https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69517
--- Comment #18 from Jakub Jelinek <jakub at gcc dot gnu.org> --- (In reply to Martin Sebor from comment #17) > (In reply to Jakub Jelinek from comment #16) > > The bug here is in G++ accepting a VLA initializer with more elements than > there is room for in the VLA, and then trashing the stack at runtime with > the extra elements. It is a regression with respect to GCC 4.9.3 which > implements C++ VLAs as specified in n3639 > (http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3639.html). This > is documented in GCC 4.9 changes (https://gcc.gnu.org/gcc-4.9/changes.html) > which highlights the feature using the following example: > > void f(int n) { > int a[n] = { 1, 2, 3 }; // throws std::bad_array_length if n < 3 > ... > > VLAs were subsequently removed from C++, and also partially (but not > completely) removed from G++, which causes C++ programs developed and tested > with G++ 4.9 to break when ported to a later version. > > C++ VLAs will be safer to use with the patch referenced in comment #9. It > patch had to be reverted from GCC 6.0 because it caused problems in Java. > Java has been removed and I plan/hope to resubmit the patch for GCC 8. (I > wanted to do it for GCC 7 but didn't get to it.) I don't see why it would be a bug. There is no standard covering VLAs in C++, it is all extensions, it is defined however we want. UB when storing something larger into something smaller is perfectly fine, users can put their own checks if they want to avoid it. You want to slow all the code down by doing the checks mandatory.
