Expression tree vs .NET 9 - compilation error

[fandrei]

Description

The following code compiles with .NET 8, but not with .NET 9.

Expression<Func<string>> expression = () => string.Join("", "");

Reproduction Steps

compile

Expected behavior

no errors

Actual behavior

Compilation errors:

Expression tree cannot contain value of ref struct or restricted type 'ReadOnlySpan'.
An expression tree may not contain an expanded form of non-array params collection parameter.

Regression?

worked in .NET 8

Known Workarounds

No response

Configuration

No response

Other information

No response

[dotnet-policy-service]

Tagging subscribers to this area: @cston
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[cston]

See also discussion of the breaking change in C# LDM-2024-06-17.

[stephentoub]

cc: @jaredpar, @jjonescz, @333fred

[cston]

Logged breaking change issue: dotnet/docs#43949