Python 3.7.0b3 fails to build with clang 6.0 (implicit cast from void* to a different pointer type is an error now):

/usr/bin/clang++ -c -Wno-unused-result -Wsign-compare -Wunreachable-code -DDYNAMIC_ANNOTATIONS_ENABLED=1 -DNDEBUG -Os -gdwarf-4 -Wstrict-aliasing=2 -pipe -Wformat -Werror=format-security -D_FORTIFY_SOURCE=2 -fstack-protector-strong --param=ssp-buffer-size=4  -fPIC -flto -O3 -g -Os -gdwarf-4 -Wstrict-aliasing=2 -pipe -Wformat -Werror=format-security -D_FORTIFY_SOURCE=2 -fstack-protector-strong --param=ssp-buffer-size=4  -fPIC -flto -O3 -D_GNU_SOURCE -fPIC -fwrapv -I/usr/include/ncursesw -flto -Os -gdwarf-4 -Wstrict-aliasing=2 -pipe -Wformat -Werror=format-security -D_FORTIFY_SOURCE=2 -fstack-protector-strong --param=ssp-buffer-size=4  -fPIC -flto -O3 -D_GNU_SOURCE -fPIC -fwrapv -I/usr/include/ncursesw  -Wextra -Wno-unused-result -Wno-unused-parameter -Wno-missing-field-initializers -Werror=implicit-function-declaration -Os -gdwarf-4 -Wstrict-aliasing=2 -pipe -Wformat -Werror=format-security -D_FORTIFY_SOURCE=2 -fstack-protector-strong --param=ssp-buffer-size=4  -fPIC -flto -O3 -D_GNU_SOURCE -fPIC -fwrapv -I/usr/include/ncursesw -fprofile-instr-generate  -I. -I./Include -Os -gdwarf-4 -Wstrict-aliasing=2 -pipe -Wformat -Werror=format-security -D_FORTIFY_SOURCE=2 -fstack-protector-strong --param=ssp-buffer-size=4  -fPIC -flto -O3 -D_GNU_SOURCE -fPIC -fwrapv -I/usr/include/ncursesw -Os -gdwarf-4 -Wstrict-aliasing=2 -pipe -Wformat -Werror=format-security -D_FORTIFY_SOURCE=2 -fstack-protector-strong --param=ssp-buffer-size=4  -fPIC -flto -O3 -D_GNU_SOURCE -fPIC -fwrapv -I/usr/include/ncursesw -fPIC -DPy_BUILD_CORE -o Programs/_testembed.o ./Programs/_testembed.c
clang-6.0: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated]
./Programs/_testembed.c:173:34: warning: ISO C++11 does not allow conversion from string literal to 'wchar_t *' [-Wwritable-strings]
    wchar_t *static_warnoption = L"once";
                                 ^
./Programs/_testembed.c:174:31: warning: ISO C++11 does not allow conversion from string literal to 'wchar_t *' [-Wwritable-strings]
    wchar_t *static_xoption = L"also_not_an_option=2";
                              ^
./Programs/_testembed.c:177:14: error: cannot initialize a variable of type 'wchar_t *' with an rvalue of type 'void *'
    wchar_t *dynamic_once_warnoption = calloc(warnoption_len+1, sizeof(wchar_t));
             ^                         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./Programs/_testembed.c:178:14: error: cannot initialize a variable of type 'wchar_t *' with an rvalue of type 'void *'
    wchar_t *dynamic_xoption = calloc(xoption_len+1, sizeof(wchar_t));
             ^                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2 warnings and 2 errors generated.
make[3]: *** [Makefile:777: Programs/_testembed.o] Error 1

Huh, I thought we had a clang builder in the pre-merge CI. Is that running an older version of clang, perhaps?

New changeset 69f5c73311a61b05485b19626935bf240ad31c5b by Nick Coghlan in branch 'master':
bpo-33182: Fix pointer types in _testembed (GH-6310)
https://github.com/python/cpython/commit/69f5c73311a61b05485b19626935bf240ad31c5b

New changeset 2961717986201d639b60de51e5f2e9aa2573856c by Nick Coghlan (Miss Islington (bot)) in branch '3.7':
bpo-33182: Fix pointer types in _testembed (GH-6310) (GH-6311)
https://github.com/python/cpython/commit/2961717986201d639b60de51e5f2e9aa2573856c

Thanks for the bug report - the 3.7 branch should be buildable again now.

My assumption would be that Travis are still running clang 5, since 6 was only released very recently.