do full type check in BARF_UNLESS_COPYABLE

Use __builtin_types_compatible_p to perform a full type check if
possible.  Otherwise fall back to the old size comparison, but add a
non-evaluated assignment to catch more type mismatches.  It doesn't flag
copies between arrays with different signedness, but that's as close to
a full type check as it gets without the builtin, as far as I can see.

Helped-by: Junio C Hamano <[email protected]>
Signed-off-by: René Scharfe <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rscharfe

git-compat-util.h

@@ -97,8 +97,14 @@ struct strbuf;
 # define BARF_UNLESS_AN_ARRAY(arr)						\
 	BUILD_ASSERT_OR_ZERO(!__builtin_types_compatible_p(__typeof__(arr), \
 							   __typeof__(&(arr)[0])))
+# define BARF_UNLESS_COPYABLE(dst, src) \
+	BUILD_ASSERT_OR_ZERO(__builtin_types_compatible_p(__typeof__(*(dst)), \
+							  __typeof__(*(src))))
 #else
 # define BARF_UNLESS_AN_ARRAY(arr) 0
+# define BARF_UNLESS_COPYABLE(dst, src) \
+	BUILD_ASSERT_OR_ZERO(0 ? ((*(dst) = *(src)), 0) : \
+				 sizeof(*(dst)) == sizeof(*(src)))
 #endif
 /*
  * ARRAY_SIZE - get the number of elements in a visible array
@@ -1093,9 +1099,6 @@ int xstrncmpz(const char *s, const char *t, size_t len);
 #define CALLOC_ARRAY(x, alloc) (x) = xcalloc((alloc), sizeof(*(x)))
 #define REALLOC_ARRAY(x, alloc) (x) = xrealloc((x), st_mult(sizeof(*(x)), (alloc)))
 
-#define BARF_UNLESS_COPYABLE(dst, src) \
-	BUILD_ASSERT_OR_ZERO(sizeof(*(dst)) == sizeof(*(src)))
-
 #define COPY_ARRAY(dst, src, n) copy_array((dst), (src), (n), sizeof(*(dst)) + \
 	BARF_UNLESS_COPYABLE((dst), (src)))
 static inline void copy_array(void *dst, const void *src, size_t n, size_t size)