Improve error on non-ambient class and function merge

[weswigham]

In #32584, we're allowing ambient classes and functions to merge, but non-ambient classes and functions still issue the same Duplicate identifier error. An improved, more specific, error would be useful. Something mentioning that either the function can have an implementation or both need to be ambient.

[austincummings]

I can work on this one.

[austincummings]

@weswigham here's the error message I've come up with. Any thoughts?

Class declaration cannot implement overload list for '{0}'. Function may have implementation or both class and function declarations must be ambient.

[weswigham]

At at glance I'd say that looks good, but I'm wondering if the term ambient is something we've used in an error message yet. @DanielRosenwasser ?

[DanielRosenwasser]

At a glance it's fine to say "ambient" as long as you have something actionable.

On the function: Function with bodies can only merge with classes that are ambient.

Related span on the class: Consider adding a 'declare' modifier to this class.

[austincummings]

@DanielRosenwasser would that look something like this then? Or should the "Function with bodies..." messages only appear when hovering over the function name?

[weswigham]

Function with bodies can only merge with classes that are ambient. is supposed to be the primary error on the functions, instead of Class declaration cannot implement overload list for '{0}'. Function may have implementation or both class and function declarations must be ambient.. The related spans are both supposed to be referring to the same thing.

[austincummings]

The related spans are both supposed to be referring to the same thing.

@weswigham sorry, I'm a bit confused as to what "same thing" refers to. Here's my understanding.

function Foo(): any;
function Foo(n: number): any {} 
//       ~~~  Function with bodies can only merge with classes that are ambient.
//       No related messages/spans?

class Foo {} 
//    ~~~ Class declaration cannot implement overload list for '{0}'
//    And related "Consider adding 'declare'..." referring to the span of this class?

[weswigham]

The function Foo should have the same related spans as the other error. Otherwise it looks good.