> if string|int is not a valid return type
`string | int` is a valid return type, in particular in your case if you know
statically the boolean you can use this (which is a valid alternative to the
variant where you assume you know stuff at compile time):
proc test(b: static[bool]) : int|string =
when(b): 1
else: "hello"
echo test true
echo test false
Run
> shouldn't the compiler emit an error
it does emit the error: `Error: type mismatch: got 'string' for '"hello"' but
expected 'int literal(1)'`. Since the function `test` is a generic (the return
type being a type class means that for every return type a new proc will be
generated), the specific proc is generated when it is instantiated. This
happens when you call `echo test true` and in that case the compiler realizes
that the generic proc definition is not valid (in principle in this case it
could have realized it earlier but it is not true in general) and throws the
correct error: the if expression has `int` as first expression returned, so all
other branches must have an `int`, I am expecting an int (which I derived being
an int from a literal 1), and now I find an "hello"
