[Bug c++/83818] New: g++ class template parameter deduction discards const qualifier

[bugzilla.gcc.karo at cupdev dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83818

            Bug ID: 83818
           Summary: g++ class template parameter deduction discards const
                    qualifier
           Product: gcc
           Version: 7.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bugzilla.gcc.karo at cupdev dot net
  Target Milestone: ---

The following code should fail to compile, since `s` is declared const; the
assignment to u should fail because it discards the const qualifier.
The assignment to s.x should also fail because `s` is const, but the code
compiles successfully;

# Code

```
template<typename T>
struct tstruct {
  T x;
  tstruct(const T &x_) : x{x_} {}
};

int main() {
  const tstruct s{2};
  tstruct<int> &u = s;
  s.x = 2;
  return 0;
}
```

# Compiling the code above (with class template parameter deduction)

```
$ g++ --std=c++17 -Wall -Wextra -Wpedantic g.cc -o g                            
g.cc: In function ‘int main()’:                                                 
g.cc:9:17: warning: unused variable ‘u’ [-Wunused-variable]                     
   tstruct<int> &u = s; 
$ ls -al g                                                                      
-rwxr-xr-x 1 karo karo 8.3K Jan 12 18:11 g*
```

# With <int> explicitly added to avoid template parameter deduction

If I explicitly add the `<int>` template parameter to the declaration of `s`,
gcc yields the correct errors and does not produce a file:

```
$ g++ --std=c++17 -Wall -Wextra -Wpedantic g.cc -o g                            
g.cc: In function ‘int main()’:                                                 
g.cc:9:21: error: binding reference of type ‘tstruct<int>&’ to ‘const
tstruct<int>’ discards qualifiers                                 
   tstruct<int> &u = s;                                                         
                     ^                                                          
g.cc:10:9: error: assignment of member ‘tstruct<int>::x’ in read-only object    
   s.x = 2;                                                                     
         ^                                                                      
g.cc:9:17: warning: unused variable ‘u’ [-Wunused-variable]                     
   tstruct<int> &u = s;
```

# GCC Version

```
$ gcc -v                                                                        
Using built-in specs.                                                           
COLLECT_GCC=gcc                                                                 
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.1/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++ --enable-shared
--enable-threads=posix --enable-libmpx --with-system-zlib --with-isl
--enable-__cxa_atexit --disable-libunwind-exceptions --enable-clocale=gnu
--disable-libstdcxx-pch --disable-libssp --enable-gnu-unique-object
--enable-linker-build-id --enable-lto --enable-plugin
--enable-install-libiberty --with-linker-hash-style=gnu
--enable-gnu-indirect-function --enable-multilib --disable-werror
--enable-checking=release --enable-default-pie --enable-default-ssp
Thread model: posix
gcc version 7.2.1 20171128 (GCC)   
```