The OFFSETOF_VAR(var, member) macro is implemented in terms of
offsetof(typeof(*var), member) with compilers that know typeof(),
but its fallback implemenation compares &(var->member) and (var) and
count the distance in bytes, i.e.

    ((uintptr_t)&(var)->member - (uintptr_t)(var))

MSVC's runtime check, when fed an uninitialized 'var', flags this as
a use of an uninitialized variable (and that is legit---uninitialized
contents of 'var' is subtracted) in a debug build.

After auditing all 6 uses of OFFSETOF_VAR(), 1 of them does feed a
potentially uninitialized 'var' to the macro in the beginning of the
for() loop:

    #define hashmap_for_each_entry(map, iter, var, member) \
            for (var = hashmap_iter_first_entry_offset(map, iter, \
                                                    OFFSETOF_VAR(var, member)); \
                    var; \
                    var = hashmap_iter_next_entry_offset(iter, \
                                                    OFFSETOF_VAR(var, member)))

We can work around this by making sure that var has _some_ value
when OFFSETOF_VAR() is called.  Strictly speaking, it invites
undefined behaviour to use NULL here if we end up with pointer
comparison, but MSVC runtime seems to be happy with it, and most
other systems have typeof() and don't even need pointer comparison
fallback code.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>