User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:64.0) Gecko/20100101 Firefox/64.0 Steps to reproduce: I build with GCC9. Actual results: I got warnings: /builds/worker/workspace/build/src/toolkit/mozapps/extensions/AddonManagerStartup-inlines.h:31:26: error: moving a local object in a return statement prevents copy elision [-Werror=pessimizing-move] /builds/worker/workspace/build/src/toolkit/mozapps/extensions/AddonManagerStartup-inlines.h:31:26: error: moving a local object in a return statement prevents copy elision [-Werror=pessimizing-move] /builds/worker/workspace/build/src/toolkit/mozapps/extensions/AddonManagerStartup-inlines.h:91:25: error: moving a local object in a return statement prevents copy elision [-Werror=pessimizing-move] /builds/worker/workspace/build/src/toolkit/mozapps/extensions/AddonManagerStartup-inlines.h:91:25: error: moving a local object in a return statement prevents copy elision [-Werror=pessimizing-move] these failed my build and saddened my day. Expected results: It should have worked :)