[cfe-dev] 'braces around scalar initializer' warning, should be error?

[will wray via cfe-dev]

(^^^ sorry for the duff formatting)

Promoting warnings to errors doesn't work as I'd hoped
as the resulting errors are not detected in requires clauses:

https://concept.godbolt.org/z/TBz2Bl
using flags
  -Werror=braced-scalar-init
  -Werror=many-braces-around-scalar-init

    struct S { int i; };

    template <class S>
    inline constexpr bool braces0 = requires { S{0}; };

    static_assert(braces0<S>);

    template <class S>
    inline constexpr bool braces1 = requires { S{{0}}; };

    static_assert(!braces1<S>);

The second assert fails on Clang (experimental concepts build)


On Tue, Dec 18, 2018 at 3:16 PM will wray <wjwray at gmail.com> wrote:

> *    struct S { int i; };*
>
> *    S s{{0}};*
> *Clang*: `*warning*: braces around scalar initializer
> [-Wbraced-scalar-init]`
>
> or, with more braces:
>
>  `*warning*: too many braces around scalar initializer
> [-Wmany-braces-around-scalar-init]
> (consistent as a warning since at least clang 3.0.0)
>
> *GCC*: `*error*: braces around scalar initializer for type 'int'`
>
> (consistent as an error since ~gcc 4.4.7)
>
> *MSVC*: no warning (with \Wall)
> *Intel*: warning #1119: extra braces are nonstandard
>
> I believe that this is a non-standard extension in Clang. Can anyone
> confirm?
> Does the standard does mandate an error here?
> If so then is there any flag to make this an error in Clang?
> (-pedantic-errors flag makes no difference, for gcc or for clang.)
> Maybe it's for MSVC / EDG compatibility? Or C compatibility? What reason?
>
> An error allows compile-time code to distinguish between scalar and
> non-scalar,
> to count the number of allowable braces and so to detect array rank & dims.
>
> On the other hand, an error forces different syntax for initializing
> scalar and non-scalar
> so disallows generic 'uniform' braced-init syntax where there may be
> scalars.
>
> Thanks for any thoughts,
> Will
>
> Here are some issues/DRs & commits that may or may not be relevant
> An old pre-11 issue
> 632. Brace-enclosed initializer for scalar member of aggregate
> <http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#632>
> explains that this was a C/C++ incompatibility, says it was resolved by Initializer
> Lists <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2672.htm>
> (LOL) (I can't see how or where it is resolved)
> h1501. Nested braces in list-initialization
> <http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_closed.html#1501>
> As part of changes for DR1467 List-initialization of aggregate from
> same-type object
> <http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1467>
> this Clang commit
> r228896
> <http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20150209/123171.html>-
> Improve the "braces around scalar init" warning to determine whether to warn
>
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