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Add comment for comparable relationship

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Kanchalai Tanglertsampan
2017-03-21 14:18:12 -07:00
parent 8fd660bcba
commit f04f7b5afd
1 changed files with 6 additions and 0 deletions
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6
src/compiler/checker.ts
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@@ -7434,6 +7434,12 @@ namespace ts {
/**
* This is *not* a bi-directional relationship.
* If one needs to check both directions for comparability, use a second call to this function or 'checkTypeComparableTo'.
*
* A type S is comparable to a type T if some (but not necessarily all) of the possible values of S are also possible values of T.
* It is used to check following cases:
* - the types of the left and right sides of equality/inequality operators (`===`, `!==`, `==`, `!=`).
* - the types of `case` clause expressions and their respective `switch` expressions.
* - the type of an expression in a type assertion with the type being asserted.
*/
function isTypeComparableTo(source: Type, target: Type): boolean {
return isTypeRelatedTo(source, target, comparableRelation);