Function assignment compatibility differs from spec.

[ghost]

Consider this:

var foo: (number) => void = (bar: string) => bar.substr(0);

According to spec, 3.10.4 Assignment Compatibility:

• M is a non-specialized call or construct signature and S has an apparent call or construct signature N where, when M and N are instantiated using type Any as the type argument for all type parameters declared by M and N (if any),
  • the signatures are of the same kind (call or construct)
  • M has a rest parameter or the number of non-optional parameters in N is less than or equal to the total number of parameters in M
  • for parameter positions that are present in both signatures, each parameter type in N is assignable to or from the corresponding parameter type in M, and
  • the result type of M is Void, or the result type of N is assignable to that of M.

Here, M = (number) => void, N = (string) => string. This is not a case of function argument bi-variance as in #2170, for string isn't assignable to or from number, and the assignment should fail to compile. Meanwhile, neither the playground nor TypeScript 1.5 for VS2013 complains.

Is this a bug in the compiler, or the spec is to be updated?

[DanielRosenwasser]

This is an unfortunate consequence of the way function type literal syntax works in our language. In a function type literal, parameter names are mandatory and the types are optional. So in your example, you don't have a parameter of type number, you have a parameter named number of type any. Here, you can try it out on the TypeScript Playground where you'll see the correct error.

[DanielRosenwasser]

For the record, if you use TypeScript with the --noImplicitAny flag, we do catch this problem.