Consider making it an error to implicitly fall through without __fallthrough; annotation in switch cases

[Gnbrkm41]

I've just got bitten by implicit fallthroughs in #35713; That makes me wonder how hard it would be to make implicit fallthroughs be warnings / errors, or if it would be worth to do so?

The JIT coding convention states that fallthroughs should be annotated with __fallthrough; (https://github.com/dotnet/runtime/blob/master/docs/coding-guidelines/clr-jit-coding-conventions.md#124-switch-statements):

Fall-through between two case statements should be indicated with the __fallthrough; annotation. It should be surrounded by blank lines to make it maximally visible.

Given how we encourage people to flag fallthroughs explicitly and C# already makes it an error to omit break statements, would it be beneficial enough to make it an error/warning?

It appears that C++17 introduces [[fallthrough]]; attribute, and GNU/Clang recognises this when -Wimplicit-fallthrough argument is specified and makes these into warnings, but I'm not sure if VS has some flag to allow that, apart from this: https://docs.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warnings-by-compiler-version?view=vs-2019#warnings-introduced-in-visual-studio-2017-rtm-compiler-version-1910250170

[sharwell]

@stephentoub This is a C++ feature request. I'm not sure we can do anything about it here.

[stephentoub]

Oops. Moved the wrong one. Thanks.

[danmoseley]

@Gnbrkm41 would you be interested in enabling the warning to see how many violations it flags?

[danmoseley]

Setting coreclr label since this is largely a CoreCLR dev decision.

[Gnbrkm41]

I'm not exactly sure how one would enable the analyzer. I heard that for MSVC it's part of the "C++ Core" Style checks (C++ Core Style Rules, ES.78) but presumably we don't want to enable the whole suite of analyzers?

[danmoseley]

Hmm, I'll leave this to area owners to consider.

[jkotas]

We can start with -Wimplicit-fallthrough and clang.

[janvorli]

We can start with -Wimplicit-fallthrough and clang.

This would mean switching to C++17
Actually, I've tried that and clang seems to support it with C++11 as well.

[janvorli]

I've started working on it.