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Always assigning to an un-initialized variable in an exhaustive switch #30000
Description
TypeScript Version: 3.4.0-dev.20190220
Search Terms:
switch case exhaustive assignment assign undefined TS2322
Code
Compile with the flags --noImplicitReturns --noImplicitAny --strictNullChecks
enum Test {
A,
B,
C
}
// this example correctly understands that all possible values of t are handled
function foo(t: Test): number {
switch (t) {
case Test.A:
return 1;
case Test.B:
return 2;
case Test.C:
return 3;
}
}
// this should be identical to foo, since a is always assigned as the switch is exhaustive
function bar(t: Test): number {
let a;
switch (t) {
case Test.A:
a = 1;
case Test.B:
a = 2;
case Test.C:
a = 3;
}
return a; // I don't expect an error here
}Expected behavior:
Code compiles with no errors. When a is returned in bar, its type is number. Typescript understands that an exhaustive switch with each case assigning to a type-inferred variable means that the variable's type is the union of each assigned value's type. I'd expect this to work because returns within an exhaustive switch are understood in a similar manner.
Actual behavior:
When a is returned in bar, its type is number | undefined
TS2322: Type 'number | undefined' is not assignable to type 'number'.
Type 'undefined' is not assignable to type 'number'.
Playground Link:
link here
Turn on noImplicitReturns, noImplicitAny, and strictNullChecks.
Related Issues:
- Initializing
letvariables and exhaustiveness checking #22470 (different, since the default doesn't assign) - TS2454 after assigning a value from an exhaustive switch statement #18362 (this is pretty old, from TS 2.4.2. Since
fooin my example works I think the closure reason might be invalid)
Activity
RyanCavanaugh commentedon Apr 1, 2019
Comments in #18362 still apply. We have some special-casing we're capable of doing for
return, but it doesn't generalize to assignments.zpdDG4gta8XKpMCd commentedon Apr 2, 2019
how is it not covered by officially recommended:
?
zpdDG4gta8XKpMCd commentedon Apr 2, 2019
http://www.typescriptlang.org/docs/handbook/advanced-types.html