When function return type is inferred, the displayed signature's return type is missing a type parameter

[reify-thomas-smith]

Search terms

Signature, return type, type parameter

Expected Behavior

Signature should be capitalize<T>(string: T): Capitalize<T>

Actual Behavior

Signature is capitalize<T>(string: T): Capitalize

Steps to reproduce the bug

/**
 * Capitalize the first letter of a string.
 */
export function capitalize<T extends string>(string: T) {
  return (
    string === '' ? '' : string[0].toUpperCase() + string.slice(1)
  ) as Capitalize<T>;
}

The bug does not occur if you add an explicit return type annotation.

Environment

• Typedoc version: 0.23.17
• TypeScript version: 4.7.4
• Node.js version: 16.14.0
• OS: macOS 12.6

[Gerrit0]

Deep in typescript.js...

if (type.flags & 268435456 /* TypeFlags.StringMapping */) {
    var typeNode = typeToTypeNodeHelper(type.type, context);
    return symbolToTypeNode(type.symbol, context, 788968 /* SymbolFlags.Type */, [typeNode]);
    //   override type arguments                                                 ^^^^^^^^^^
}

Thanks TS. You do it magically, so just don't bother filling out the typeArguments property for anyone using the compiler API :/