[libcxx] r294612 - Fix PR31916 - std::visit rejects visitors accepting lvalue arguments

[Eric Fiselier via cfe-commits]

I'm planning to merge this fix into the 4.0 release branch.

Although not a regression, since <variant> is a new feature, it would still
be nice to ship without this bug.

I'll merge these changes later today if there are no objections, and once
all the bots pass.

/Eric

On Thu, Feb 9, 2017 at 12:01 PM, Eric Fiselier via cfe-commits <
cfe-commits at lists.llvm.org> wrote:

> Author: ericwf
> Date: Thu Feb  9 13:01:22 2017
> New Revision: 294612
>
> URL: http://llvm.org/viewvc/llvm-project?rev=294612&view=rev
> Log:
> Fix PR31916 - std::visit rejects visitors accepting lvalue arguments
>
> A static assertion was misfiring since it checked
> is_callable<Visitor, decltype(__variant_alt<T>.value)>. However
> the decltype expression doesn't capture the value category as
> required. This patch applies extra braces to decltype to fix
> that.
>
> Modified:
>     libcxx/trunk/include/variant
>     libcxx/trunk/test/std/utilities/variant/variant.visit/visit.pass.cpp
>
> Modified: libcxx/trunk/include/variant
> URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/
> variant?rev=294612&r1=294611&r2=294612&view=diff
> ============================================================
> ==================
> --- libcxx/trunk/include/variant (original)
> +++ libcxx/trunk/include/variant Thu Feb  9 13:01:22 2017
> @@ -578,7 +578,7 @@ private:
>      constexpr decltype(auto) operator()(_Alts&&... __alts) const {
>        __std_visit_exhaustive_visitor_check<
>            _Visitor,
> -          decltype(_VSTD::forward<_Alts>(__alts).__value)...>();
> +          decltype((_VSTD::forward<_Alts>(__alts).__value))...>();
>        return __invoke_constexpr(_VSTD::forward<_Visitor>(__visitor),
>                                  _VSTD::forward<_Alts>(__alts).
> __value...);
>      }
>
> Modified: libcxx/trunk/test/std/utilities/variant/variant.
> visit/visit.pass.cpp
> URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/test/std/
> utilities/variant/variant.visit/visit.pass.cpp?rev=
> 294612&r1=294611&r2=294612&view=diff
> ============================================================
> ==================
> --- libcxx/trunk/test/std/utilities/variant/variant.visit/visit.pass.cpp
> (original)
> +++ libcxx/trunk/test/std/utilities/variant/variant.visit/visit.pass.cpp
> Thu Feb  9 13:01:22 2017
> @@ -283,9 +283,20 @@ void test_exceptions() {
>  #endif
>  }
>
> +// See http://llvm.org/PR31916
> +void test_caller_accepts_nonconst() {
> +  struct A {};
> +  struct Visitor {
> +    void operator()(A&) {}
> +  };
> +  std::variant<A> v;
> +  std::visit(Visitor{}, v);
> +}
> +
>  int main() {
>    test_call_operator_forwarding();
>    test_argument_forwarding();
>    test_constexpr();
>    test_exceptions();
> +  test_caller_accepts_nonconst();
>  }
>
>
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