llvm dev - Dec 2021 - clang-tidy: hard to repro bug in the 'modernize-use-default-member-init' check

Folks, I'm trying understand a `clang-tidy` bug where the
'modernize-use-default-member-init' check misbehaves on this (reduced)
input:

struct Thing {
    Thing(int arg) : a(arg), b(0) {}
    int a, b;
};

The check works correctly when running `clang-tidy` without a compilation
DB. The tool complains, but works correctly. Things also work correctly in
the LIT tests...

Yet when running with a compilation DB, I get this output after fixes are
applied (notice the trailing comma, which makes the code ill-formed):

struct Thing {
    Thing(int arg) : a(arg), {}
    int a, b{0};
};

How do I write a unit test for this? I mean how do I create a reliable
repro?

Thanks,
Oleg.
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I'd probably start by analyzing the bug, maybe figuring out the root cause
- then work backwards for the test case to commit along with the test.
(figuring out the root cause might reveal easier ways to reproduce the bug,
once it's understood)

To root cause the bug, I'd try parallel debugging sessions - with and
without a compilation DB, for instance - stepping through each, and seeing
where the behavior diverges, and why it diverges.

On Mon, Dec 6, 2021 at 3:33 PM Oleg Smolsky via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Folks, I'm trying understand a `clang-tidy` bug where the
> 'modernize-use-default-member-init' check misbehaves on this
(reduced)
> input:
>
> struct Thing {
>     Thing(int arg) : a(arg), b(0) {}
>     int a, b;
> };
>
> The check works correctly when running `clang-tidy` without a compilation
> DB. The tool complains, but works correctly. Things also work correctly in
> the LIT tests...
>
> Yet when running with a compilation DB, I get this output after fixes are
> applied (notice the trailing comma, which makes the code ill-formed):
>
> struct Thing {
>     Thing(int arg) : a(arg), {}
>     int a, b{0};
> };
>
> How do I write a unit test for this? I mean how do I create a reliable
> repro?
>
> Thanks,
> Oleg.
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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