Clang does not warn about missing returns when a switch statement covers all enum values

[jeremy-rifkin]

The following code triggers a missing return warning by gcc, msvc, and edg but not clang:

enum class E { a, b };

int foo(E e) {
    switch(e) {
        case E::a:
            return 20;
        case E::b:
            return 30;
    }
}

https://godbolt.org/z/b5r34nWf6

The main issue here is safety: This code is bug-prone due to extraneous enum values. The most common situation this could lead to bugs is for flag enums, e.g. https://godbolt.org/z/zx6nnPYMq.

Secondarily: This can lead to some annoyances where code compiles fine on clang but has a flood of warnings on gcc (possibly fatal ones, if using -Werror=return-type). This leads to PRs such as #122382 and #105520.

I'd like to propose changing the behavior to warn on this case by default. I briefly discussed this with folks on the llvm discord as well: https://discord.com/channels/636084430946959380/636732781086638081/1329124923322531850.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Jeremy Rifkin (jeremy-rifkin)

The following code triggers a missing return warning by gcc, msvc, and edg but not clang: ```cpp enum class E { a, b };

int foo(E e) {
switch(e) {
case E::a:
return 20;
case E::b:
return 30;
}
}

https://godbolt.org/z/b5r34nWf6

The main issue here is safety: This code is bug-prone due to extraneous enum values. The most common situation this could lead to bugs is for flag enums, e.g. https://godbolt.org/z/zx6nnPYMq.

Secondarily: This can lead to some annoyances where code compiles fine on clang but has a flood of warnings on gcc (possibly fatal ones, if using -Werror=return-type). This leads to PRs such as https://github.com/llvm/llvm-project/pull/122382 and https://github.com/llvm/llvm-project/pull/105520.

I'd like to propose changing the behavior to warn on this case by default. I briefly discussed this with folks on the llvm discord as well: https://discord.com/channels/636084430946959380/636732781086638081/1329124923322531850.
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