Issue with Template literal type

Question

I dont know what's wrong with my typing :

type t_firstChar<T extends string > = T extends `${infer A}${infer R}` ? A : T 
let checkExtUnionFirstChar : t_firstChar<"Test1" > extends t_firstChar<"Test1" | "Function" >  ? true :false  //true , expected true 

let checkExtUnionJson_1 : {firstC:t_firstChar<"Test1">} extends {firstC:(t_firstChar<"Test1">|t_firstChar<"Function">)} ? true :false  //true , expected true 
let checkExtUnionJson_2 : {firstC:t_firstChar<"Test1">} extends {firstC:t_firstChar<"Test1">}|{firstC:t_firstChar<"Function">} ? true :false  //true , expected true 

type IFirstChar<T extends string >  = {firstC:t_firstChar<T>}
let checkExtUnionInterface : IFirstChar<"Test1" > extends IFirstChar<"Test1" | "Function" > ? true :false  // false , expected true

Playground link

I expect that typeof checkExtUnionInterface is true but it's false. Can anyone explain what I'm missing please?

Did you mean to use `infer A | infer R` instead of `\`${infer A}${infer R}\``? — Vivick, Aug 20 '23 at 17:04
@Vivick This is a minimal example that is not necessarily meaningful , but here i m trying to retrieve first character of T using `${infer A}${infer R}` , thank you for your time — issueWebpack, Aug 20 '23 at 17:18
This is a limitation in *variance measurement* for conditional types. See [ms/TS#48936](//github.com/microsoft/TypeScript/issues/48936) for a similar issue. The compiler marked `IFirstChar` as *invariant* in `T`, so `IFirstChar` is compatible with `IFirstChar` if `A` and `B` are the same type. This is incorrect; `IFirstChar` "should" be marked as having unreliable or unmeasurable variance, so that the check would be made structurally, but that would hurt performance. Does that fully address the question? If so I'll write up an answer explaining; if not, what am I missing? — jcalz, Aug 20 '23 at 18:37
@jcalz Thank to you i find a lot of article and stuff about it , thank you so much !!! — issueWebpack, Aug 20 '23 at 20:14
On second thought I see that I already have an answer to a similar question that would be nearly identical here; see [that one instead](https://stackoverflow.com/questions/73583314/why-are-these-generics-being-treated-differently). — jcalz, Aug 20 '23 at 21:34

Issue with Template literal type

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