https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82570
Bug ID: 82570
Summary: Lambda fails to compile because it doesn't meet
constexpr requirements
Product: gcc
Version: 7.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: andysem at mail dot ru
Target Milestone: ---
Created attachment 42380
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=42380&action=edit
A testcase that shows the problem.
The attached program fails to compile with the following command line:
g++-7 -std=gnu++1z -o test_constexpr_lambda test_constexpr_lambda.cpp
test_constexpr_lambda.cpp: In instantiation of ‘foo(Pred)::<lambda(int)> [with
Pred = main()::<lambda(int)>]’:
test_constexpr_lambda.cpp:12:7: required from ‘struct foo(Pred) [with Pred =
main()::<lambda(int)>]::<lambda(int)>’
test_constexpr_lambda.cpp:10:9: required from ‘void foo(Pred) [with Pred =
main()::<lambda(int)>]’
test_constexpr_lambda.cpp:25:33: required from here
test_constexpr_lambda.cpp:14:18: error: uninitialized variable ‘buf’ in
‘constexpr’ function
unsigned char buf[4];
^~~
test_constexpr_lambda.cpp:2:13: error: ‘void iterate(Body) [with Body =
foo(Pred) [with Pred = main()::<lambda(int)>]::<lambda(int)>]’, declared using
local type ‘foo(Pred) [with Pred = main()::<lambda(int)>]::<lambda(int)>’, is
used but never defined [-fpermissive]
inline void iterate(Body body)
^~~~~~~
test_constexpr_lambda.cpp:2:13: warning: ‘void iterate(Body) [with Body =
foo(Pred) [with Pred = main()::<lambda(int)>]::<lambda(int)>]’ used but never
defined
The program compiles with -std=gnu++14. It also compiles successfully with gcc
6.3.0 and -std=gnu++1z (i.e. this is a regression since gcc 6).
I believe, this is a compiler bug because (a) neither the lambda nor the
calling code are marked constexpr and (b) according to
[expr.prim.lambda.closure]/4 the lambda should only be considered constexpr if
it meets constexpr requirements (otherwise it should be compiled as a regular
runtime function).
g++-7 -v
Using built-in specs.
COLLECT_GCC=g++-7
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/7/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 7.2.0-1mind1'
--with-bugurl=file:///usr/share/doc/gcc-7/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr
--with-gcc-major-version-only --program-suffix=-7
--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 --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-offload-targets=nvptx-none --without-cuda-driver
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 7.2.0 (Ubuntu 7.2.0-1mind1)
