Test plan for "improved definite assignment"

[RikkiGibson]

Championed issue: dotnet/csharplang#4465
Spec: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-10.0/improved-definite-assignment.md#specification

• definite assignment and nullability improvements for ??
  • <null-conditional> ?? expr_second
• definite assignment and nullability improvements for ? :
  • expr_cond ? expr_true : expr_false can result in conditional state
• definite assignment and nullability improvements for ==
  • <null-conditional> == <null-constant>
  • <null-conditional> == <not-null>
  • expr == <constant-true>
  • expr == <constant-false>
  • corresponding rules for !=
  • predefined vs. user-defined == operators
  • DataFlowsOut API needs tests to demonstrate that the usage of Meet() between ==/!= operands is sound
• definite assignment and nullability improvements for is
  • <null-conditional> is <not-null-only-matching-pattern>
  • <null-conditional> is <null-only-matching-pattern>
  • <null-conditional> is <true-only-matching-pattern>
  • <null-conditional> is <false-only-matching-pattern>
• true, false operators

Needs some discussion:

• definite assignment and nullability improvements for switch
• definite assignment and nullability improvements for relational operators (>, ...)

Productivity

• change x != null && x?.TryGetValue(out y) to x?.TryGetValue(out y) == true

[RikkiGibson]

We concluded that before feature merge, we will just get the nullable 'is' PR (#53311) merged, then another PR to ensure test coverage for the implemented aspects of the feature.

We intend to do the 'switch' and '<=' aspects in PRs to main after the feature is merged before shipping C# 10.

[jcouv]

I think we can close this now. Thanks