Template literal wrong inferred value when multiple placeholders

[marcj]

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.

[ahejlsberg]

This is working as intended, though I'll grant it's a bit odd.

The key behavior to understand is that for two immediately adjacent template literal type placeholders we always match a single character from the source to the first placeholder. For example, when matching 'abcd' against `${string}${infer T}`, we'll match the first character against ${string} and thus infer 'bcd' for T. This holds even when the placeholder is ${number}, so matching '1234' against `${number}${infer T}` will match '1' against ${number} and infer '234' for T.

When resolving the d9 conditional type, we first perform inference for T. So, we match '2234' against `${number}${infer T1}${number}`, which matches the first '2' against the first ${number}, infers the second '2' for T, and matches '34' against the last ${number}. Following inference, we instantiate with the type inferred for T, leaving us with the conditional check '2234' extends `${number}2${number}`. Now, because the placeholders have intervening text, we scan the source string for the first occurrence of the separator '2'. That means we end up with an matching an empty string to the first ${number} and the string '234' to the second ${number}. This fails, and therefore the conditional type resolves to never.

Our matching is not particularly sophisticated, but that is in part by design because we don't want the compiler to be a full regex engine. In general, immediately adjacent placeholders are really only useful for taking strings apart one character at a time.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.