GCC expands the pointer type in this conditional expression even for
template types _Up that are not arrays. This raises an error when
std::decay<> is used with reference types (as is done in LLVM's
sources). Using add_pointer<> causes GCC to only instantiate a
pointer type for array types.
Upstream LLVM has at least one commit aimed at addressing this GCC
behavior, but it is not directly back-portable as upstream has also
split out type_traits into several internal headers. The upstream
commit (5fab33af7f083a0043112742027172e9f297c07f) also does not
include this fix here. I attempted to backport the equivalent
upstream fix (just using __remove_extent_t<> instead of typename
remove_extent<>) but it did not fix the compile error with GCC.
```
In file included from /usr/obj/usr/src/amd64.amd64/tmp/usr/include/c++/v1/__compare/ordering.h:13,
from /usr/obj/usr/src/amd64.amd64/tmp/usr/include/c++/v1/__compare/common_comparison_category.h:12,
from /usr/obj/usr/src/amd64.amd64/tmp/usr/include/c++/v1/tuple:168,
from /usr/src/contrib/llvm-project/llvm/include/llvm/ADT/DenseMapInfo.h:20,
from /usr/src/contrib/llvm-project/llvm/include/llvm/ADT/DenseMap.h:17,
from /usr/src/contrib/llvm-project/llvm/lib/Transforms/Scalar/GVNHoist.cpp:36:
/usr/obj/usr/src/amd64.amd64/tmp/usr/include/c++/v1/type_traits: In instantiation of 'struct std::__1::__decay<llvm::CHIArg&, true>':
/usr/obj/usr/src/amd64.amd64/tmp/usr/include/c++/v1/type_traits:1591:89: required from 'struct std::__1::decay<llvm::CHIArg&&>'
/usr/obj/usr/src/amd64.amd64/tmp/usr/include/c++/v1/__utility/pair.h:132:16: required by substitution of 'template<class _Tuple, typename std::__1::enable_if<typename std::__1::conditional<(std::__1::__tuple_like_with_size<_Tuple, 2, typename std::__1::__uncvref<_Tp>::type>::value && (! std::__1::is_same<typename std::__1::decay<_Tp>::type, std::__1::pair<llvm::BasicBlock*, llvm::SmallVector<llvm::CHIArg, 2> > >::value)), std::__1::pair<llvm::BasicBlock*, llvm::SmallVector<llvm::CHIArg, 2> >::_CheckTupleLikeConstructor, std::__1::__check_tuple_constructor_fail>::type::__enable_implicit<_Tuple>(), void>::type* <anonymous> > constexpr std::__1::pair<llvm::BasicBlock*, llvm::SmallVector<llvm::CHIArg, 2> >::pair(_Tuple&&) [with _Tuple = llvm::CHIArg&&; typename std::__1::enable_if<typename std::__1::conditional<(std::__1::__tuple_like_with_size<_Tuple, 2, typename std::__1::__uncvref<_Tp>::type>::value && (! std::__1::is_same<typename std::__1::decay<_Tp>::type, std::__1::pair<llvm::BasicBlock*, llvm::SmallVector<llvm::CHIArg, 2> > >::value)), std::__1::pair<llvm::BasicBlock*, llvm::SmallVector<llvm::CHIArg, 2> >::_CheckTupleLikeConstructor, std::__1::__check_tuple_constructor_fail>::type::__enable_implicit<_Tuple>(), void>::type* <anonymous> = <missing>]'
/usr/src/contrib/llvm-project/llvm/lib/Transforms/Scalar/GVNHoist.cpp:892:51: required from here
/usr/obj/usr/src/amd64.amd64/tmp/usr/include/c++/v1/type_traits:1582:30: error: forming pointer to reference type 'std::__1::remove_extent<llvm::CHIArg&>::type' {aka 'llvm::CHIArg&'}
1582 | >::type type;
| ^~~~
```