to!string with inout enum argument fails to compile

[dlangBugzillaToGithub]

snarwin+bugzilla (@pbackus) reported this on 2024-11-25T15:46:38Z

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

Description

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
---

[dlangBugzillaToGithub]

dfj1esp02 commented on 2024-11-26T18:45:25Z

private template enumRep(T, S, S value)
if (is (T == immutable) && isExactSomeString!T && is(S == enum))
{
    static T enumRep = toStr!T(value);
}

If I mark the argument as immutable, then it compiles:
private template enumRep(T, S, immutable S value)

Presumably for enums it should be fine.

Invoking with enumRep!(immutable(T), immutable(S), member) works too.