[Issue 23238] New: Cannot write a const Nullable(T, T nullValue)

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23238

          Issue ID: 23238
           Summary: Cannot write a const Nullable(T, T nullValue)
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: regression
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: kirsybuu at gmail.com

The following code does not compile anymore. It worked in 2.097.2, and broke by
2.099 but possibly earlier.

import std.stdio, std.typecons;
    void main() {
    const Nullable!(ulong, 0) x = 1;
    writeln(x);
}

The error (compiled using DMD 2.100.0), with all but the struct overload
omitted for brevity:

/src/phobos/std/format/write.d(1239): Error: none of the overloads of template
`std.format.internal.write.formatValueImpl` are callable using argument types
`!()(LockingTextWriter, const(Nullable!(ulong, 0LU)), const(FormatSpec!char))`
[omitted]
/src/phobos/std/format/internal/write.d(2665):                       
`formatValueImpl(Writer, T, Char)(auto ref Writer w, auto ref T val, ref scope
const FormatSpec!Char f)`
  with `Writer = LockingTextWriter,
       T = const(Nullable!(ulong, 0LU)),
       Char = char`
  whose parameters have the following constraints:
  `~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~`
`        is(T == struct)
        or:
        is(T == union)
      > hasToString!(T, Char)
        or:
      > !is(BuiltinTypeOf!T)
  - !is(T == enum)
`  `~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~`
[omitted]
/src/phobos/std/format/write.d(1239):        All possible candidates are marked
as `deprecated` or `@disable`
  Tip: not satisfied constraints are marked with `>`
/src/phobos/std/format/write.d(632): Error: template instance
`std.format.write.formatValue!(LockingTextWriter, const(Nullable!(ulong, 0LU)),
char)` error instantiating
/src/phobos/std/stdio.d(1710):        instantiated from here:
`formattedWrite!(LockingTextWriter, char, const(Nullable!(ulong, 0LU)))`
/src/phobos/std/stdio.d(4227):        instantiated from here:
`write!(const(Nullable!(ulong, 0LU)), char)`
nullable_write.d(4):        instantiated from here:
`writeln!(const(Nullable!(ulong, 0LU)))`

It is also not possible to call toString() manually on x. I get this error:

nullable_write.d(4): Error: none of the overloads of template
`std.typecons.Nullable!(ulong, 0LU).Nullable.toString` are callable using
argument types `!()() const`
/src/phobos/std/typecons.d(3780):        Candidate is: `toString()`
nullable_write.d(4):        All possible candidates are marked as `deprecated`
or `@disable`

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