impy | Python3 library with features | Machine Learning library

First of all: if you want to use the output of fillna you should add inplace=True to your command or assign this line to another variable. If you'll assign it to antoher variable (new_df = df.fillna(0)) you will also avoid displaying the output.

Instead you can add another line to the block with output that can help you understand the data:

Here's roughly how overload resolution works:

1. Name lookup to find candidate functions and function templates.
2. Deduce template arguments for each function template, and substitute deduced arguments into the template to generate a single function template specialization as the candidate. Throw out any function template for which deduction fails (including substitution failure).
3. Compare the candidates. Pick the best one or complain if there isn't one.

If step 2 triggers a hard error - by forming an invalid construct outside the immediate context of the function template signature - then your program is ill-formed; you never get to step 3. It doesn't matter if the candidate wouldn't have been chosen in step 3 if the error wasn't there.

Here, decltype(T_() + U_()) was in the immediate context originally. Therefore, when U_ got deduced to Wrapper<...> and substituted into the signature, that expression is ill-formed, but the error is in the immediate context and so it's a substitution failure. But when you move that expression into a separate class template Result, the error is no longer in the immediate context of the function template's signature, so it's a hard error instead.

If you don't want to repeat the expression multiple times, use an alias template: