[clang-tidy] cppcoreguidelines-pro-type-member-init check flags anonymous union member

[paulaltin]

Running clang-tidy on this code:

struct s {
    union {
        int a = 0;
        int b;
    };
};

results in the following warning:

warning: constructor does not initialize these fields: b [cppcoreguidelines-pro-type-member-init]

However, adding an initializer to b (e.g. int b = 0) results in a clang diagnostic:

error: initializing multiple members of union [clang-diagnostic-error]

[llvmbot]

@llvm/issue-subscribers-clang-tidy

[paulaltin]

The cppcoreguidelines-pro-type-member-init.cpp file has a test that ensures the check does not flag a union with only one member initialized:

llvm-project/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines-pro-type-member-init.cpp

Line 296 in 6f73bd7

union NegativeUnionInClass {

and a test that ensures the check does flag members of anonymous unions when none are initialized:

llvm-project/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines-pro-type-member-init.cpp

Line 332 in 6f73bd7

union {

(both authored by @alexfh).

[paulaltin]

A similar issue was reported in 2017: #34232

At the time, @JonasToth suggested the bug might be related to this commit: llvm-mirror/clang-tools-extra@b7c157e

Since then, a commit by @Sockke was added to fix an apparently related issue: e490248

[dfrib]

The same warning is emitted also for when initializing one of the union members in a mem-init-list (as opposed to default mem initializers in the union), flagging e.g. the idiomatic std::optional backports for constexpr default-construction of disengaged optionals:

struct Optional {
    // warning: constructor does not initialize these fields: value_ 
    // [cppcoreguidelines-pro-type-member-init,hicpp-member-init]
    constexpr Optional() : dummy_{} {}
    union {
        char dummy_;
        int value_;
    };
};

[movie-travel-code]

Btw, the review differential of the fix patched by @Sockke is https://reviews.llvm.org/D108370. IMHO, clang-tidy should be conservative when emitting the diagnostics, if the clang-tidy cannot afford the overhead or has no confidence to cover all the cases, it should give up emitting the warnings.

[Sockke]

The latest patch is here: https://reviews.llvm.org/D127293, and there are still omissions in the processing of a union.

[Sockke]

Fixed

[paulaltin]

Thanks for your work fixing this and getting it through review @Sockke!