[Issue 20574] New: Ambigious parameter names should trigger an error

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20574

          Issue ID: 20574
           Summary: Ambigious parameter names should trigger an error
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: puremagic at zoadian.de

````
import std.stdio;
import std.traits;

void foo(int b);
void foo(int a);

void foo(int a) {
    writeln(a);
}

pragma(msg, ParameterIdentifierTuple!foo); // prints "b"

void main() {
        foo(5);
}
```

Ambigious parameter names should trigger an error, as per spec they _are_ part
of the public API.
Also if you imagine foo beeing declared in two seperate modules i suspect
import oder would change the behaviour of ParameterIdentifierTuple.
If D gets named parameters this will probably also matter.

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