[Bug c++/82336] New: GCC requires but does not emit defaulted constructors in certain cases

["m101010a at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82336

            Bug ID: 82336
           Summary: GCC requires but does not emit defaulted constructors
                    in certain cases
           Product: gcc
           Version: 7.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: m101010a at gmail dot com
  Target Milestone: ---

When compiling the following source file, gcc requires a definition for
bar::bar() but does not emit one, causing a linker error:

$ cat x.cpp
struct foo { int x = 5; };
struct bar : foo { bar() = default; };
struct baz { bar x; };
void qux(baz = {}){}
int main() { qux(); }
$ g++ -v -Wall -Wextra x.cpp
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-multilib/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 7.1.1 20170630 (GCC) 
COLLECT_GCC_OPTIONS='-v' '-std=c++11' '-Wall' '-Wextra' '-g' '-shared-libgcc'
'-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/cc1plus -quiet -v -D_GNU_SOURCE x.cpp
-quiet -dumpbase x.cpp -mtune=generic -march=x86-64 -auxbase x -g -Wall -Wextra
-std=c++11 -version -o /tmp/ccwPoekN.s
GNU C++11 (GCC) version 7.1.1 20170630 (x86_64-pc-linux-gnu)
        compiled by GNU C version 7.1.1 20170630, GMP version 6.1.2, MPFR
version 3.1.5-p2, MPC version 1.0.3, isl version isl-0.18-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/../../../../x86_64-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/../../../../include/c++/7.1.1

/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/../../../../include/c++/7.1.1/x86_64-pc-linux-gnu
 /usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/../../../../include/c++/7.1.1/backward
 /usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/include
 /usr/local/include
 /usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/include-fixed
 /usr/include
End of search list.
GNU C++11 (GCC) version 7.1.1 20170630 (x86_64-pc-linux-gnu)
        compiled by GNU C version 7.1.1 20170630, GMP version 6.1.2, MPFR
version 3.1.5-p2, MPC version 1.0.3, isl version isl-0.18-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 53125f87ce78f0cf5a177bd50a2fd619
COLLECT_GCC_OPTIONS='-v' '-std=c++11' '-Wall' '-Wextra' '-g' '-shared-libgcc'
'-mtune=generic' '-march=x86-64'
 as -v --64 -o /tmp/ccXARTmy.o /tmp/ccwPoekN.s
GNU assembler version 2.28.0 (x86_64-pc-linux-gnu) using BFD version (GNU
Binutils) 2.28.0.20170506
COMPILER_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/:/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/:/usr/lib/gcc/x86_64-pc-linux-gnu/:/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/:/usr/lib/gcc/x86_64-pc-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/:/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-std=c++11' '-Wall' '-Wextra' '-g' '-shared-libgcc'
'-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/collect2 -plugin
/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/liblto_plugin.so
-plugin-opt=/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/lto-wrapper
-plugin-opt=-fresolution=/tmp/ccvOLjqj.res -plugin-opt=-pass-through=-lgcc_s
-plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lc
-plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lgcc --build-id
--eh-frame-hdr --hash-style=gnu -m elf_x86_64 -dynamic-linker
/lib64/ld-linux-x86-64.so.2 -pie
/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/../../../../lib/Scrt1.o
/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/../../../../lib/crti.o
/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/crtbeginS.o
-L/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1
-L/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/../../../../lib -L/lib/../lib
-L/usr/lib/../lib -L/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/../../..
/tmp/ccXARTmy.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc
/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/crtendS.o
/usr/lib/gcc/x86_64-pc-linux-gnu/7.1.1/../../../../lib/crtn.o
/tmp/ccXARTmy.o: In function `main':
/home/m42a/tmp/gcc-bug/x.cpp:5: undefined reference to `bar::bar()'
collect2: error: ld returned 1 exit status


Since bar's constructor is declared and defaulted, it should exist.

This behavior appears in every version since 4.8, but not in 4.7.4.  It occurs
at all optimization levels, and on both Linux and MinGW.

Removing the explicit declaration of bar's constructor does not fix the issue,
but many other small changes do.  Removing foo::x's initializer fixes it, and
so does adding an initializer to baz::x ("bar x{}", "bar x = {}", and "bar x =
bar()" all work), and changing qux's parameter to "baz = baz{}" or "baz =
baz()", and providing an explicit parameter when calling qux, even if that
parameter is just "{}".