Unconstrained Null Coalescing

Unconstrained type parameter in null coalescing operator

Champion issue: https://github.com/dotnet/csharplang/issues/1284

In C# 8.0 we introduced a feature that permits a null coalescing operator to have a left operand that is not known to be either a reference or value type (i.e. an unconstrained type parameter). This is a placeholder for its specification.