Tracking F# language feature requests and improvements

[vzarytovskii]

Note
This is a tracking issue for all language feature requests and improvements which are "proposed priority" in the language design.

https://github.com/fsharp/fslang-suggestions/labels/proposed-priority

Please, note that some of the issues are in fslang repo, tracking issues will need to be created for them here.

Of these, the following are of particularly high interest:

• Make FSharp.Core collection functions for Array.Parallel more regular fsharp/fslang-suggestions#187
• Support nested type definitions fsharp/fslang-suggestions#277
• try/with in seq expressions fsharp/fslang-suggestions#1027
• Allow _.Property / _.MethodCall() / _.IndexerAccess[idx] shorthand for accessor functions #13907
• [WIP] RFC-TBD Ease restrictions on static members and let in union and records #14132
• WIP: with- keyword support for anonymous records #11911
• Better generic unmanaged structs handling ++ bidirectional F#/C# interop for 'unmanaged' constraint #12154
• [WIP] RFC FS-1092: Anonymous Type-tagged Union types #10896 #12265
• [RFC FS-1043] Extension members visible to trait constraints #6805
• [RFC FS-1060] Nullness checking  #6804
• Make the fun keyword optional fsharp/fslang-suggestions#168
• Support markdown in documentation comments fsharp/fslang-suggestions#237
• Checked arithmetic operators fsharp/fslang-suggestions#1035
• Allow type providers to report location, warnings and informationals to the compiler fsharp/fslang-suggestions#155
• Allow double dollar signs for interpolated strings as in C# 11 fsharp/fslang-suggestions#1150

There is a larger set of approved-in-principle though many of these are "niggles". It's certainly good to make progress on these as well though often they're a surprising amount of work.