[Bug c++/80587] New: Incorrect type from outer scope inside lambda under some conditions

["jasonr at 3db-labs dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80587

            Bug ID: 80587
           Summary: Incorrect type from outer scope inside lambda under
                    some conditions
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jasonr@3db-labs.com
  Target Milestone: ---

Created attachment 41292
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41292&action=edit
Test case source file

I observed this error on an Ubuntu 17.04 system; here is the output of `g++
-v`:

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Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/6/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 6.3.0-12ubuntu2'
--with-bugurl=file:///usr/share/doc/gcc-6/README.Bugs
--enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-6 --program-prefix=x86_64-linux-gnu- --enable-shared
--enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/
--enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie
--with-system-zlib --disable-browser-plugin --enable-java-awt=gtk
--enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-6-amd64/jre
--enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-6-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-6-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--with-target-system-zlib --enable-objc-gc=auto --enable-multiarch
--disable-werror --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 6.3.0 20170406 (Ubuntu 6.3.0-12ubuntu2) 

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The nature of the bug is tough to explain, so see the attached test case. I
minimized it as much as possible, but I ran into it when using Boost.Hana from
Boost v1.64. Based on the test case, this appears to be a compiler bug, not an
issue with the library.

In the test, I create a `boost::hana::tuple<>` object, then use
`boost::hana::for_each()` to invoke a callable with each element of the tuple.
In my test case, I provide a lambda as the callable. I've noticed that, if I
try to use the enclosing tuple type from within the lambda, g++ uses the
incorrect type in some cases. I would expect the program to print:

-----

working example:
tuple type before for_each: boost::hana::tuple<int, int>
tuple type inside for_each: boost::hana::tuple<int, int>
tuple type inside for_each: boost::hana::tuple<int, int>

non-working example:
tuple type before for_each: boost::hana::tuple<int, int>
tuple type inside for_each: boost::hana::tuple<int, int>
tuple type inside for_each: boost::hana::tuple<int, int>

-----

However, when I compile and run the program using g++ as follows:

g++ tuple_test.cc -o tuple_test -std=c++14

I get the following output:

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working example:
tuple type before for_each: boost::hana::tuple<int, int>
tuple type inside for_each: boost::hana::tuple<int, int>
tuple type inside for_each: boost::hana::tuple<int, int>

non-working example:
tuple type before for_each: boost::hana::tuple<int, int>
tuple type inside for_each: int
tuple type inside for_each: int

-----

I see this error on every version of gcc that I've tried: 6.1, 6.2, 6.3, 7.0,
and 8.0 trunk (https://wandbox.org/permlink/4zKtIIZ9k7sI7Ckv). All versions of
clang that I've tried that are supported by Hana seem to work fine. I've
minimized the test case as much as I can, but I haven't been able to identify a
workaround.