This code doesn't work!
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struct S{ @safe int opApply(scope int delegate(ref int) dg){return 0;} }
@safe void main()
{
    S s;
    foreach (x; s) {}
}
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@safe delegate can implicit convert to @system delegate.
Following code can compile:
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struct S{ @safe void foo(){} }
@safe void main()
{
    S s;
    void delegate() dg = &s.foo;
}
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And of course, "int delegate(ref int) @safe" should convert to "int delegate(ref int)".

Therefore, safe function should allow to call opApply.

Workaround is here:
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struct S{
    @safe int opApply(scope int delegate(ref int) dg){return 0;}
    @safe int opApply(scope int delegate(ref int)@safe dg){return 0;}
}
@safe void foo()
{
    S s;
    foreach (x; s) {}
}
@system void main()
{
    S s;
    foreach (x; s) {}
    foo();
}
--------------------------------------------