sealed when -> post 1.0 syntax for exhaustive w...

# language-proposals

[mikehearn]

sealed when -> post 1.0 syntax for exhaustive when, i.e. the compiler flags an error if you don't cover all the possible cases. when is already exhaustive when you use the result as an expression, but if you just have a free-standing when as a statement, there's currently no way to make it exhaustive