[Issue 19278] New: extern(C++, "name") doesn't accept expressions

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19278

          Issue ID: 19278
           Summary: extern(C++, "name") doesn't accept expressions
           Product: D
           Version: D2
          Hardware: x86
                OS: Windows
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: turkeyman at gmail.com

Consider:

```
version (A)
  enum NS = "something";
else
  enum NS = "something_else";
extern(C++, NS):

//...
```

In this example, the parser sees an identifier and assumes that to be the name
for the namespace, rather than check if it's an expression that evaluates to a
string.
I think it should attempt to evaluate the expression as a string.

It could be argued that may be ambiguous, but this isn't:
```
string GetNs() { version (A) { return "A"; } else { return "B"; } }
extern(C++, GetNs()):
```

There's also this:
```
alias NS = AliasSeq!("A", "B");
extern(C++, NS):
```
That is, do want `extern(C++, "A", "B")` in this case, but again, it looks like
an identifier to the parser.


Various work-around constructions don't work:
```
version (A)
  extern(C++, "A"):
else
  extern(C++, "B"):
void f(); // <- f is not in any namespace, the externs are scoped inside the
version cases
```

```
mixin(`extern(C++, "NS"):`);
void f(); // <- f is not namespaced; it seems the mixin statement has some sort
of scope which the extern doesn't escape
```


One of the goals of the string namespace was that you could synthesise the
name, so it really just needs to work with expressions.

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