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

[Richard Smith via cfe-dev]

(For clarity, since you didn't say, I'm assuming this is a question about
C++, and I'm going to assume you mean Clang's default C++ mode, C++14.)

On Tue, 18 Dec 2018 at 12:17, will wray via cfe-dev <cfe-dev at lists.llvm.org>
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?
>

Nope, this is valid, and I'm frankly stunned that GCC rejects.

C++14 [dcl.init.aggr]/2 says: "Each member is copy-initialized from the
corresponding initializer-clause. [...] [ Note: If an initializer-clause is
itself an initializer list, the member is list-initialized, which will
result in a recursive application of the rules in this section if the
member is an aggregate.  — end note ]"
C++14 [dcl.init.list]/3.5: "Otherwise, if the initializer list has a single
element of type E and either T is not a reference type or its referenced
type is reference-related to E, the object or reference is initialized from
that element;"

In fact, it's valid in every revision of C++ since C++11. (It was
ill-formed in C++98, which allowed int n = {0}, but not S s = {{0}};, but
the syntax you used was added in C++11.)

The fact that clang accepts

struct S { int i; };
S s = {{0}};

in C++98 mode under -pedantic-errors is a (minor) bug.

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)
>

See above.


> h1501. Nested braces in list-initialization
> <http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_closed.html#1501>
>

As noted here, one set of braces around a scalar initialization is
permitted, and two or more sets is invalid. So:

S s = {{{0}}}; // error

and indeed for that Clang reports

error: too many braces around scalar initializer
[-Werror,-Wmany-braces-around-scalar-init]
S s = {{{0}}};
        ^~~

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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