https://issues.dlang.org/show_bug.cgi?id=24880

          Issue ID: 24880
           Summary: to!string with inout enum argument fails to compile
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: phobos
          Assignee: nobody@puremagic.com
          Reporter: snarwin+bugzilla@gmail.com

As of DMD 2.109.1, the following program fails to compile:

---
enum E { One }

inout(E) fun(inout(E) e)
{
    import std.conv;
    auto s = e.to!string;
    return e;
}
---

The error message is:

---
/usr/include/dmd/phobos/std/conv.d(1227): Error: variable `std.conv.value` -
only parameters or stack-based variables can be `inout`
/usr/include/dmd/phobos/std/conv.d(210): Error: template instance
`std.conv.toImpl!(string, inout(E))` error instantiating
bug.d(6):        instantiated from here: `to!(inout(E))`
---

This error occurs because toImpl attempts to pass an inout(E) value as an argument to the template enumRep. The instantiation fails because inout values cannot be used as template arguments:

---
enum E { One }
enum example(T, T value) = true;
alias test = example!(inout(E), E.One);
// Error: only parameters or stack-based variables can be `inout
---

--