GCC -Wimplicit-fallthrough is not warning when I miss [[fallthrough]]

user_997112 0
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I am using GCC version 8.3 and have a switch statement:

#include <iostream>

enum class Type : char
{
    Value1 = 'A',
    Value2,
    Value3,
    Value4,
};

int main()
{
    Type t;

    switch(t)
    {
        case Type::Value1:
        case Type::Value2:
        case Type::Value3:
            std::cout << "hw" << std::endl;
        case Type::Value4:
            break;
    }
}

I want to be forced to use [[fallthrough]] otherwise generate a warning.

I added gcc flags to detect missing enums and implict fallthrough:

g++ -o main.cc -O3 -Wswitch-enum -Wimplicit-fallthrough -std=c++1z

The new flags have been detected because I now get warnings I'm missing enums. I fixed those but I still receive no warnings about implicit fallthrough.

I've put the code here:

https://godbolt.org/z/rj5sTPWz4

Am I missing something? I was expecting to be warned?

UPDATE:

I changed the code to this:

struct Object
{
    int a;
};

int main()
{
    Type t;
    Object o;

    switch(t)
    {
        case Type::Value1:
        case Type::Value2:
        case Type::Value3:
        case Type::Value4:
            o.a = 5;
            break;
    }
}

and it still doesn't generate a warning.

gcc-warning switch-statement gcc c++
2 Answers

I don't think it's considered fallthrough if you've got no statements in between the labels—that's just multiple case labels on a statement.

Gcc doesn't even warn on

    case Type::Value1:
    x++; //real fallthrough here
    case Type::Value2:
    case Type::Value3:
    case Type::Value4:
        break;

though (unlike clang), but it does warn on

    case Type::Value1:
    x++; //real fallthrough here
    case Type::Value2:
    x++;
    case Type::Value3:
    case Type::Value4:
        break;

https://godbolt.org/z/s8Ka84Gq6

so it seems you need both actual fallthrough (on both gcc and clang) and into something that has an effect in order to elicit the warning (on gcc).

user_10847740 
    case Type::Value4:
        break;

Because there is just break there, there is no difference, if you fall through or not. Add something there.

    case Type::Value3:
        std::cout << "something";
    case Type::Value4:
        std::cout << "actually something else";
        break;
user_90727530
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