[Bug c++/108414] New: template meta programming

["amewo22370 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108414

            Bug ID: 108414
           Summary: template meta programming
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: amewo22370 at gmail dot com
  Target Milestone: ---

Created attachment 54276
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54276&action=edit
archive with source code and preprocessed files

gcc of version 12.2.0 and linux 5.15.83-1-lts

I have attached the source code. If you compile test code with line 36
commented out and line 37 uncommented, then the compiler gives an error. But if
you uncomment line 36 and comment out line 37, the compiler compiles the code
without errors, but the result is incorrect.

Description of the code: template alias 'template<std::size_t I, typename T>
using nth_type_t;' must return type with index I, that contains in type list
'T'.

Comilator commands: 
gcc -v -save-temps -std=c++20 -lstdc++ test_line36.cpp -o test_line36
gcc -v -save-temps -std=c++20 -lstdc++ test_line37.cpp -o test_line37

Also added attachments according to bug reporting instructions.

Compilation error with 'gcc -v -save-temps -std=c++20 -lstdc++ test_line37.cpp
-o test_line37':

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/12.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++,d --enable-bootstrap
--prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/
--with-build-config=bootstrap-lto --with-linker-hash-style=gnu
--with-system-zlib --enable-__cxa_atexit --enable-cet=auto
--enable-checking=release --enable-clocale=gnu --enable-default-pie
--enable-default-ssp --enable-gnu-indirect-function --enable-gnu-unique-object
--enable-libstdcxx-backtrace --enable-link-serialization=1
--enable-linker-build-id --enable-lto --enable-multilib --enable-plugin
--enable-shared --enable-threads=posix --disable-libssp --disable-libstdcxx-pch
--disable-werror
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.2.0 (GCC) 
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++20' '-o' 'test_line37'
'-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-pc-linux-gnu/12.2.0/cc1plus -E -quiet -v -D_GNU_SOURCE
test_line37.cpp -mtune=generic -march=x86-64 -std=c++20 -fpch-preprocess -o
test_line37.ii
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-pc-linux-gnu/12.2.0/../../../../x86_64-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/x86_64-pc-linux-gnu/12.2.0/../../../../include/c++/12.2.0

/usr/lib/gcc/x86_64-pc-linux-gnu/12.2.0/../../../../include/c++/12.2.0/x86_64-pc-linux-gnu

/usr/lib/gcc/x86_64-pc-linux-gnu/12.2.0/../../../../include/c++/12.2.0/backward
 /usr/lib/gcc/x86_64-pc-linux-gnu/12.2.0/include
 /usr/local/include
 /usr/lib/gcc/x86_64-pc-linux-gnu/12.2.0/include-fixed
 /usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++20' '-o' 'test_line37'
'-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-pc-linux-gnu/12.2.0/cc1plus -fpreprocessed test_line37.ii
-quiet -dumpbase test_line37.cpp -dumpbase-ext .cpp -mtune=generic
-march=x86-64 -std=c++20 -version -o test_line37.s
GNU C++20 (GCC) version 12.2.0 (x86_64-pc-linux-gnu)
        compiled by GNU C version 12.2.0, GMP version 6.2.1, MPFR version
4.1.0-p13, MPC version 1.2.1, isl version isl-0.25-GMP

warning: MPFR header version 4.1.0-p13 differs from library version 4.1.1-p1.
warning: MPC header version 1.2.1 differs from library version 1.3.0.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C++20 (GCC) version 12.2.0 (x86_64-pc-linux-gnu)
        compiled by GNU C version 12.2.0, GMP version 6.2.1, MPFR version
4.1.0-p13, MPC version 1.2.1, isl version isl-0.25-GMP

warning: MPFR header version 4.1.0-p13 differs from library version 4.1.1-p1.
warning: MPC header version 1.2.1 differs from library version 1.3.0.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 402ce889a414e2a3abbbe3146fa0a6cb
test_line37.cpp: In substitution of ‘template<long unsigned int I, class T>
using nth_type_t = typename nth_type::type [with long unsigned int I = 0; T =
type_list<int, float, double>]’:
test_line37.cpp:49:36:   required from here
test_line37.cpp:41:7: error: no type named ‘type’ in ‘struct nth_type<0,
type_list<int, float, double> >’
   41 | using nth_type_t = typename nth_type<I, T>::type;
      |       ^~~~~~~~~~
test_line37.cpp: In substitution of ‘template<long unsigned int I, class T>
using nth_type_t = typename nth_type::type [with long unsigned int I = 1; T =
type_list<int, float, double>]’:
test_line37.cpp:50:36:   required from here
test_line37.cpp:41:7: error: no type named ‘type’ in ‘struct nth_type<1,
type_list<int, float, double> >’
test_line37.cpp: In substitution of ‘template<long unsigned int I, class T>
using nth_type_t = typename nth_type::type [with long unsigned int I = 2; T =
type_list<int, float, double>]’:
test_line37.cpp:51:36:   required from here
test_line37.cpp:41:7: error: no type named ‘type’ in ‘struct nth_type<2,
type_list<int, float, double> >’
test_line37.cpp: In function ‘int main()’:
test_line37.cpp:53:25: error: ‘elm0’ was not declared in this scope
   53 |     std::cout << typeid(elm0).name() << std::endl;
      |                         ^~~~
test_line37.cpp:54:25: error: ‘elm1’ was not declared in this scope
   54 |     std::cout << typeid(elm1).name() << std::endl;
      |                         ^~~~
test_line37.cpp:55:25: error: ‘elm2’ was not declared in this scope
   55 |     std::cout << typeid(elm2).name() << std::endl;
      |