int_max is an imprecise value on platforms where sizeof(long) >= sizeof(double). Most 64-bit systems have long's larger than a double's 53-bit mantissa (and likely all platforms when considering long long per the above comment).

Will it be truncated in the right direction (towards zero) to avoid this triggering on values with undefined conversion behavior?

the previous code used LONG_MIN < v and v < LONG_MAX directly rather than LONG_MAX + 1 stored into a double. (I believe the C promotion will promoted those values to a double before comparison as all floating point types have a higher rank than integer types)

The original comment explains why you should use < LONG_MAX. I would keep the original comment and the code, and just move it into PyLong_FromDouble().

I think I had to add comment about this: I assumed that LONG_MAX == 2 ** (CHAR_BIT * sizeof(long) - 1) - 1 and LONG_MIN == -2 ** (CHAR_BIT * sizeof(long) - 1), i.e. (unsigned long)LONG_MAX + 1 is a power of two and can be exactly represented by double (assuming that FLT_RADIX == 2). Does that make sense?

(Originally I wrote it like this: const double int_max = pow(2, CHAR_BIT * sizeof(long) - 1), see #15611 (comment))

Here I'm trying to demonstrate correctness of this approach:

In [66]: SIZEOF_LONG = 8; CHAR_BITS = 8   
In [67]: LONG_MAX = (1 << (SIZEOF_LONG * CHAR_BITS - 1)) - 1; LONG_MIN = -LONG_MAX - 1
In [68]: int_max = float(LONG_MAX + 1)        

In [69]: int_max == LONG_MAX + 1
Out[69]: True

In [70]: def cast_to_long(dval):
    ...:     assert isinstance(dval, float)
    ...:     wholepart = math.trunc(dval)
    ...:     if LONG_MIN <= wholepart <= LONG_MAX:
    ...:         return wholepart
    ...:     raise RuntimeError('undefined behavior')
In [71]: def long_from_double(dval):
    ...:     assert isinstance(dval, float)
    ...:     if -int_max <= dval < int_max:
    ...:         return cast_to_long(dval)
    ...:     raise ValueError('float is out of range, use frexp()')

In [72]: long_from_double(int_max)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-72-280887471997> in <module>()
----> 1 long_from_double(int_max)

<ipython-input-71-ccaef6014bf1> in long_from_double(dval)
      3     if -int_max <= dval < int_max:
      4         return cast_to_long(dval)
----> 5     raise ValueError('float is out of range, use frexp()')

ValueError: float is out of range, use frexp()

In [73]: int_max.hex()
Out[73]: '0x1.0000000000000p+63'

In [74]: long_from_double(float.fromhex('0x1.fffffffffffffp+62'))
Out[74]: 9223372036854774784

In [75]: long_from_double(float.fromhex('-0x1.fffffffffffffp+62'))
Out[75]: -9223372036854774784

In [76]: long_from_double(-int_max)
Out[76]: -9223372036854775808

In [77]: long_from_double(float.fromhex('-0x1.0000000000001p+63'))
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-77-de5e9e1eba23> in <module>()
----> 1 long_from_double(float.fromhex('-0x1.0000000000001p+63'))

<ipython-input-71-ccaef6014bf1> in long_from_double(dval)
      3     if -int_max <= dval < int_max:
      4         return cast_to_long(dval)
----> 5     raise ValueError('float is out of range, use frexp()')

ValueError: float is out of range, use frexp()

I think this is fine, under reasonable assumptions on the platform. LONG_MAX + 1 must be a power of 2 (follows from C99 §6.2.6.2p2), and while it's theoretically possible that double will be unable to represent LONG_MAX + 1 exactly, that seems highly unlikely in practice. So the conversion to double must be exact (C99 §6.3.1.4p2).

It's not safe based purely on the C standard to assume that LONG_MIN = -LONG_MAX - 1: the integer representation could be ones' complement or sign-magnitude, in which case LONG_MIN = -LONG_MAX. But that assumption is safe in practice for any platform that Python's likely to meet, and we make the assumption of two's complement for signed integers elsewhere in the codebase. If we're worried enough about this, we could change the -int_max <= dval comparison to -int_max < dval. On balance, I'd suggest making that change (partly just for the aesthetics of the symmetry).

Believe it or not, it's also not safe based purely on the C standard to assume that (unsigned long)LONG_MAX + 1 is representable as an unsigned long: C99 §6.2.5p9 only guarantees that nonnegative long values are representable as unsigned long But the chance of that not being true in practice is negligible (at least until someone tries to port CPython to the DS9000). And the failure mode is benign: we'd just end up never taking the fast path.

Re-reading all this, I had one more worry (which is why I dismissed my own review): what happens if the exact value of dval lies strictly between LONG_MAX and LONG_MAX + 1? In that case we could end up converting a double that, strictly speaking, is outside the range of long. But it turns out that we're safe, because C99 is quite explicit here: §6.3.1.4p1 says (emphasis mine):

If the value of the integral part cannot be represented by the integer type, the behavior is undefined.

So any double value that's strictly smaller than LONG_MAX + 1 should be fine.

it's also not safe based purely on the C standard to assume that (unsigned long)LONG_MAX + 1 is representable as an unsigned long

Then I think we could use ((double)(LONG_MAX / 2 + 1)) * 2, but does it worth it?

It's not safe based purely on the C standard to assume that LONG_MIN = -LONG_MAX - 1: the integer representation could be ones' complement or sign-magnitude, in which case LONG_MIN = -LONG_MAX. But that assumption is safe in practice for any platform that Python's likely to meet, and we make the assumption of two's complement for signed integers elsewhere in the codebase.

Shouldn't we formally state that we support only two's complement representation?
BTW it was proposed to abandon other representations and it looks like committee is agree with that.

Then I think we could use ((double)(LONG_MAX / 2 + 1)) * 2, but does it worth it?

Definitely not worth it! The C standard permits LONG_MAX == ULONG_MAX, but I'd be astonished if you ever find a real implementation (now or in the future) that has this property.

Shouldn't we formally state that we support only two's complement representation?

Yes, we should, though I'm not sure where would be the best place. But I think it's a non-issue in practice.