Left arrow ligature in JavaScript + Flow

[SergioMorchon]

Environment

Cascadia Code version number: 1909.16
Application (with version) used to display text: Visual Studio Code 1.38.1
OS platform and version: Windows 10 Pro, 18362
Screen resolution (i.e. 220dpi): 1920x1080 @ 100%

Any other software? No

Steps to reproduce

1. Open a JavaSript file that will be used with Flow (https://flow.org/).
2. Open the following flow file: react-redux.
3. You'll see declare type MapStateToProps<-S, -OP, +SP> =

Expected behavior

Don't apply a ligature in that case. It should keep as <-, because < is used to declare a generic, and - for write-only (see https://flow.org/en/docs/types/interfaces/#interface-property-variance-read-only-and-write-only-).

Actual behavior

That <-S is changed by a left arrow.

[ExE-Boss]

Same with TypeScript (and other programming languages with generics):

let foo: Bar<-3>;

[aaronbell]

It sounds like this is not something that can be special cased out in the font code (as in, ignore ligature when X occurs). That said, I can provide a stylistic set that turns off that specific ligature generally that would need to be accessible via font settings.

[ExE-Boss]

Another option would be to improve the TextMate language grammar so that, - followed by a number has a different TextMate scope from the enclosing generics. Same with Flow’s read‑only and write‑only modifiers (+ and -).

This will fix it in VS Code, but not on GitHub:

[bterlson]

FWIW, I think flow is uniquely impacted by this given its variance sigils for generic parameters. TypeScript is impacted, but I suspect only in very niche cases where users are passing a negative number literal type. I think C# generic syntax doesn't allow <-.