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When function return type is inferred, the displayed signature's return type is missing a type parameter #2079

reify-thomas-smith opened this issue Oct 18, 2022 · 1 comment
bug Functionality does not match expectation


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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.


  • Typedoc version: 0.23.17
  • TypeScript version: 4.7.4
  • Node.js version: 16.14.0
  • OS: macOS 12.6
@reify-thomas-smith reify-thomas-smith added the bug Functionality does not match expectation label Oct 18, 2022
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Gerrit0 commented Oct 23, 2022

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 :/

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