Language proposal: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-8.0/null-coalescing-assignment.md
LDM notes: https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-07-16.md
Implementation: https://github.com/dotnet/roslyn/tree/features/null-operator-enhancements

• Check language version 8.0
  • Upgrade Project available
• Parsing
  • parses regardless of language version
  • has same precedence as simple assignment
  • associativity
  • ??= is a token
• a ??= b is an r-value of type A, even if A is nullable and B is not
• a must be a reference type or nullable value type or unconstrained type parameter
• Test is a is null, operator== is ignored
• Conversions of b only; no conversions of A
  • implicit conversion from B to A or A0 (where A is A0?)
  • implicit user-defined conversion from B to A or A0
    • operator can be defined in either A or B
  • null conversion
  • default conversion
    • test conversion of default is to A not A0; issue warning since conversion to Nullable<A0> was probably not intended
• a is an l-value and r-value
  • by-val or ref local, possibly captured
  • by-val or ref parameter, possibly captured
  • writable field
  • read/write property or readable ref property
  • read/write indexer or readable ref indexer
  • read/write COM indexed property
  • read/write event
  • ref returning invocation
  • member of dynamic receiver
  • this is not allowed, for class and struct
  • pointers are not allowed
• b is an r-value expression including
  • by-val or ref local, possibly captured
  • by-val or ref parameter, possibly captured
• a ?? b is supported for unconstrained type parameters (no reference or value type constraints)
  • A is an unconstrained type parameter and B is implicitly convertible to A, with result type A
  • A is an unconstrained type parameter and A is implicitly convertible to B, with result type B
  • A is an unconstrained type parameter and B is dynamic, with result type dynamic
• a ??= b is supported for unconstrained type parameters (no reference or value type constraints)
  • A is an unconstrained type parameter and B is implicitly convertible to A, with result type A
• await is supported within a and b
  • with side-effects in a and b
• ref
  • ref a ??= ref b; // error
  • ref a ??= b; // error
• in parameter: void M(in object x) { }
  • M(a ??= b); // ok
  • M(in (a ??= b)); // error
• a ??= throw expr is not allowed
• Val escape
  • Span<int>? a = expr; a ??= stackAllocIntoSpan;
• Flow analysis
  • Variables definitely assigned in a are definitely assigned before b and after expression
  • Variables only assigned in b are not definitely assigned after expression
  • out var in b is definitely assigned in remainder of b, but not definitely assigned after ??=
• Lowering
  • a ??= b is lowered to tmpA ?? (tmpA = b)
  • Side-effects are executed exactly once for a, even if a ??= b is used as an expression
  • Side-effects are only executed for b if a is null
• SemanticModel
  • GetTypeInfo includes the type and converted type of b
  • IOperation
  • CFG
• IDE
  • Auto-formatting does not add space between ?? and =