clang-tidy v. -Weffc++

Context

Recent versions of clang-tidy added readability-redundant-member-init: https://clang.llvm.org/extra/clang-tidy/checks/readability-redundant-member-init.html

This is handy! When a class member has a default initializer it is redundent to initialize it in the member list.

But, because some types don't have default initializers, forgetting to initialize them in the member list (or using c++11 initialization in the class definition) can lead to serious trouble like mapbox/wagyu#69 - refs mapbox/wagyu#70.

So, this is the reason, at https://github.com/mapbox/cpp/issues/37#issuecomment-336200744, that we recommend using g++ and the -Weffc++ flag because it can catch this (note, clang++ plus -Weffc++ cannot):

-Weffc++ - useful when building with g++ (does not do much with clang++). With g++ it can catch uninitialized class members and prevent crashes like mapbox/wagyu#69 - refs mapbox/wagyu#70

Problem

• -Weffc++ will warn on all class members not explicitly initialized in the initializer list
• clang-tidy will automatically remove variables from the initializer list that have default constructors

So, the two will fight: causing each other warnings. For this reason I think we should likely:

• let clang-tidy win
• recommend no longer using g++ with -Weffc++
• figure out what alternative way we can catch when members, without default initializers, are uninitialized (without needing to use -Weffc++). Maybe another clang-tidy check?

Start with clang-tidy's readability-redundant-member-init documentation and the linked mapbox/cpp#37, wagyu#69, and wagyu#70 discussions; no repository file or test is identified. Determine an agreed approach for -Weffc++, clang-tidy guidance, and detecting uninitialized members; done requires a documented recommendation and configuration.