https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84491
Bug ID: 84491 Summary: Multiple inheritance and covariant return types causes internal compiler error Product: gcc Version: 7.3.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: christopher.hood at gtri dot gatech.edu Target Milestone: --- With respect to GCC 7.3.0, but I have tested on other GCCs down to 4.7 with the same result. Using built-in specs. COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/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 Thread model: posix gcc version 7.3.0 (GCC) This example using multiple inheritance and two branches of covariant virtual functions causes the internal compiler error: test-thunk-error.cc: In instantiation of ‘struct Parents<Constituent<Foo, Bar> >’: test-thunk-error.cc:8:8: required from ‘struct InterfaceBase<FooWithBar, Constituent<Foo, Bar> >’ test-thunk-error.cc:37:24: required from here test-thunk-error.cc:4:8: internal compiler error: in make_thunk, at cp/method.c:115 struct Parents : P::Interface... ^~~~~~~ Judicious commenting of certain seemingly non-offensive lines will cause compilation to succeed (any of lines commented a,b,c,d). The code which breaks GCC does compile without error on Clang. Example listing: template <class ... P> struct Parents : P::Interface... {}; template <class T, class ... P> struct InterfaceBase : Parents<P...> { virtual T* Model ()=0; // a }; struct Foo { struct Interface : InterfaceBase<Foo> {}; virtual Interface* Iface ()=0; // b }; struct Bar { struct Interface : InterfaceBase<Bar> {}; virtual Interface* Iface ()=0; // c }; template <class T, class U> struct Constituent : T, U { struct Interface : InterfaceBase<Constituent, T, U> {}; virtual Interface* Iface ()=0; // d }; struct FooWithBar : Constituent<Foo, Bar> { struct Interface : InterfaceBase<FooWithBar, Constituent> {}; }; int main () { return 0; }