fix(core): promote to BigInt instead of panicking on neg/abs of i64::MIN 💥

ndc_core/src/int.rs

@@ -149,7 +149,10 @@ impl Int {
     #[must_use]
     pub fn abs(&self) -> Self {
         match self {
-            Self::Int64(i) => Self::from(i.abs()),
+            // `i64::MIN.abs()` overflows, so promote to BigInt in that case.
+            Self::Int64(i) => i
+                .checked_abs()
+                .map_or_else(|| Self::from(BigInt::from(*i).abs()), Self::Int64),
             Self::BigInt(b) => Self::from(b.abs()),
         }
     }
@@ -218,7 +221,10 @@ impl Neg for Int {
 
     fn neg(self) -> Self::Output {
         match self {
-            Self::Int64(i) => Self::Int64(i.neg()),
+            // `-i64::MIN` overflows, so promote to BigInt in that case.
+            Self::Int64(i) => i
+                .checked_neg()
+                .map_or_else(|| Self::BigInt(BigInt::from(i).neg()), Self::Int64),
             Self::BigInt(i) => Self::BigInt(i.neg()),
         }
     }

tests/functional/programs/001_math/032_neg_abs_i64_min.ndc

@@ -0,0 +1,8 @@
+// Negating or taking the absolute value of `i64::MIN` overflows the i64
+// range (`-i64::MIN` does not fit in an i64). `Int::neg` and `Int::abs` used
+// to call `i.neg()` / `i.abs()` directly, which panics in debug builds and
+// silently wraps in release. They now promote to BigInt, yielding the correct
+// positive magnitude.
+// expect-output: 9223372036854775808 9223372036854775808 9223372036854775808
+let min = -9223372036854775807 - 1;
+print(-min, abs(min), abs(-min));

tests/functional/programs/900_bugs/bug0026_neg_abs_i64_min_overflow.ndc

@@ -0,0 +1,6 @@
+// `abs(i64::MIN)` and `-(i64::MIN)` overflow the i64 range because the
+// positive magnitude `2^63` does not fit in an i64. `Int::abs`/`Int::neg`
+// used to evaluate `i.abs()`/`i.neg()` directly, panicking with "attempt to
+// negate with overflow" in debug builds. They now promote to BigInt.
+// expect-output: 9223372036854775808
+print(abs(-9223372036854775807 - 1));