The sibling comment covers it a bit more in detail, but it's largely just some guessing and as much an art to figuring out what the types are or could be. This

The sibling comment covers it a bit more in detail, but it's largely just some guessing and as much an art to figuring out what the types are or could be. This particular one looks like it's taking a function pointer in and checking if it's a valid function (not null) and then checking the first two bytes of the function. From my memory, the windows ABI uses the first two bytes of functions for installing hooks/debugging by patching the first two bytes into some kind of jump (while originally being nops). The 0x2b part I'm not sure about myself but it looks like some other kind of similar checks. And actually then thinking about the way it's calling it, i'm wondering if this is actually from some C++ standard library code for doing stuff with a vtable, looking up the vtable entry and checking it's validity before calling it (in this case, location 0x18, and checking some kind of RTTI at 0x28 and 0x2b) and storing that it's been initialized in 0x21. So I think this might be part of an initialization function for some property on top of a object that exists at *param_1. and the decompiler has misinterpreted it as a longlong because of the access patterns (64bit pointers).