Test plan for "recursive patterns"

[jcouv]

This is a place to collect test ideas for the v2 patterns work.

Championed issue: dotnet/csharplang#45
Open LDM issues: dotnet/csharplang#2095

Before Initial Merge

• Specification checked into csharplang.
• IOperation and CFG should not "blow up" on any pattern stuff
• Disable switch expression in expression trees [See ErrorCode.ERR_ExpressionTreeContainsSwitchExpression]
• EE test that uses switch expression
• Update the compiler test plan (CFG, IOperation, switch expression, new category for patterns)

After initial merge

• Need API get get Deconstruct method (like GetDeconstructionInfo). Are any conversions involved in a positional pattern? [See SymbolInfoForPositionalSubpattern. There are no conversions involved.]
• Although the language assumes that Deconstruct and properties used in switch statement are idempotent/pure, the compiler tests should capture the actual evaluation order (to avoid inadvertent changes) [See RecordOrderOfEvaluation]
• Test switch on (dynamic)someITupleInstance [See ITupleFromDynamic]
• Verify IDE formatting on switch expressions (once syntax is finalized)
• Verify new syntax nodes can be registered to be triggered in analyzers
• edit-and-continue [see https://github.com/Debugging and edit-and-continue for pattern-matching: action items #25547]
• Test the SemanticModel for the names referenced in patterns, including parameter names, tuple element names, field names, and property names.
• IOperations
• Discuss the "decompilation" feature in the IDE (likely broken)
• 0- and 1- tuple patterns
• Ref-struct types and lifetimes
• Test capturing on recursive pattern variable
  • Worth noting when the closure class gets allocated here, even if it's not observable
• Property pattern on anonymous type
• Should we allow trailing commas? (issue Switch expressions currently disallow trailing comma csharplang#2098)

Missing tests (not exhaustive):

• Test various combinations of omitted parts for recursive patterns.
• Test recursive pattern matching in scripts (e.g. pattern variable promoted to field).
• Tests should ensure code coverage of parsing, binding, lowering, and code generation changes. [Done]
• Test flow analysis for recursive patterns.
• Test flow analysis for switch expression.
• Test type inference for switch expression.
• Lazily binding types for variables declared in the var pattern
• Scope of variables declared using the var pattern
• Definite assignment for variables declared using the var pattern
• Error when there is a type or constant named var in scope where the var pattern is used

Would like tests that ensure code coverage for

• Building the decision dag [Done]

IDE scenarios:

• How should we format is { }? (is there a space?)
• Formatting should not insert a space between type and positional pattern (is Point(1, 2)) (issue Patterns: Formatting should not insert space between Type and positional sub-pattern #31975)
• Convert switch statement to switch expression (PR Add feature to convert switch statement to switch expression #31812)
• Should we have some refactoring between switch expression and switch statement? (a switch statement with returns or assignments in all branches, maybe?)
• Should we have a refactoring from conditional e ? x : y to e switch { true: x, false: y} (so that you can then make e an enum instead of a bool
• Test the name completion System.Array -> array in e is (System.Array $$ and e is (_, System.Array $$
• Should some existing (or new) formatting options control formatting switch expressions?
• Confirm syntax for property pattern: is { Prop: 0 } or is { Prop is 0 }?

BCL:

• Add MatchFailureException to Core 3 (design https://github.com/dotnet/corefx/issues/33284)
• Port MatchFailureException to mono

From the meeting:

• Test for space in positional Deconstruct in formatter
• Unsupported EnC items should be rude edits
• Verify breakpoint design with IDE
• IOperation support for switch expressions and recursive patterns, CFG support for switch expressions
• await
  • await e switch { ... }
  • e switch { pat => await ... }
  • Other tests for unary & binary operator precedence in op e switch { }
• Nullable reference types
  • Test for warnings appearing/disappearing based on null checks in patterns
• FAR for Deconstruct should find patterns doing deconstruction
• LanguageVersion check
• Deconstruct extension method with null receiver
• Lambda in switch expression and testing that common type inference works appropriately
• Switch expression in LINQ query syntax
• Switch expression with stackalloc expression
• Update compiler test plan with new expression and pattern forms after merging (Eric's checklist)
• is {} spacing in the formatter

[gafter]

@jcouv what is #25400? ;)

[alrz]

Convert switch statement to switch expression?

This is close to completion: #31812

[jcouv]

@agocke @gafter Do you have some plan to verify and close this test plan?