[Bug c++/114446] New: Spurious "inconsistent parameter pack deduction" error when constraining template function

[Bug c++/114446] New: Spurious "inconsistent parameter pack deduction" error when constraining template function

[gccbugzilla at maycontaincode dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114446

            Bug ID: 114446
           Summary: Spurious "inconsistent parameter pack deduction" error
                    when constraining template function
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gccbugzilla at maycontaincode dot com
  Target Milestone: ---

Created attachment 57790
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=57790&action=edit
Minimal C++ source file that reproduces the issue

I have a file "bug.cpp", a 24-line C++ source file that does not include any
headers, its full content is at the bottom of this description. Within, there
are two template functions that each take a typename, a parameter pack of types
deduced from the function arguments, and a non-type template parameter with
default value. One of them uses an enable_if construct for the nontype template
argument to constrain based on the first template argument, but in this file it
never causes SFINAE (which is correct). I try a few methods of calling these
functions, each time passing the same two template arguments and the same one
function argument.

The behavior that seems incorrect to me is that GCC apparently gives different
treatment for the template function that is constrained unless the function
argument is casted to the type it would have been implicitly converted to
anyway. The unconstrained template doesn't care that the function parameter
doesn't exactly match the explicitly-specified template parameter, it happily
performs the implicit conversion. Everything else is the same so I do not
understand what could be causing this behavior. I also tried replacing the
enable_if construct with a template using alias that doesn't constrain, and GCC
accepts that without error, so it seems like the issue only happens when using
template class partial specialization.

I am unable to reproduce the issue in MSVC 14.40.33617 or Clang 18.1.0, they
both accept "bug.cpp" without incident.

I did some searching through other bug reports, but could not find anything
that looked related, and searching the exact error message in summary-only mode
actually gave me zero results.

Output of gcc -v:

> Using built-in specs.
> COLLECT_GCC=gcc
> COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/12/lto-wrapper
> OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
> OFFLOAD_TARGET_DEFAULT=1
> Target: x86_64-linux-gnu
> Configured with: ../src/configure -v --with-pkgversion='Debian 12.2.0-14' --with-bugurl=file:///usr/share/doc/gcc-12/README.Bugs --enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-12 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --enable-libphobos-checking=release --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none=/build/gcc-12-bTRWOB/gcc-12-12.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-12-bTRWOB/gcc-12-12.2.0/debian/tmp-gcn/usr --enable-offload-defaulted --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
> Thread model: posix
> Supported LTO compression algorithms: zlib zstd
> gcc version 12.2.0 (Debian 12.2.0-14)

I have also used Compiler Explorer to try "x86-64 gcc 5.1", "x86-64 gcc 13.2"
and "x86-64 gcc (trunk)", all three produce the same issue as my local GCC
version: https://gcc.godbolt.org/z/9b6faWanW

Command line that produces the issue:

> g++ -std=c++17 bug.cpp

Output of above command:

> bug.cpp: In function ‘void runtest2b()’:
> bug.cpp:22:25: error: no matching function for call to ‘test2<int, int*>(std::nullptr_t)’
>    22 |         test2<int, int*>(nullptr); //3
>       |         ~~~~~~~~~~~~~~~~^~~~~~~~~
> bug.cpp:11:6: note: candidate: ‘template<class T, class ... Args, typename enable_if_not_void<T, int>::type <anonymous> > void test2(Args&& ...)’
>    11 | void test2(Args&&...){}
>       |      ^~~~~
> bug.cpp:11:6: note:   template argument deduction/substitution failed:
> bug.cpp:22:25: note:   inconsistent parameter pack deduction with ‘int*’ and ‘std::nullptr_t’
>    22 |         test2<int, int*>(nullptr); //3
>       |         ~~~~~~~~~~~~~~~~^~~~~~~~~

Adding -save-temps to the command line generates a file "a-bug.ii" with this
content, which is identical to the "bug.cpp" file after the first several
lines, other than omitting irrelevant comments:

# 0 "bug.cpp"
# 0 "<built-in>"
# 0 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 0 "<command-line>" 2
# 1 "bug.cpp"

template<typename, typename T> struct enable_if_not_void { using type = T; };
template<typename T> struct enable_if_not_void<void, T> {};

template<typename V, typename T>
using enable_if_not_void_t = typename enable_if_not_void<V, T>::type;

template<typename T, typename... Args, int = 0>
void test1(Args&&...){}
template<typename T, typename... Args, enable_if_not_void_t<T, int> = 0>
void test2(Args&&...){}
void runtest1()
{
 test1<int, int*>(nullptr);
}
void runtest2a()
{
 test2<int, int*>(static_cast<int*>(nullptr));
}
void runtest2b()
{
 test2<int, int*>(nullptr);
}