Note regarding thenew-api-needs-documentation label:

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We won't be able to merge this until the C# compiler is updated to guard overload resolution with params span on LangVersion.

/azp run

Addedneeds-breaking-change-doc-created label because this PR has thebreaking-change label.

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Tagging @dotnet/compat for awareness of the breaking change.

@jozkee, why did you mark this as breaking? What is breaking about it?

source breaking?

Can you elaborate?

There's a case where a collection expression can't resolve betweenROS<string> andROS<object>, seedotnet/sdk#41089 (comment). It may be a compiler bug, though.

Additionally, do we not want to document cases of types with conditions similar to StringValues?

There's a case where a collection expression can't resolve betweenROS<string> andROS<object>, seedotnet/sdk#41089 (comment). It may be a compiler bug, though.

Currently, thebetter conversion rules for collection expressions treat this case as ambiguous.

Perhaps there should be an additional rule to preferSystem.ReadOnlySpan<E₁> overSystem.ReadOnlySpan<E₂> if an implicit conversion exists fromE₁ toE₂.

There's a case where a collection expression can't resolve betweenROS<string> andROS<object>, seedotnet/sdk#41089 (comment). It may be a compiler bug, though.

Currently, thebetter conversion rules for collection expressions treat this case as ambiguous.

Perhaps there should be an additional rule to preferSystem.ReadOnlySpan<E₁> overSystem.ReadOnlySpan<E₂> if an implicit conversion exists fromE₁ toE₂.

I thought that was part of the first-class span support. Is it not,@333fred?

Perhaps there should be an additional rule to preferSystem.ReadOnlySpan<E₁> overSystem.ReadOnlySpan<E₂> if an implicit conversion exists fromE₁ toE₂.

I thought that was part of the first-class span support. Is it not,@333fred?

Unfortunately, implicit conversions from first-class span support may not be sufficient for a collection expression argument since implicit conversions are only considered inbetter conversion when the types arenot span types.

• E is acollection expression and one of the following holds:
  • ...
  • T₁ is not aspan_type, andT₂ is not aspan_type, and an implicit conversion exists fromT₁ toT₂

Currently, thebetter conversion rules for collection expressions treat this case as ambiguous.

This is tracked bydotnet/roslyn#73857.

Note for breaking change doc: If this ship as is, it should be pointed out thatstring.Trim*(params ROS<char>) will hideextension methods which usually match the whole sequence while the one added here matches any of the characters.

See#102822 (comment).