raise a type error on attempt to strict-equal-compare a value of string | null to undefined

the following examples are almost certainly bugs:

declare var one: string | undefined;
// expected a type error: Operator '===' cannot be applied to types 'string | undefined' and 'null'
// actual no problem
if (one === null) { 
   // unreachable, because one can't ever be null, undefined was likely meant instead
}

declare var another: string | null;
// expected a type error: Operator '===' cannot be applied to types 'string | null' and 'undefined'
// actual no problem
if (another === undefined) {
   // unreachable, because one can't ever be undefined, null was likely meant instead
}

consider raising a type error for the above situations

[nippur72]

experienced the same problem today: I refactored a number|null to number|undefined and my code stopped to work because there were ===null checks still left behind.

[jfirebaugh]

Ironically, I work on a codebase where we’re incrementally enabling strictNullChecks and was bit by this.

To support strictNullChecks , the following change was made:

- let applyMemoryInitializer: () => void;
+ let applyMemoryInitializer: (() => void) | null = null;

A later condition was overlooked, and became always-true.

if (applyMemoryInitializer !== undefined) {
   ...
}