How do I identify which type a union is currently storing, in dlang?

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I have a union, which stores either a string or a double. AS you know, unions store only one type, how do I identify which type was last assigned to it, or in other words, which type is it currently storing? Plus, I'm using the D language, there isn't a tag for it, so I had to write it here.

What I have tried:

I've tried it with sizeof. I thought that each type would change the sizeof the union, but it doesn't happen, you can check the following code:

tvar qa;
qa.d = 20.4;
writeln(qa.sizeof);
qa.i = 20;
writeln(qa.sizeof);
qa.s = "qa";
writeln(qa.sizeof);
readln;

tqvar is the variant here.