No error when using uninitialized value in noImplicitAny & strictNullChecks mode

[DanielRosenwasser]

From master as of right now (7b34b612beda66b0812462a3feeabc63852cd842)

var x;
var y = 100 + x;

Compile with strictNullChecks and noImplicitAny

Expected: Some sort of error for using x.
Actual: No error at all?

[ahejlsberg]

This is actually by design. Your var x is an implicit any variable that is tracked by control flow analysis (#11263). Because it hasn't been initialized, its control flow type in the 100 + x expression is undefined, and the + operator permits undefined as an operand value. If anything, we should discuss whether the numeric operators should be stricter with respect to undefined and null, but that's a different discussion.

[DanielRosenwasser]

Sure, that issue definitely sounds like something we should look into; is the type of x still going to be undefined under noImplicitAny?