[abhiishek340]

Fixes #197.

The negative overflow bounds were generated as the negated positive bounds, but min_value has one more unit of magnitude than max_value. Separately, int_pow and int63_pow_on_int64 used abs base, which is negative for min_value, so the bounds check was skipped.

	before	after
Int.pow Int.min_value 2	0	raises
Int.pow (-4) 31 (= Int.min_value)	raises	Int.min_value
Int64.pow (-2L) 63L	raises	Int64.min_value
Int64.pow (-8L) 21L	raises	Int64.min_value
Int64.pow Int64.min_value 0L	raises	1L

One note on the issue: on 63-bit int, (-2) ** 62 = 2 ** 62, which does overflow, so raising there is correct. The int case that reaches min_value is (-4) ** 31.

Changes

• generate_pow_overflow_bounds.ml: compute negative bounds against -(max_value + 1) for odd exponents (and for 0 and 1), and against max_value for even ones. Emit negative tables for every type, using the same per-architecture structure as the positive int table. Negative int entries are parenthesized inside Stdlib.Int64.to_int (...).
• pow_overflow_bounds.ml / .mli: regenerated. Positive tables are unchanged. In int64_negative_overflow_bounds, only exponents 0, 1, 3, 7, 9, 21 and 63 change.
• int_math.ml: int_pow and int63_pow_on_int64 compare signed bases against the positive and negative bounds, as int64_pow already does. This also removes the now-unused abs_local.

Testing

I don't have an OxCaml toolchain, so I haven't built Base itself. What I did check:

• Ran the patched generator with OCaml 5.2.1 + zarith. The unpatched generator reproduces the committed pow_overflow_bounds.ml. Every new negative bound b at exponent e satisfies min_value <= b ** e, and b - 1 overflows.
• Compiled the regenerated table file (with small stand-ins for Iarray / Portability_hacks) together with the new checks and an OCaml port of Base_int_math_int_pow_stub. Compared int, int64 and int63-on-int64 results against Zarith for about 210k (base, exponent) pairs: bases from -300 to 300, ±2^k ± 1, every table bound ± 1, and min_value / max_value; exponents 0 to 70. There were no mismatches.