Incorrect 'never' type inference in TypeScript 2.6

[sharwell]

TypeScript Version: 2.6.2

Code

https://github.com/sharwell/antlr4ts/blob/c9a8ef86fbdfc7f027916b5b281dac322495ca55/src/atn/LL1Analyzer.ts#L251

Expected behavior:

When the line is reached, t has the type Transition.

Actual behavior:

When the line is reached, t has the inferred type never. We are forced to work around the issue by explicitly casting and reassigning the variable.

[HerringtonDarkholme]

Please patiently await TS2.7. #19671

It is probably because your concrete class have structurally identical type with your abstract class.

https://github.com/sharwell/antlr4ts/blob/c9a8ef86fbdfc7f027916b5b281dac322495ca55/src/atn/WildcardTransition.ts#L13

[the8thavatar]

I also see this problem from TypeScript v2.5.2 onwards where it was ok in 2.4.1..
var existingSymbol: SASymbol,...
nodes = canvas.getNodes();
for (i = 0; i < nodes.length; i++) {
existingSymbol = nodes[i] as SASymbol;
if ((existingSymbol.isSASymbol || existingSymbol instanceof PlaceableText
|| existingSymbol instanceof DiagramNode || existingSymbol instanceof ShapeNode || existingSymbol instanceof SvgNode
)
...)
It reports existingSymbol to be type never from this point: existingSymbol instanceof ShapeNode
.. but I can delete the preceding cases and it becomes the expected SASymbol type again.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.