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Template literal expression is not assignable to matching template literal type #43243
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[-]Type '`${string} abc`' is not assignable to type '`${string} ${string}`'.[/-][+]Template literal expression is not assignable to matching template literal type[/+]on Mar 14, 2021 We currently don't have any assignability rules to allow assigning one arbitrary template type to another (which is what's being suggested here). It's worth thinking about what rules, if any, would make sense to add.
Ah, thanks Ryan Cavanaugh (@RyanCavanaugh), that makes sense.
This appears to be a generalization of the inferencing operation. Inferencing is able to do a kind of matching, but only when all parts are closed.
If so, then this is related to #43060. If templates with open components could be inferred, then the same logic could be used to determine assignability.
For example, this ad-hoc expression could check assignability to
${string} ${string}:type Assignable<T> = T extends `${infer A} ${infer B}` ? A extends string ? B extends string ? true : never : never : never; type V1 = Assignable<`${string} abc`>; // never type V2 = Assignable<`abc def`>; // true
Intuitively, it seems that if a type can evaluate to
truein an expression like the above (where the innerextendsclauses represent the primitive or generic constraints in the template), then it is assignable to the template.My respect and thanks, Anders Hejlsberg (@ahejlsberg). ☀️
Reacted by Anders Hejlsberg
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template literal types, template literal assignability
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Note in the second case, where
value2is a literal string, the assignment works okay.🙁 Actual behavior
🙂 Expected behavior
I would expect this value to be assignable, since the statically known
valuetype matches the template pattern.It is also odd that the two cases above behave differently.