[Bug c++/88804] New: Inconsistently diagnosed unused but set static variable in lambda

[m101010a at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88804

            Bug ID: 88804
           Summary: Inconsistently diagnosed unused but set static
                    variable in lambda
           Product: gcc
           Version: 8.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: m101010a at gmail dot com
  Target Milestone: ---

$ cat x.cpp
void foo(int);
template <class T> void call(T t) {
    t(0);
}
void bar() {
        static int x=3;
        call([](auto){
                foo(x);
        });
}
$ cat y.cpp
void foo(int);
template <class T> void call(T t) {
    t(0);
}
void bar() {
        static int x=3;
        call([](int){
                foo(x);
        });
}
$ g++ -fsyntax-only -Wunused-but-set-variable x.cpp
x.cpp: In function ‘void bar()’:
x.cpp:6:13: warning: variable ‘x’ set but not used [-Wunused-but-set-variable]
  static int x=3;
             ^
$ g++ -fsyntax-only -Wunused-but-set-variable y.cpp
$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/8.2.1/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 --enable-multilib --disable-werror
--enable-checking=release --enable-default-pie --enable-default-ssp
--enable-cet=auto
Thread model: posix
gcc version 8.2.1 20181127 (GCC) 

x is incorrectly diagnosed as unused when used in a lambda with an auto
parameter, but not in a lambda with a type parameter (or no parameter).  This
happens even if the lambda isn't called, but does not happen if the lambda is
called in the same function where it is declared.