Another technique I often employ is to grep for the error message, in the case of #16486 "cannot deduce function from argument types".  Once the location of where the error message is emitted, I work back from there to answer the question "why?".

Here's where the error is emitted from:  https://github.com/dlang/dmd/blob/e58579a73438e62a16c0c43dc33ce8d36b520edd/src/dmd/func.d#L2788-L2790

Unfortunately, that resolution code is very difficult to follow and will likely require some lengthy debugging.

Mike