strictNullChecks and arithmetic expressions

[bobappleyard]

TypeScript Version: 2.1.4

With strictNullChecks on:

Code

    let y = undefined + 1;

Expected behavior:

An error telling me this is the wrong thing to do.

Actual behavior:

Compiles fine, y is a number.

This does the same thing for all the arithmetic operators.

[bobappleyard]

    let y = undefined + "";

Leads to y being a string.

Tried with {} and I get Operator '+' cannot be applied to types '{}' and '{}'

[RyanCavanaugh]

@ahejlsberg not spec'd this way but I think the arithmatic operators should disallow null / undefined in strictNullChecks. Thoughts?

Adding to a string is always allowed since people use this for concatenation of arbitrary values, especially for debugging code.

[ghost]

undefined + 1 evaluates as NaN. Maybe y should be typed as the literal NaN value. NaN is almost as troublesome as null in places, and harder to deal with (NaN !== NaN, typeof NaN === 'number'). It's one of those cases of the compiler being technically correct but not very useful. Perhaps we could consider treating it specially as well.

value+"" is often used in place of String(value) and so this seems expected.