When compiling a c++ project after system upgrade I got an error telling me to submit a full bug report. Here's the smallest program to which I could reduce the original source:

$ cat foo.cpp
#include <utility>

struct T {
int x = 0, y = x;
};

std::pair<int, T> f() { return {0, {}}; }
$ pacman -Qi gcc | grep Version
Version : 7.1.1-4
$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/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 --disable-multilib --disable-werror --enable-checking=release --enable-default-pie --enable-default-ssp
Thread model: posix
gcc version 7.1.1 20170630 (GCC)
$ g++ foo.cpp
foo.cpp: In function ‘std::pair<int, T> f()’:
foo.cpp:7:38: internal compiler error: in replace_placeholders_r, at cp/tree.c:2795
std::pair<int, T> f() { return {0, {}}; }
^
Please submit a full bug report,
with preprocessed source if appropriate.
See <https://bugs.archlinux.org/> for instructions.

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It doesn't seem to me that this is an arch-specific bug but I can't compare with another distro with the same compiler version.
The code compiles fine with gcc 6.3 (running in Void linux, though).

Downgrading to 7.1.1-2 gives the same error, and unfortunately I can't check with an earlier version as I cleaned the pacman cache recently.