greenlet does not compile with main branch with clang

[tacaswell]

Bug report

Bug description:

The error is:

  clang++ -fno-strict-overflow -Wsign-compare -Wunreachable-code -DNDEBUG -g -O3 -Wall -fPIC -I/home/tcaswell/.virtualenvs/cp315-clang/include -I/home/tcaswell/.pybuild/cp315-clang/include/python3.15 -c src/greenlet/greenlet.cpp -o build/temp.linux-x86_64-cpython-315/src/greenlet/greenlet.o
  In file included from src/greenlet/greenlet.cpp:19:
  In file included from src/greenlet/greenlet_internal.hpp:18:
  In file included from src/greenlet/TGreenlet.hpp:35:
  In file included from /home/tcaswell/.pybuild/cp315-clang/include/python3.15/internal/pycore_interpframe.h:8:
  In file included from /home/tcaswell/.pybuild/cp315-clang/include/python3.15/internal/pycore_code.h:11:
  /home/tcaswell/.pybuild/cp315-clang/include/python3.15/internal/pycore_backoff.h:65:30: error: non-constant-expression cannot be narrowed from type 'int' to 'uint16_t' (aka 'unsigned short') in initializer list [-Wc++11-narrowing]
     65 |         .value_and_backoff = MAKE_VALUE_AND_BACKOFF(value, backoff)
        |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  /home/tcaswell/.pybuild/cp315-clang/include/python3.15/internal/pycore_backoff.h:43:5: note: expanded from macro 'MAKE_VALUE_AND_BACKOFF'
     43 |     ((value << BACKOFF_BITS) | backoff)
        |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  /home/tcaswell/.pybuild/cp315-clang/include/python3.15/internal/pycore_backoff.h:65:30: note: insert an explicit cast to silence this issue
     65 |         .value_and_backoff = MAKE_VALUE_AND_BACKOFF(value, backoff)
        |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        |                              static_cast<uint16_t>(                )
  /home/tcaswell/.pybuild/cp315-clang/include/python3.15/internal/pycore_backoff.h:43:5: note: expanded from macro 'MAKE_VALUE_AND_BACKOFF'
     43 |     ((value << BACKOFF_BITS) | backoff)
        |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  In file included from src/greenlet/greenlet.cpp:43:
  src/greenlet/PyGreenlet.cpp:62:23: warning: unused variable 'c' [-Wunused-variable]
     62 |         UserGreenlet* c = new UserGreenlet(o, GET_THREAD_STATE().state().borrow_current());
        |                       ^
  1 warning and 1 error generated.
  error: command '/lib/ccache/bin/clang++' failed with exit code 1
  error: subprocess-exited-with-error

which was introduce via https://github.com/python/cpython/pull/141591/files#diff-2772c99555ca0a4a3987e20582200c6eb6314fea3e5f3a5f7f2cbc2e371e22adL51 which inlined tho computation of the new value with creating the struct to hold it:

cpython/Include/internal/pycore_backoff.h

Lines 59 to 67 in 726e8e8

	static inline _Py_BackoffCounter
	make_backoff_counter(uint16_t value, uint16_t backoff)
	{
	assert(backoff <= UNREACHABLE_BACKOFF);
	assert(value <= MAX_VALUE);
	return ((_Py_BackoffCounter){
	.value_and_backoff = MAKE_VALUE_AND_BACKOFF(value, backoff)
	});
	}

This is apparently fine with clang's c complier and gcc/g++, but clang++ warns. I'm out of my c/++ depth to understand why.

The following patch fixes the issue:

diff --git a/Include/internal/pycore_backoff.h b/Include/internal/pycore_backoff.h
index 7f60eb49508..feb5f3e11e6 100644
--- a/Include/internal/pycore_backoff.h
+++ b/Include/internal/pycore_backoff.h
@@ -61,8 +61,9 @@ make_backoff_counter(uint16_t value, uint16_t backoff)
 {
     assert(backoff <= UNREACHABLE_BACKOFF);
     assert(value <= MAX_VALUE);
+    uint16_t result = MAKE_VALUE_AND_BACKOFF(value, backoff);
     return ((_Py_BackoffCounter){
-        .value_and_backoff = MAKE_VALUE_AND_BACKOFF(value, backoff)
+        .value_and_backoff = result
     });
 }

however I am not sure if this should be accounted for here of if greenlet should be working around it.

To reproduce this you may also need to account for python-greenlet/greenlet#481

@ clang++ --version
clang version 21.1.6
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

CPython versions tested on:

CPython main branch

Operating systems tested on:

linux (Arch)