[Bug c++/70774] New: constexpr function with reference parameter gives reinterpret_cast from integer to pointer error

[steven.spark at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70774

            Bug ID: 70774
           Summary: constexpr function with reference parameter gives
                    reinterpret_cast from integer to pointer error
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: steven.spark at gmail dot com
  Target Milestone: ---

Here is the simplified code:

--------------
#define PORTX (*(volatile unsigned char *)(0x1B))

constexpr volatile unsigned char * testRef(volatile unsigned char & x) {
        return &x;
}
volatile unsigned char* addr = testRef(PORTX); /// OK

constexpr volatile unsigned char* addr2 = testRef(PORTX); /// error: 
   /// reinterpret_cast from integer to pointer
--------------


This compiled without problem with gcc 4.8.1, but fails with 4.9.2.


Using built-in specs.
COLLECT_GCC=avr-g++.exe
Target: avr
Configured with:
/home/toolsbuild/workspace/avr8-gnu-toolchain/src/gcc/configure
LDFLAGS=-L/home/toolsbuild/workspace/avr8-gnu-toolchain/avr8-gnu-toolchain-win32_x86-hostlibs/lib
CPPFLAGS= --target=avr --host=i686-w64-mingw32 --build=x86_64-pc-linux-gnu
--prefix=/home/toolsbuild/workspace/avr8-gnu-toolchain/avr8-gnu-toolchain-win32_x86
--libdir=/home/toolsbuild/workspace/avr8-gnu-toolchain/avr8-gnu-toolchain-win32_x86/lib
--enable-languages=c,c++ --with-dwarf2 --enable-doc --disable-shared
--disable-libada --disable-libssp --disable-nls --with-avrlibc=yes
--with-mpfr=/home/toolsbuild/workspace/avr8-gnu-toolchain/avr8-gnu-toolchain-win32_x86-hostlibs
--with-gmp=/home/toolsbuild/workspace/avr8-gnu-toolchain/avr8-gnu-toolchain-win32_x86-hostlibs
--with-mpc=/home/toolsbuild/workspace/avr8-gnu-toolchain/avr8-gnu-toolchain-win32_x86-hostlibs
--enable-win32-registry=avrtoolchain
--with-pkgversion=AVR_8_bit_GNU_Toolchain_3.5.0_1662
--with-bugurl=http://www.atmel.com
Thread model: single
gcc version 4.9.2 (AVR_8_bit_GNU_Toolchain_3.5.0_1662) 



I'm writing code for AVR micro controller where every byte counts, so compile
time evaluation is key. I was trying out the new Atmel Studio 7 IDE which comes
with the new 4.9.2 gcc when I came across this problem. In my real world code
I'm using constexpr constructor and constexpr instances of my pin handler class
- to make life easier and encapsulate port, data direction and other registers. 

Note: If I use pointer instead of reference then it compiles fine:

constexpr volatile unsigned char * testAddr(volatile unsigned char * x) {
        return x;
}
constexpr volatile unsigned char* addr3 = testAddr(&PORTX); /// OK


Why is this? Am I doing something wrong? How can I use (port) references in
constexpr functions?

This also does NOT work:

constexpr volatile unsigned char& ref5 = PORTX; /// error