Test plan for parameterless struct constructors

[cston]

Proposal: parameterless-struct-constructors.md
Branch: features/struct-ctors

• -langversion:
  • for parameterless ctor (see PublicParameterlessConstructor)
  • for field initializers (see FieldInitializers_01)
• public and non-public ctors declared in source (see PublicParameterlessConstructor vs. NonPublicParameterlessConstructor_01)
• extern and unsafe are valid ctor modifiers
• : this() and : base() initializers:
  • : base() initializer disallowed
  • : this() initializer allowed from other ctors (see ThisInitializer_01)
  • : this(arg0, ...) initializer allowed from parameterless ctor (see ThisInitializer_02)
• record struct parameterless ctor must include : this(arg0, ...) (see TypeDeclaration_ParameterlessConstructor_OtherConstructors)
• default ignores parameterless ctor
• new() invokes parameterless ctor:
  • from new S() and new S { ... } (see PublicParameterlessConstructor)
  • from new S() and new S { ... } in expression tree (see ExpressionTrees)
  • from new T() with where T : struct and where T : new() (see PublicParameterlessConstructor)
• all fields assigned:
  • all fields have initializers if any have initializers and no ctors (see FieldInitializers_01 and FieldInitializers_02)
  • all fields have initializers or are explicitly assigned in each explicit ctor (see FieldInitializers_01 and FieldInitializers_02)
• field initializers and initialization order with:
  • no constructors (see FieldInitializers_None)
  • explicit parameterless ctor (see FieldInitializers_None)
  • explicit ctor with parameters (see FieldInitializers_None)
  • explicit ctor with : this() initializer with/without parameterless ctor (see ThisInitializer_03)
  • explicit parameterless ctor with : this(arg0, ...) initializer (see ThisInitializer_03)
• field initializers and initialization order on record struct with cases above and: (see TypeDeclaration_ParameterlessConstructor_Initializers)
  • no ()
  • ()
  • (arg0, ...)
• nullability analysis (initial state of fields in ctor bodies) for field initializer cases above (see NullableAnalysis_*)
• error for new S() in parameter default value if S has parameterless ctor (see ParameterDefaultValues_*)
• analyzer checks runtime capability and warns for new T() with where T : struct or where T : new() on desktop framework: dotnet/roslyn-analyzers/issues/5272

[tmat]

• EnC analyzer needs to be updated