[Bug c++/79180] New: Nested lambda-capture causes segfault for parameter pack

[markus at dreseler dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79180

            Bug ID: 79180
           Summary: Nested lambda-capture causes segfault for parameter
                    pack
           Product: gcc
           Version: 6.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: markus at dreseler dot de
  Target Milestone: ---

Created attachment 40561
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40561&action=edit
Preprocessed file

The following code compiles but prints garbage and then segfaults under gcc
6.3.0 on OS X:

        #include <iostream>
        #include <string>

        void foo(const std::string &a) {
          std::cout << a << std::endl;
        }

        template <typename... Args>
        void bar(Args &&... args) {
          [&]() {
            [&]() {
                  foo(args...);
              }();
          }();
        }

        int main() {
         const std::string x("Hello World!");
         bar(x);
        }

Expected result: Print "Hello World!" and exit.

Compiling with `-Wextra -Wall` gives this warning:

        param.cpp: In function 'int main()':
        param.cpp:11:11: warning: 'args#0' is used uninitialized in this
function [-Wuninitialized]
                   foo(args...);
                   ^~~
        param.cpp:16:5: note: 'args#0' was declared here
         int main() {
             ^~~~

Clang and VC++ do not have any issues here. People at StackOverflow seem to
agree that this is valid code and appears to be a bug in GCC:
http://stackoverflow.com/questions/41769851/gcc-causes-segfault-for-lambda-captured-parameter-pack

Additional information:
        [:~/tmp] 1 $ g++-6 param.cpp && ./a.out
        # Garbage here
        Segmentation fault: 11

        [:~/tmp] 139 $ g++-6 -v
        Using built-in specs.
        COLLECT_GCC=g++-6
       
COLLECT_LTO_WRAPPER=/usr/local/Cellar/gcc/6.3.0_1/libexec/gcc/x86_64-apple-darwin16.3.0/6.3.0/lto-wrapper
        Target: x86_64-apple-darwin16.3.0
        Configured with: ../configure --build=x86_64-apple-darwin16.3.0
--prefix=/usr/local/Cellar/gcc/6.3.0_1
--libdir=/usr/local/Cellar/gcc/6.3.0_1/lib/gcc/6
--enable-languages=c,c++,objc,obj-c++,fortran --program-suffix=-6
--with-gmp=/usr/local/opt/gmp --with-mpfr=/usr/local/opt/mpfr
--with-mpc=/usr/local/opt/libmpc --with-isl=/usr/local/opt/isl
--with-system-zlib --enable-libstdcxx-time=yes --enable-stage1-checking
--enable-checking=release --enable-lto --with-build-config=bootstrap-debug
--disable-werror --with-pkgversion='Homebrew GCC 6.3.0_1'
--with-bugurl=https://github.com/Homebrew/homebrew-core/issues --enable-plugin
--disable-nls --enable-multilib
        Thread model: posix
        gcc version 6.3.0 (Homebrew GCC 6.3.0_1)

        [:~/tmp] 1 $ sw_vers
        ProductName:    Mac OS X
        ProductVersion: 10.12.2
        BuildVersion:   16C67