = new HashSet<string>()
{
	"str1",
	"str2",
	"str3",
}.ToFrozenSet(StringComparer.OrdinalIgnoreCase);
var alt = hs.GetAlternateLookup<ReadOnlySpan<char>>();//This line won't compile

Expected behavior

This should compile because the documentation says it's been added to FrozenSet.

https://learn.microsoft.com/en-us/dotnet/api/system.collections.frozen.frozenset-1.getalternatelookup?view=net-9.0

Actual behavior

The build fails with this error:

error CS9244: The type 'ReadOnlySpan' may not be a ref struct or a type parameter allowing ref structs in order to use it as parameter 'TAlternate' in the generic type or method 'FrozenSet.TryGetAlternateLookup(out FrozenSet.AlternateLookup)'

Regression?

No.

Known Workarounds

None.

Configuration

Visual Studio Community 2022 17.11.3
Windows 10
.NET 9 RC 1
<LangVersion>preview</LangVersion>

Other information

No response

[svick]

I believe the problem here is that the reference assembly version of FrozenSet<T> is missing where TAlternate : allows ref struct.

Compare:

runtime/src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/FrozenDictionary.AlternateLookup.cs

Line 24 in 4c10eff

public AlternateLookup<TAlternateKey> GetAlternateLookup<TAlternateKey>() where TAlternateKey : notnull, allows ref struct

runtime/src/libraries/System.Collections.Immutable/ref/System.Collections.Immutable.net9.cs

Line 11 in 4c10eff

public System.Collections.Frozen.FrozenDictionary<TKey,TValue>.AlternateLookup<TAlternateKey> GetAlternateLookup<TAlternateKey>() where TAlternateKey : notnull, allows ref struct { throw null; }

runtime/src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/FrozenSet.AlternateLookup.cs

Line 24 in 4c10eff

public AlternateLookup<TAlternate> GetAlternateLookup<TAlternate>() where TAlternate : allows ref struct

runtime/src/libraries/System.Collections.Immutable/ref/System.Collections.Immutable.net9.cs

Line 25 in 4c10eff

public System.Collections.Frozen.FrozenSet<T>.AlternateLookup<TAlternate> GetAlternateLookup<TAlternate>() { throw null; }

[mfeemster]

This sounds correct. Do you know of a workaround? Or can it only be fixed when the next RC is released?

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[stephentoub]

Thank you for reporting this.

Do you know of a workaround? Or can it only be fixed when the next RC is released?

A hack would be to reference the implementation assembly rather than the ref assembly.

The only real fix is to fix the ref assembly, though.

#107846
#107847

[jeffhandley]

Fixed by #107846