Champion: Pattern-based `fixed` statement (15.7)

[jcouv]

Introduce a pattern that would allow types to participate in fixed statements.

• Proposal added (https://github.com/dotnet/csharplang/blob/master/proposals/pattern-based-fixed.md)
• Discussed in LDM
• Decision in LDM
• Finalized (done, rejected, inactive)
• Spec'ed

API portion discussed in LDM: https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-12-04.md
Tagging @VSadov

[aluanhaddad]

With local variable type inference explicitly prohibited in fixed statements it seems weird that they would support patterns especially since var is a pattern.

Maybe that restriction should be lifted.

[PathogenDavid]

The proposal link is broken since the file got moved, here's a working link: https://github.com/dotnet/csharplang/blob/d437a2a6aa5e2d3deb6bb7f1faf03967d0f3d6ab/proposals/csharp-7.3/pattern-based-fixed.md