8000 number & { __brand } is not working with the comparison operator in nightly build · Issue #11643 · microsoft/TypeScript · GitHub
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number & { __brand } is no 8000 t working with the comparison operator in nightly build #11643

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zpdDG4gta8XKpMCd opened this issue Oct 14, 2016 · 3 comments
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number & { __brand } is not working with the comparison operator in nightly build #11643

zpdDG4gta8XKpMCd opened this issue Oct 14, 2016 · 3 comments
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Design Limitation Constraints of the existing architecture prevent this from being fixed

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@zpdDG4gta8XKpMCd
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declare var x: number & { __brand: any };
x > 0 // <-- [ts] Operator '>' cannot be applied to types 'number & { __brand: any; }' and '0'.
@aozgaa aozgaa self-assigned this Oct 17, 2016
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aozgaa commented Oct 17, 2016

The error first appeared in a63a0d8.

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aozgaa commented Oct 19, 2016

Hey Aleksey,

This ties into an ongoing discussion about what we think about the use/addition of nominal types in our type system. There is a long list of issues related to nominal types.

It looks like this feature is actually working as designed, though the behavior is admittedly a bit counter-intuitive at first glance. Based on the recent addition of the comparability relation (issue #5517), specifically here, x isn't comparable to '0' because '0' is a numeric literal, and thus a subtype of 'number'. Here are two workarounds (inspired by your comment here :) ):

type BrandedNum = number & { __numberBrand: any };

declare var x : BrandedNum;
x > 0; // error: 0 is of numeric literal '0', a subtype of number
x > <number>0; // okay
x > <BrandedNum>0; // okay, in my opinion the clearest

In fact, this 8000 should be a general solution for dealing with "nominal typing" via brands: upcasting the other argument to a comparison operation to that of the two types' common ancestor will make the comparison pass our type check.

@aozgaa aozgaa added By Design Deprecated - use "Working as Intended" or "Design Limitation" instead Design Limitation Constraints of the existing architecture prevent this from being fixed labels Oct 19, 2016
@aozgaa aozgaa closed this as completed Oct 19, 2016
@aozgaa aozgaa reopened this Oct 19, 2016
@zpdDG4gta8XKpMCd
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interesting, i will consider that

@aozgaa aozgaa removed the By Design Deprecated - use "Working as Intended" or "Design Limitation" instead label Oct 19, 2016
@aozgaa aozgaa mentioned this issue Oct 20, 2016
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@aozgaa aozgaa closed this as completed Oct 25, 2016
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