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template literal types starting with ${string} don't work when the string has more than 1 character #42903
Description
Bug Report Suggestion
🔎 Search Terms
template literal types string
🕗 Version & Regression Information
4.3.0-dev.20210221
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about trmplate literal types
⏯ Playground Link
💻 Code
const foo: `${string}${number}` = 'a123' //no error
const bar: `${string}${number}` = 'foo123' //Type '"foo123"' is not assignable to type '`${string}${number}`'.🙁 Actual behavior
Type '"foo123"' is not assignable to type '`${string}${number}`'
🙂 Expected behavior
no error
Activity
[-]template literal types starting with ${string} don't work[/-][+]template literal types starting with ${string} don't work when the string is more than 1 character[/+][-]template literal types starting with ${string} don't work when the string is more than 1 character[/-][+]template literal types starting with ${string} don't work when the string has more than 1 character[/+]ahejlsberg commentedon Feb 23, 2021
This is working as intended. The pattern matching we use in template literal type inference and assignability checking is (by design) pretty simple. From #40336:
For your example code to behave as you expected we'd have to scan ahead and find the point where the remainder of the string is a valid number. But valid numbers include things like
1e1,Nan,Infinity, and so on. So, we simply don't attempt that and instead always match exactly one character for a placeholder immediately followed by another placeholder.DetachHead commentedon Feb 24, 2021
i see. can this be changed to a feature suggestion instead of a bug then?
typescript-bot commentedon Feb 26, 2021
This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.