build: Disable -Wbraced-scalar-init, which is incompatible with -Wc++11-narrowing #23191

To test:

void Fun(int) {}
int main() {
    unsigned a(-1);
    Fun({a});
}

Before:

$ clang++ -Wbraced-scalar-init -Wc++11-narrowing -std=c++17 /tmp/1.cpp -o /tmp/exe 
/tmp/1.cpp:4:10: error: non-constant-expression cannot be narrowed from type 'unsigned int' to 'int' in initializer list [-Wc++11-narrowing]
    Fun({a});
         ^
/tmp/1.cpp:4:10: note: insert an explicit cast to silence this issue
    Fun({a});
         ^
         static_cast<int>( )
/tmp/1.cpp:4:9: warning: braces around scalar initializer [-Wbraced-scalar-init]
    Fun({a});
        ^~~
1 warning and 1 error generated.

After:

$ clang++ -Wno-braced-scalar-init -Wc++11-narrowing -std=c++17 /tmp/1.cpp -o /tmp/exe 
/tmp/1.cpp:4:10: error: non-constant-expression cannot be narrowed from type 'unsigned int' to 'int' in initializer list [-Wc++11-narrowing]
    Fun({a});
         ^
/tmp/1.cpp:4:10: note: insert an explicit cast to silence this issue
    Fun({a});
         ^
         static_cast<int>( )
1 error generated.

Concept ACK

Why is it incompatible? In the output above there is 1 error due to Wc++11-narrowing and 1 warning due to Wbraced-scalar-init. When the latter is disabled the warning is not shown. I don't see the relation (incompatibility) between the two options.

OTOH, F({a}) looks wrong, I would like the compiler to point it to me so that I can fix it to F(a), no?

Is the intention to use F({a}) with Wc++11-narrowing to get a warning for passing "incompatible" types? Could the same be achieved by F(a) with Wimplicit-int-conversion and/or Wsign-conversion?

OTOH, F({a}) looks wrong, I would like the compiler to point it to me so that I can fix it to F(a), no?

F(a) will disable -Wc++11-narrowing.

Could the same be achieved by F(a) with Wimplicit-int-conversion and/or Wsign-conversion?

It is currently not possible to compile Bitcoin Core with either of those settings.

<!--e57a25ab6845829454e8d69fc972939a-->

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

<!--174a7506f384e20aa4161008e828411d-->

Conflicts

Reviewers, this pull request conflicts with the following ones:

• #23269 (build: remove redundant warning flags by fanquake)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

<!--cf906140f33d8803c4a75a2196329ecb-->

🐙 This pull request conflicts with the target branch and needs rebase.

_{Want to unsubscribe from rebase notifications on this pull request? Just convert this pull request to a "draft".}