[Bug c++/97341] New: Implicit casting from char** to const char* const* in template argument failing

[reikdas at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97341

            Bug ID: 97341
           Summary: Implicit casting from char** to const char* const* in
                    template argument failing
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: reikdas at gmail dot com
  Target Milestone: ---

Created attachment 49332
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49332&action=edit
preprocessed file

Implicit casting of char** to a const char* const* by passing a char** variable
to a class template's argument like - 

char *bar = (char*)"Hello1";
constexpr char** baz = &bar;
template <const char* const*> class foo {};

int main()
{
    foo<baz> Foo;
    return 1;
}

gives a compilation error - 

test.cpp:7:12: error: the address of the ‘const char* const’ subobject of ‘bar’
is not a valid template argument
    7 |     foo<baz> Foo;
      |            ^


Output of g++ -v -save-temps -std=c++17 test.cpp - 

Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --prefix=/usr --libdir=/usr/lib
--libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++,d --with-isl
--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-install-libiberty --enable-linker-build-id
--enable-lto --enable-multilib --enable-plugin --enable-shared
--enable-threads=posix --disable-libssp --disable-libstdcxx-pch
--disable-libunwind-exceptions --disable-werror
gdc_include_dir=/usr/include/dlang/gdc
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 10.2.0 (GCC) 
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++17' '-shared-libgcc'
'-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/cc1plus -E -quiet -v -D_GNU_SOURCE
test.cpp -mtune=generic -march=x86-64 -std=c++17 -fpch-preprocess -o test.ii
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../x86_64-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../include/c++/10.2.0

/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../include/c++/10.2.0/x86_64-pc-linux-gnu

/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../include/c++/10.2.0/backward
 /usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include
 /usr/local/include
 /usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include-fixed
 /usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++17' '-shared-libgcc'
'-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/cc1plus -fpreprocessed test.ii -quiet
-dumpbase test.cpp -mtune=generic -march=x86-64 -auxbase test -std=c++17
-version -o test.s
GNU C++17 (GCC) version 10.2.0 (x86_64-pc-linux-gnu)
        compiled by GNU C version 10.2.0, GMP version 6.2.0, MPFR version
4.1.0, MPC version 1.1.0, isl version isl-0.21-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C++17 (GCC) version 10.2.0 (x86_64-pc-linux-gnu)
        compiled by GNU C version 10.2.0, GMP version 6.2.0, MPFR version
4.1.0, MPC version 1.1.0, isl version isl-0.21-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 2affcd71b0797e2421aae734ab600c81
test.cpp: In function ‘int main()’:
test.cpp:7:12: error: the address of the ‘const char* const’ subobject of ‘bar’
is not a valid template argument
    7 |     foo<baz> Foo;
      |            ^