Sorry, unimplemented: 64 bit mode not compiled in - c++

Question

This is what I'm trying to do(on Windows 7 64-bits):
g++ -m64 -o main main.cpp
And the error I get is:
Sorry, unimplemented: 64 bit mode not compiled in ...
I installed mingw-64-install.exe from here: https://sourceforge.net/projects/mingw-w64/files/Toolchains%20targetting%20Win32/Personal%20Builds/mingw-builds/installer/
But I still get the same error. Do I also need to add the path to this folder in the environment variable?
This is what I get when I type g++ -v:
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=c:/mingw/bin/../libexec/gcc/mingw32/4.9.3/lto-wrapper.exe
Target: mingw32
Configured with: ../src/gcc-4.9.3/configure --build=x86_64-pc-linux-gnu --host=m
ingw32 --prefix=/mingw --disable-win32-registry --target=mingw32 --with-arch=i58
6 --enable-languages=c,c++,objc,obj-c++,fortran,ada --enable-static --enable-sha
red --enable-threads --with-dwarf2 --disable-sjlj-exceptions --enable- version-sp
ecific-runtime-libs --enable-libstdcxx-debug --with-tune=generic --enable-nls
Thread model: win32​
gcc version 4.9.3 <GCC>

Answer 1

When running mingw-64-install.exe, you have to choose the x86_64 architecture on the settings screen. With this architecture, 64-bit is also the default target. If you run g++ -v, you should get something like:
C:\>g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=C:/Program\ Files/mingw-w64/x86_64-5.3.0-posix-seh-rt_v4-rev0/mingw64/bin/../libexec/gcc/x86_64-w64-mingw32/5.3.0/lto-wrapper.exe
Target: x86_64-w64-mingw32
Configured with: ../../../src/gcc-5.3.0/configure --host=x86_64-w64-mingw32 --build=x86_64-w64-mingw32 --target=x86_64-w64-mingw32 --prefix=/mingw64 --with-sysroot=/c/mingw530/x86_64-530-posix-seh-rt_v4-rev0/mingw64 --with-gxx-include-dir=/mingw64/x86_64-w64-mingw32/include/c++ --enable-shared --enable-static --disable-multilib --enable-languages=c,c++,fortran,lto --enable-libstdcxx-time=yes --enable-threads=posix --enable-libgomp --enable-libatomic --enable-lto --enable-graphite --enable-checking=release --enable-fully-dynamic-string --enable-version-specific-runtime-libs --disable-isl-version-check --disable-libstdcxx-pch --disable-libstdcxx-debug --enable-bootstrap --disable-rpath --disable-win32-registry --disable-nls --disable-werror --disable-symvers --with-gnu-as --with-gnu-ld --with-arch=nocona --with-tune=core2 --with-libiconv --with-system-zlib --with-gmp=/c/mingw530/prerequisites/x86_64-w64-mingw32-static --with-mpfr=/c/mingw530/prerequisites/x86_64-w64-mingw32-static --with-mpc=/c/mingw530/prerequisites/x86_64-w64-mingw32-static --with-isl=/c/mingw530/prerequisites/x86_64-w64-mingw32-static --with-pkgversion='x86_64-posix-seh-rev0, Built by MinGW-W64 project' --with-bugurl=http://sourceforge.net/projects/mingw-w64 CFLAGS='-O2 -pipe -I/c/mingw530/x86_64-530-posix-seh-rt_v4-rev0/mingw64/opt/include -I/c/mingw530/prerequisites/x86_64-zlib-static/include -I/c/mingw530/prerequisites/x86_64-w64-mingw32-static/include' CXXFLAGS='-O2 -pipe -I/c/mingw530/x86_64-530-posix-seh-rt_v4-rev0/mingw64/opt/include -I/c/mingw530/prerequisites/x86_64-zlib-static/include -I/c/mingw530/prerequisites/x86_64-w64-mingw32-static/include' CPPFLAGS= LDFLAGS='-pipe -L/c/mingw530/x86_64-530-posix-seh-rt_v4-rev0/mingw64/opt/lib -L/c/mingw530/prerequisites/x86_64-zlib-static/lib -L/c/mingw530/prerequisites/x86_64-w64-mingw32-static/lib '
Thread model: posix
gcc version 5.3.0 (x86_64-posix-seh-rev0, Built by MinGW-W64 project)
The Target line should say x86_64.

Answer 2

Had this problem with Windows 10 Home / AMD Ryzen 5 3400G.
For some reason the mingw64 install program decided for host i686-8.1.0-win32-dwarf-rt_v6-rev0
Solution was copying the mingw64 x86_64-w64-mingw32 installation from another notebook.