[Bug c++/79175] New: incorrect visibility of variables in if-else if blocks with dynamic_cast

[md346906 at students dot mimuw.edu.pl]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79175

            Bug ID: 79175
           Summary: incorrect visibility of variables in if-else if blocks
                    with dynamic_cast
           Product: gcc
           Version: 5.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: md346906 at students dot mimuw.edu.pl
  Target Milestone: ---

Created attachment 40559
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40559&action=edit
source code

The attached program does not give an error "undeclared identifier 'b'" despite
being undeclared in that scoped - a variable declared using a dynamic_cast in
an if condition should be only visible to the block corresponding to that
condition and should not be visible in the following else-if blocks.

Expected behaviour: Error "use of undeclared identifier 'b' in line 16".

Actuall behaviour: Program accepted and compiled.

Compiled with: g++ a.cpp -Wall -Wextra 

Compiler output:

a.cpp: In function ‘int main()’:
a.cpp:15:17: warning: unused variable ‘c’ [-Wunused-variable]
   } else if (C* c = dynamic_cast<C*>(a)) {
                 ^

g++ -v

Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/5/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
5.4.0-6ubuntu1~16.04.4' --with-bugurl=file:///usr/share/doc/gcc-5/README.Bugs
--enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-5 --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 --with-system-zlib
--disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-5-amd64/jre --enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-5-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-5-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --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 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.4)