Fix Bugzilla 22293: opCast!bool for Nullable

[pbackus]

The second opCast overload is necessary to allow built-in implicit conversions (e.g., mutable to const) to be performed explicitly via a cast expression. Nullable.opEquals depends on this behavior.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22293	enhancement	Nullable should define opCast!bool

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#9039"

[pbackus]

src/text/json/ParserMarker.d(31,29): Error: none of the overloads of template `std.typecons.Nullable!(const(JSONParserRange!(JSONLexerRange!(string, LexOptions.noTrackLocation)))).Nullable.opCast` are callable using argument types `!(immutable(Nullable!(JSONParserRange!(JSONLexerRange!(string, LexOptions.noTrackLocation)))))()`
/buildkite/builds/zaphod-3/dlang/phobos/buildkite-ci-build/distribution/bin/../imports/std/typecons.d(3558,10):        Candidates are: `opCast(T : bool)() const`
/buildkite/builds/zaphod-3/dlang/phobos/buildkite-ci-build/distribution/bin/../imports/std/typecons.d(3564,16):                        `opCast(T, this This)()`
  with `T = immutable(Nullable!(JSONParserRange!(JSONLexerRange!(string, LexOptions.noTrackLocation)))),
       This = Nullable!(const(JSONParserRange!(JSONLexerRange!(string, LexOptions.noTrackLocation))))`
  must satisfy the following constraint:
`       is(This : T)`

If I'm reading this right, the cast that's failing is Nullable!(const(T)) to immutable(Nullable!(T)), which is an unsafe type coercion.

I think maybe the right solution here is for the compiler to fall back to built-in cast behavior when no suitable opCast overload is present. Forcing every library author who overloads opCast to handle edge cases like this one is not really reasonable.

[pbackus]

Further examination reveals that the D compiler will let you cast between more or less any two types as long as they're the same size:

struct A { ubyte[8] x; }
struct B { int* p; }

static assert(A.sizeof == B.sizeof);

void main()
{
    A a;
    auto b = cast(B) a;
}

So I guess the correct template constraint for the second opCast overload here would be is(This : T) || (This.sizeof == T.sizeof)...as crazy as that sounds.

[pbackus]

LDC build error appears to be transient (didn't happen the first time); force-pushing to re-run.

[schveiguy]

As much as I hate this opCast boiler plate, it appears to be necessary.