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strictNullChecks thinks a value may be null if null is assigned to it in its non-null branch #7630
Description
TypeScript Version:
3853bb8 (master after merging #7140 )
Code
class Prop {
value: any;
}
var x: Prop | null = null;
if (x === null) {
;
}
else if (x.value === null) { // Line 9
x = null;
}tsc -t es5 --strictNullChecks ./foo.ts
Expected behavior:
Compiles.
Actual behavior:
foo.ts(9,10): error TS2531: Object is possibly 'null' or 'undefined'.
If the x = null; assignment (line 10) is removed, it does compile, and x is correctly inferred to be non-null in the condition and body of the else-if block. But this assignment should not have any effect on the nullability of x in the else-if condition.
Activity
weswigham commentedon Mar 22, 2016
Im pretty sure the reason is this: Assignment within a block disabled narrowing within it. If so, the solution is likely already in the works: Control flow based type guards (See PR #6959)
Arnavion commentedon Mar 22, 2016
Disabling narrowing within the block is one thing, but line 9 is before the block.
RyanCavanaugh commentedon Mar 22, 2016
Assignments anywhere within the scope of a variable disable type guards (for now).
As you've noticed, this makes null checks extremely cumbersome, which is why control flow analysis was bumped up in priority.