Bug Report

🔎 Search Terms

template literal wrong infer

🕗 Version & Regression Information

Tested in version 4.1.5, 5.2.3, 4.3.5, 4.4.0, 4.5.2, 4.6.0 nightly, everywhere the same behaviour.

⏯ Playground Link

Playground link with relevant code

💻 Code

type d8 = `1234` extends `${number}${infer T1}${number}` ? T1 : never; //'2'
type d9 = `2234` extends `${number}${infer T1}${number}` ? T1 : never; //never

🙁 Actual behavior

d9 is computed as never.

🙂 Expected behavior

d9 should be computed as '2'.

After debugging a bit inferFromLiteralPartsToTemplateLiteral in checker.ts, it seems p = getSourceText(s).indexOf(delim, p) searches for 2 in a targetTexts being ['', 2, ''] yielding a wrong result.

It's not clear to me what the actual algorithm should be here as I haven't found any spec or fundamental rules behind inferring template placeholders like above.