Open issues for pattern-matching extensions for C# 8

Specification

• The specification needs to be updated with the additional syntax forms being added. See https://github.com/dotnet/csharplang/issues/1054 for a summary of the proposed changes vs C# 7.

Unimplemented parts

• Give an error when a wildcard is used but something named _ is in scope.

• Implement the switch statement in the presence of recursive patterns

  • Parsing
  • Binding
  • Lowering
  • Code-gen
  • Debug info
  • Edit-and-continue

• Implement the match expression

  • Parsing
  • Binding
  • Lowering
  • Code-gen

• It is an error if the name var binds to a type in a var pattern. Implement and test.

Test plan needed

• We need a test plan for these additions. Here is a high-level list of some tests that are needed

  • Scoping mechanism works for pattern variables in recursive patterns
  • In scripts top-level pattern variables in recursive patterns become fields.
  • IDE scenarios need to be identified tested.
  • Fuzz generator should be extended to support recursive patterns.
  • Test each of the kinds of symbols that should be supported in property patterns
  • Test all of the ways that a name would not be valid to name a property in a property pattern
  • Need to design the data representation for edit-and-continue for temps in patterns
  • Need to precisely share temps per the edit-and-continue spec
  • Need bullets here or github issues for prototype comments.

• Need to ensure good code quality, e.g. avoid redundant null checks preceding types tests

• Need to ensure a good tradeoff between decision tree size explosion and execution of redundant tests.