Better conversion from collection expression

• Proposed
• Prototype: Not Started
• Implementation: Not Started
• Specification: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-13.0/collection-expressions-better-conversion.md

Summary

Updates to the better conversion rules to be more consistent with params, and better handle current ambiguity scenarios. For example, ReadOnlySpan<string> vs ReadOnlySpan<object> can currently cause ambiguities during overload resolution for [""]. This is an issue because .NET 9 is adding such methods to string.Concat, which we need to address in the language.

Design meetings

• https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-08-19.md#better-conversion-from-collection-expression
• https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-08-21.md#better-conversion-from-collection-expression
• https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-09-11.md#better-conversion-from-collection-expression-and-params-collections