[Issue 21662] New: Extern linkage variables cannot be of types with disabled default construction

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21662

          Issue ID: 21662
           Summary: Extern linkage variables cannot be of types with
                    disabled default construction
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: n8sh.secondary at hotmail.com

The real-world place where this got in the way was trying to use `extern(C++)
extern __gshared core.stdcpp.basic_string!char` to refer to a variable of type
std::string defined in a C++ library. The problem is not specific to C++
compatibility however.

Example:

---
struct S { @disable this(); }
extern __gshared S someVal;
void main() {}
---
Error: variable onlineapp.someVal default construction is disabled for type S

Can we fix this by giving `someVal` a void initializer? No.

---
struct S { @disable this(); }
extern __gshared S someVal = void;
void main() {}
---
Error: variable onlineapp.someVal extern symbols cannot have initializers

Changing `void` to `S.init` or removing `__gshared` does not change the error
message.

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