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module test;
import std.stdio;

struct S
{
    public void toString (scope void delegate(scope const(char)[]) @safe nothrow sink) @safe
    {
        sink("output");
    }
}

void main()
{
    S s;
    writeln(s);
}
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$ dmd test.d

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/Library/D/dmd/src/phobos/std/format.d(3962): Error: function `test.S.toString(scope void delegate(scope const(char)[]) nothrow @safe sink)` is not callable using argument types `(void delegate(scope const(char)[] s) @safe)`
/Library/D/dmd/src/phobos/std/format.d(3962):        cannot pass argument `__lambda4` of type `void delegate(scope const(char)[] s) @safe` to parameter `scope void delegate(scope const(char)[]) nothrow @safe sink`
/Library/D/dmd/src/phobos/std/format.d(4342): Error: template instance `std.format.formatObject!(LockingTextWriter, S, char)` error instantiating
/Library/D/dmd/src/phobos/std/format.d(1854):        instantiated from here: `formatValueImpl!(LockingTextWriter, S, char)`
/Library/D/dmd/src/phobos/std/format.d(575):        instantiated from here: `formatValue!(LockingTextWriter, S, char)`
/Library/D/dmd/src/phobos/std/stdio.d(1506):        instantiated from here: `formattedWrite!(LockingTextWriter, char, S)`
/Library/D/dmd/src/phobos/std/stdio.d(3863):        instantiated from here: `write!(S, char)`
test.d(14):        instantiated from here: `writeln!(S)`
Failed: ["/Library/D/dmd/bin/dmd", "-g", "-unittest", "-v", "-o-", "test.d", "-I."]
[Finished in 0.7s]
-----