allow use of undefined as switch key value

[shawnl]

switch (foo) {
undefined => do_stuff(),
else => do_other_stuff(),
}

While would trim the undefined key branch as unreachable.

Why is this useful? you might ask. It allows more ways to disable code paths based on compile time variables. undefined here would generally be a const. Instead you currently get

    #1 with zig trunk
    zig trunk (Editor #1, Compiler #1) Zig

/home/ubuntu/example.zig:4:5: error: use of undefined value here causes undefined behavior

Compiler returned: 1

Which is bogus (however llvm won't take an undefined here either, IIUC, so it does have to be explicitly handled).

Related: #597

[shawnl]

I'm starting to have second thoughts about this proposal, as it muddles the concept of undefined. (not the C concept, but the Zig concept) However I think my use case is sound. Any suggestions?

[daurnimator]

switching on undefined should immediately be an error: undefined means "any value could be here; it's an error to try read it"

[shawnl]

@daurnimator yeah, but it also turns into unreachable in a number of places in llvm, except that zig doesn't let it do that.

[shawnl]

Yeah, I'm going to close this as kinda sneaky, almost hidden control flow.