Use the same strategy also for memoryview

Author: encukou

Misc/NEWS.d/next/Library/2020-03-11-15-46-59.bpo-39689.uzQhJX.rst

@@ -1,3 +1,3 @@
-The struct module now loads native booleans as ``char`` rather than
-``_Bool`` to avoid triggering undefined behavior. The behavior is the same
-on all supported (tested) platforms.
+The struct module and memoryview now load native booleans as ``char`` rather
+than ``_Bool`` to avoid triggering undefined behavior. For valid native _Bool,
+the behavior is the same on all supported (tested) platforms.

Objects/memoryobject.c

@@ -1699,7 +1699,9 @@ unpack_single(const char *ptr, const char *fmt)
     case 'l': UNPACK_SINGLE(ld, ptr, long); goto convert_ld;
 
     /* boolean */
-    case '?': UNPACK_SINGLE(ld, ptr, _Bool); goto convert_bool;
+    // memcpy-ing values other than 0 or 1 to a _Bool variable triggers
+    // undefined behavior, so cast from char instead. See bpo-39689.
+    case '?': ld = (_Bool)*ptr; goto convert_bool;
 
     /* unsigned integers */
     case 'H': UNPACK_SINGLE(lu, ptr, unsigned short); goto convert_lu;