Test plan for "null-conditional assignment"

[RikkiGibson]

Championed issue: dotnet/csharplang#8677
Speclet: https://github.com/dotnet/csharplang/blob/main/proposals/null-conditional-assignment.md
Label: Feature - Null Conditional Assignment
Feature branch: https://github.com/dotnet/roslyn/tree/features/null-conditional-assignment

Compiler

• language version
  • parsing / syntax model does not depend on language version (NullConditionalAssignmentParsingTests)
  • member and element assignment (LangVersion_*)
• compound assignment: (LangVersion_03)
  • parsing (NullConditionalAssignmentParsingTests.VariousAssignmentKinds_LeftMemberAccess)
  • arithmetic operators: +=, etc. (CompoundAssignment_01)
  • logical operators: |=, etc.
  • shift operators: <<=, etc.
• increment, decrement (disallowed): a?.B++, --a?.B (IncrementDecrement_01)
• member kinds:
  • field
  • property
  • event
  • method
• null coalescing: a?.B ??= c (NullCoalescingAssignment_01)
• nested:
  • a?.b?.c = d (PropertyAccessAssignment_Nested_01)
  • a?.b = c?.d = e?.f (FieldAccessAssignment_Nested_03)
  • a?.b = c ?? d (NullConditionalAssignmentParsingTests.NullCoalescing_LeftMemberAccess)
• readonly member
• readonly indexer
• struct receiver (FieldAccessAssignment_StructReceiver_*)
  • readonly member
• ref returning member (AssignRefReturningMethod_01)
• cases from Readonly setter calls on non-variables
• spec explicitly the limitations around structs
• a?.B = c where B is init property
• inline array with conditional element assignment
• Expression trees (disallowed)
• dynamic receiver
• *s?.P = e
• use result: (a?.B = c)?.ToString()
  • member with reference type
  • member with value type
  • member with type parameter type: (TypeParameter_*)
    • unconstrained (disallowed)
    • struct constraint
    • class constraint
  • element assignment
• evaluation:
  • receiver evaluated exactly once (FieldAccessAssignment_Nested_03)
  • RHS evaluated conditionally (FieldAccessAssignment_Nested_03)
  • element index evaluated conditionally
  • member arguments evaluated conditionally
  • using result (FieldAccessAssignment_Nested_04)
• ref assignment (disallowed): a?.B = ref c (RefAssignment_01)
• deconstruction assignment (disallowed): (a?.B, c?.D) = (x, y) (DeconstructionLeft)
• await: (await a)?.B = c
• definite assignment
  • receiver not definitely assigned (DefiniteAssignment_01)
  • variable only assigned or used in RHS
  • variable only assigned or used in member arguments
• nullable analysis
  • receiver maybe null after a?.B = c (NullableAnalysis_01)
  • receiver and member not null after checking result (NullableAnalysis_01)
  • a?.B = a.C; should not warn for a.C
  • suppression operator: a?.B! = c
• Semantic model (no changes)
• IOperation (no changes)
• CFG (ConditionalAccessFlow_06, _07)
  • null-coalescing assignment: ??= (ensure assignment in correct branch of CFG)
  • conditional receiver: (c ? a : b)?.D = e
• Public API (no changes)

Productivity

• formatting
• Quick info for member
• Go to Definition for member
• Refactoring from existing code
• Generate member for a?.b = c Support generate method in null-conditional-assignment #77526