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import std.traits;

ref int foo(ref int);

void main()
{
    alias ReturnType!foo function(ParameterTypeTuple!foo) Bar;
    int x;
    Bar bar;

    static assert(!__traits(compiles, bar(5) ));  // ok, can't take r-value
    static assert(__traits(compiles, bar(x) ));  // ok, takes l-value

    pragma(msg, Bar);  // 'int function(ref int)' -- missing ref return
    bar(x) = 5;  // Error: (*bar)(x) is not an lvalue
}
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