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functions returning never are not treated the same as raw throw calls in control-flow analysis #9655
Description
TypeScript Version: 2.0.0-beta
Code
function fail(): never {
throw 'error';
}
enum E {
A,
B
}
function f(t: E) {
let s: string;
switch (t) {
case E.A:
s = 'a';
break;
case E.B:
s = 'b';
break;
default:
fail(); // but replacing with "throw 'error"' removes the TS error.
}
s += '';
}Expected behavior:
s += '' is allowed because all terminating (non-throwing) branches assign to s.
In general, I am working under the assumption that calling functions that returning never are equivalent to raw throw calls during control-flow analysis.
Actual behavior:
ERROR(21,3): : Variable 's' is used before being assigned.
Activity
RyanCavanaugh commentedon Jul 12, 2016
The limitation here has to do with the way the compiler is designed -- control flow analysis happens before typechecking, but we would need type information (as well as identifier resolution) to determine that the
fail()call points to a function that is: never, so thes += '';node is marked as reachable via a path that doesn't assign tos.The intended fix is to write
return fail();instead offail();.