https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83020
Bug ID: 83020 Summary: ('17) Class template constructor call skipped with no error when substitution fails in default argument Product: gcc Version: 7.2.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: oremanj at mit dot edu Target Milestone: --- The below C++ program is ill-formed, but gcc 7.1 and 7.2 accept it in C++17 mode, and emit code that simply doesn't call the constructor of Whoops at all. This is reproducible if the constructor is defined out-of-line as well; there's no reference to Whoops in the generated assembly. In C++14 mode or with gcc 6.3, the error is correctly diagnosed. $ cat t.cc struct NoDefault { int val = 1234; NoDefault(int v) : val(v) {} }; template <class T> struct Whoops { const char *str; T obj; Whoops(const char *s, T v = T()) : str(s), obj(v) {} }; const char *test() { return Whoops<NoDefault>("hi").str; } $ g++-7 -O3 -std=c++17 -Wall -c t.cc && objdump -d t.o t.o: file format elf64-x86-64 Disassembly of section .text: 0000000000000000 <_Z4testv>: 0: 31 c0 xor %eax,%eax 2: c3 retq $ g++-7 -O0 -std=c++17 -Wall -c t.cc && objdump -d t.o t.o: file format elf64-x86-64 Disassembly of section .text: 0000000000000000 <_Z4testv>: 0: 55 push %rbp 1: 48 89 e5 mov %rsp,%rbp 4: 5d pop %rbp 5: c3 retq $ 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='Debian 7.2.0-12+hrtdeb8u1' --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=gcc4-compatible --enable-gnu-unique-object --disable-vtable-verify --enable-libmpx --enable-plugin --with-system-zlib --with-target-system-zlib --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i586 --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 --disable-initfini-array Thread model: posix gcc version 7.2.1 20171025 (Debian 7.2.0-12)