Improve error messages for duplicate identifiers

[mhegazy]

We need to split the "Duplicate identifier" error in to a suite of more detailed errors about various types of illegal merges. "Duplicate identifier" is too general. Some examples of cases that warrant their own message:

• Would be a legal merge (like module & module) but exports don't match
• Fundule in the wrong order
• Clodule in the wrong order
• Fundule in different files
• Clodule in different files
• Interface and enum can't merge
• Interface and class can't merge
• Class and enum can't merge
• Function and enum can't merge

Basically every illegal cell in the matrix of merges should have a separate error message (or some of them can be grouped together in terms of value/type/namespace space). For example:

Class cannot merge with any other type

[robertknight]

Additional related request, it would be useful if the message included the source location of the existing identifier.

[mhegazy]

@robertknight we used to do that in the 1.0 time-frame, but then the error message was too long and not easily piped through IDEs and editors to allow jumping to the location; instead, the new behavior is to error on all instances of the duplicate definitions, this way you can chose which one to jump to.

[brandonbloom]

the new behavior is to error on all instances of the duplicate definitions, this way you can chose which one to jump to.

Unfortunately, if the duplicate identifier is not in your code, for example if you do class Node { ..., then you can't find the other definition easily. Presumably "Node" is colliding with something from the DOM typings.

[kitsonk]

The compiler outputs all locations...

$ cat test.ts
class Node { }
$ tsc test.ts
/node_modules/typescript/lib/lib.d.ts(12324,11): error TS2300: Duplicate identifier 'Node'.
/node_modules/typescript/lib/lib.d.ts(12376,13): error TS2300: Duplicate identifier 'Node'.
test.ts(1,7): error TS2300: Duplicate identifier 'Node'.

If you are in vscode, the quick info does not provide all locations, but if you were to comment out the offending code and just write:

Node;

The quick info would provide you with the definition locations you could navigate to.

[NaridaL]

• Would be a legal merge (like module & module) but exports don't match

namespace FF {
    export function foo() { // error here, not on FF: "Duplicate function implementation"
        return 0
    }
}
namespace FF {
    export function foo() { // here too
        return "2"
    }
}

which seems appropriate...?

• Fundule in the wrong order
• Clodule in the wrong order

A namespace declaration cannot be located prior to a class or function with which it is merged.

• Fundule in different files
• Clodule in different files

A namespace declaration cannot be in a different file from a class or function with which it is merged.

• Interface and class can't merge
  No error at all currently: https://www.typescriptlang.org/play/#src=interface%20FF%20%7B%0D%0A%20%20%20%20foo()%3A%20void%0D%0A%7D%0D%0Aclass%20FF%20%7B%0D%0A%20%20%20%20x%3A%20string%0D%0A%7D

The following are still an issue:

• Interface and enum can't merge
• Class and enum can't merge
• Function and enum can't merge