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)