RegExpMatchArray has lost some compatibility with Array since 2.1.0-dev.20160729

[falsandtru]

TypeScript Version: master

Code

(''.match(/ /) || []).map(s => s.toLowerCase());

Expected behavior:

$ node built/local/tsc.js --lib es6 --strictNullChecks index.ts

Actual behavior:

$ node built/local/tsc.js --lib es6 --strictNullChecks index.ts
index.ts(1,1): error TS2349: Cannot invoke an expression whose type lacks a call signature.

[RyanCavanaugh]

Same root cause as #10025 but it's much less clear how to fix this example

[falsandtru]

I understand as a breaking change, not a bug. Thanks Ryan Cavanaugh (@RyanCavanaugh) .

[ahejlsberg]

Several subtle issues at play here. With #9407 we still perform subtype reduction for the operands of the || and ?: operators. However, even after subtype reduction the expression (''.match(/ /) || []) has type RegExpMatchArray | never[] because there are extra optional properties introduced by RegExpMatchArray. In the property access for the map property we would previously perform subtype reduction again, and that would reduce the union function type of map down to a single type. This step we now omit because of #9407.

I think the real issue here is that we produce the union type RegExpMatchArray | [] instead of just RegExpMatchArray (and this issue was there all along). It may be that for || and ?: we should say that if neither operand is a subtype of the other, but one is assignable to the other, we pick the type of the one to which the other is assignable.

[falsandtru]

Thanks for the fix!