Parameter named `this` kept unchanged

[buckle2000]

dstep would generate

extern (D) auto uiControl(T)(auto ref T this)
{
    return cast(uiControl*) this;
}

for ui.h in https://github.com/andlabs/libui a0a980712e8057d86747fcf52b67f59b56932f5a

However, this is a keyword in D.