[Issue 20766] New: empty string literals passed as optional parameter should not be 0 terminated

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20766

          Issue ID: 20766
           Summary: empty string literals passed as optional parameter
                    should not be 0 terminated
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: b2.temp at gmx.com

concrete illustration:

---
void main()
{
    test();
}

void test(string s = "")
{
    if (s) {}
    else   { assert(false); }
    s ? {} : assert(false);
}
---


`""` is actually a pointer to 0x00 and a length of 0.  Since `s.ptr` will never
be null the else branch (or the last exp of a CondExpr) will never be executed,
even when calling test() without argument.

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