This is triggered when running gtests with an UBSan build.

To enable this check add the following to your mozconfig:

ac_add_options --enable-address-sanitizer
ac_add_options --enable-undefined-sanitizer="shift"
ac_add_options --disable-jemalloc

Traceback:

[ RUN      ] Cairo.Simple
TestCairo:TryArcs centerY 0.000000, radius 14.000000
gfx/cairo/cairo/src/cairo-fixed-private.h:62:14: runtime error: left shift of negative value -32768
    #0 0x7f59b7a64144 in _cairo_fixed_from_int gfx/cairo/cairo/src/cairo-fixed-private.h:62:14
    #1 0x7f59b7a7f033 in _line_exceeds_16_16 gfx/cairo/cairo/src/cairo-image-surface.c:2386:16
    #2 0x7f59b7a7e831 in _pixman_image_add_traps gfx/cairo/cairo/src/cairo-image-surface.c:2444:6
    #3 0x7f59b7a7e34e in _composite_traps gfx/cairo/cairo/src/cairo-image-surface.c:2516:5
    #4 0x7f59b7a80e31 in _clip_and_composite gfx/cairo/cairo/src/cairo-image-surface.c:2359:15
    #5 0x7f59b7a866c9 in _clip_and_composite_trapezoids gfx/cairo/cairo/src/cairo-image-surface.c:3258:12
    #6 0x7f59b7a86186 in _clip_and_composite_polygon gfx/cairo/cairo/src/cairo-image-surface.c:3625:15
    #7 0x7f59b7a70dac in _cairo_image_surface_fill gfx/cairo/cairo/src/cairo-image-surface.c:3816:15
    #8 0x7f59b7ad3af8 in _cairo_surface_fill gfx/cairo/cairo/src/cairo-surface.c:2352:11
    #9 0x7f59b7a63ed2 in _cairo_gstate_fill gfx/cairo/cairo/src/cairo-gstate.c:1290:15
    #10 0x7f59b7afc386 in _moz_cairo_fill_preserve gfx/cairo/cairo/src/cairo.c:2473:14
    #11 0x7f59af019c22 in mozilla::layers::TryCircle(double, double, double) gfx/2d/unittest/TestCairo.cpp:24:3
    #12 0x7f59af019993 in mozilla::layers::Cairo_Simple_Test::TestBody() gfx/2d/unittest/TestCairo.cpp:32:3
    #13 0x7f59aee7d55b in testing::Test::Run() testing/gtest/gtest/src/gtest.cc:2519:5
    #14 0x7f59aee7e405 in testing::TestInfo::Run() testing/gtest/gtest/src/gtest.cc:2695:11
    #15 0x7f59aee7ebf1 in testing::TestCase::Run() testing/gtest/gtest/src/gtest.cc:2813:28
    #16 0x7f59aee8c642 in testing::internal::UnitTestImpl::RunAllTests() testing/gtest/gtest/src/gtest.cc:5179:43
    #17 0x7f59aee8c106 in testing::UnitTest::Run() testing/gtest/gtest/src/gtest.cc:4788:10
    #18 0x7f59aeec54f7 in mozilla::RunGTestFunc(int*, char**) testing/gtest/mozilla/GTestRunner.cpp:158:10
    #19 0x7f59b96a1450 in XREMain::XRE_mainStartup(bool*) toolkit/xre/nsAppRunner.cpp:3788:16
    #20 0x7f59b96aa069 in XREMain::XRE_main(int, char**, mozilla::BootstrapConfig const&) toolkit/xre/nsAppRunner.cpp:4722:12
    #21 0x7f59b96aaad1 in XRE_main(int, char**, mozilla::BootstrapConfig const&) toolkit/xre/nsAppRunner.cpp:4816:21
    #22 0x556b74927cae in do_main(int, char**, char**) browser/app/nsBrowserApp.cpp:218:22
    #23 0x556b74927344 in main browser/app/nsBrowserApp.cpp:300:16

The problem is

#define CAIRO_FIXED_16_16_MIN _cairo_fixed_from_int (-32768)

where _cairo_fixed_from_int() shifts the argument left by 8.

So if you ignore the undefined behaviour I think it should be -8388608 or 1111 1111 1000 0000 0000 0000 0000 0000. But I'm not sure how to best define it. Lee?