diff --git a/src/big_int.cpp b/src/big_int.cpp
index 6db3301da..1d81a0483 100644
--- a/src/big_int.cpp
+++ b/src/big_int.cpp
@@ -489,7 +489,8 @@ gb_internal void big_int_and(BigInt *dst, BigInt const *x, BigInt const *y) {
 
 gb_internal void big_int_and_not(BigInt *dst, BigInt const *x, BigInt const *y) {
 	if (mp_iszero(x)) {
-		big_int_init(dst, y);
+		// 0 &~ y == 0 & ~y == 0
+		big_int_from_i64(dst, 0);
 		return;
 	}
 	if (mp_iszero(y)) {
@@ -505,13 +506,13 @@ gb_internal void big_int_and_not(BigInt *dst, BigInt const *x, BigInt const *y)
 			mp_decr(&x1);
 			mp_decr(&y1);
 
-			BigInt ny1 = {};
-			mp_complement(&y1, &ny1);
-			mp_and(&x1, &ny1, dst);
+			BigInt nx1 = {};
+			mp_complement(&x1, &nx1);
+			mp_and(&y1, &nx1, dst);
 
 			big_int_dealloc(&x1);
 			big_int_dealloc(&y1);
-			big_int_dealloc(&ny1);
+			big_int_dealloc(&nx1);
 			return;
 		}
 
@@ -532,6 +533,7 @@ gb_internal void big_int_and_not(BigInt *dst, BigInt const *x, BigInt const *y)
 		BigInt z1 = {};
 		big_int_or(&z1, &x1, &y1);
 		mp_add_d(&z1, 1, dst);
+		big_int_neg(dst, dst);
 
 		big_int_dealloc(&x1);
 		big_int_dealloc(&y1);
diff --git a/tests/internal/test_constant_folding.odin b/tests/internal/test_constant_folding.odin
index 385ba6835..016976201 100644
--- a/tests/internal/test_constant_folding.odin
+++ b/tests/internal/test_constant_folding.odin
@@ -2,10 +2,145 @@ package test_internal
 
 import "core:testing"
 
+// Constant folding against the answer the backend produces. A folded constant that is merely
+// wrong still compiles, so a harness comparing accept/reject sees agreement 
+// Every case here pairs a constant with the same expression on variables for that reason.
+//
+// `a &~ b` is `a & ~b`. `big_int_and_not` had three independent faults: `0 &~ y` returned `y`,
+// the both-negative branch used its operands the wrong way round, and the negative-left branch
+// dropped the sign of its result.
+
+@(test)
+and_not_constant_folding_matches_runtime :: proc(t: ^testing.T) {
+	// the zero short-circuit: 0 &~ anything is 0
+	{
+		a, b := 0, 3
+		testing.expect_value(t, 0 &~ 3, a &~ b)
+		testing.expect_value(t, 0 &~ 3, 0)
+	}
+	{
+		a, b := 0, -3
+		testing.expect_value(t, 0 &~ -3, a &~ b)
+		testing.expect_value(t, 0 &~ -3, 0)
+	}
+
+	// negative left operand: the result must stay negative
+	{
+		a, b := -7, 3
+		testing.expect_value(t, -7 &~ 3, a &~ b)
+		testing.expect_value(t, -7 &~ 3, -8)
+	}
+	{
+		a, b := -255, 5
+		testing.expect_value(t, -255 &~ 5, a &~ b)
+		testing.expect_value(t, -255 &~ 5, -256)
+	}
+
+	// both negative
+	{
+		a, b := -7, -3
+		testing.expect_value(t, -7 &~ -3, a &~ b)
+		testing.expect_value(t, -7 &~ -3, 0)
+	}
+	{
+		a, b := -3, -7
+		testing.expect_value(t, -3 &~ -7, a &~ b)
+		testing.expect_value(t, -3 &~ -7, 4)
+	}
+
+	// the cases that were already correct, so a fix cannot regress them
+	{
+		a, b := 7, 3
+		testing.expect_value(t, 7 &~ 3, a &~ b)
+		testing.expect_value(t, 7 &~ 3, 4)
+	}
+	{
+		a, b := 7, -3
+		testing.expect_value(t, 7 &~ -3, a &~ b)
+		testing.expect_value(t, 7 &~ -3, 2)
+	}
+	{
+		a, b := 7, 0
+		testing.expect_value(t, 7 &~ 0, a &~ b)
+		testing.expect_value(t, 7 &~ 0, 7)
+	}
+}
+
+@(test)
+and_not_constant_folding_every_width :: proc(t: ^testing.T) {
+	// the zero-left shape reaches unsigned types too
+	{
+		a, b := u8(0), u8(1)
+		testing.expect_value(t, u8(0) &~ u8(1), a &~ b)
+		testing.expect_value(t, u8(0) &~ u8(1), u8(0))
+	}
+	{
+		a, b := u64(0), u64(255)
+		testing.expect_value(t, u64(0) &~ u64(255), a &~ b)
+		testing.expect_value(t, u64(0) &~ u64(255), u64(0))
+	}
+
+	// signed, at the extremes of each width
+	{
+		a, b := i8(-128), i8(1)
+		testing.expect_value(t, i8(-128) &~ i8(1), a &~ b)
+		testing.expect_value(t, i8(-128) &~ i8(1), i8(-128))
+	}
+	{
+		a, b := i8(-128), i8(127)
+		testing.expect_value(t, i8(-128) &~ i8(127), a &~ b)
+		testing.expect_value(t, i8(-128) &~ i8(127), i8(-128))
+	}
+	{
+		a, b := i8(-7), i8(-128)
+		testing.expect_value(t, i8(-7) &~ i8(-128), a &~ b)
+		testing.expect_value(t, i8(-7) &~ i8(-128), i8(121))
+	}
+	{
+		a, b := i16(-7), i16(3)
+		testing.expect_value(t, i16(-7) &~ i16(3), a &~ b)
+		testing.expect_value(t, i16(-7) &~ i16(3), i16(-8))
+	}
+	{
+		a, b := i32(-255), i32(5)
+		testing.expect_value(t, i32(-255) &~ i32(5), a &~ b)
+		testing.expect_value(t, i32(-255) &~ i32(5), i32(-256))
+	}
+	{
+		a, b := i64(-7), i64(-3)
+		testing.expect_value(t, i64(-7) &~ i64(-3), a &~ b)
+		testing.expect_value(t, i64(-7) &~ i64(-3), i64(0))
+	}
+}
+
+// `&~` was the only operator found divergent; the rest of the bitwise family shares the sign
+// handling and must stay agreeing.
+
+@(test)
+bitwise_constant_folding_matches_runtime :: proc(t: ^testing.T) {
+	{
+		a, b := -7, 3
+		testing.expect_value(t, -7 & 3, a & b)
+		testing.expect_value(t, -7 | 3, a | b)
+		testing.expect_value(t, -7 ~ 3, a ~ b)
+	}
+	{
+		a, b := -7, -3
+		testing.expect_value(t, -7 & -3, a & b)
+		testing.expect_value(t, -7 | -3, a | b)
+		testing.expect_value(t, -7 ~ -3, a ~ b)
+	}
+	{
+		a, b := 0, -3
+		testing.expect_value(t, 0 & -3, a & b)
+		testing.expect_value(t, 0 | -3, a | b)
+		testing.expect_value(t, 0 ~ -3, a ~ b)
+	}
 // `<=` and `<` on a `bit_set` are subset and proper subset, `>=` and `>` superset. The folder
 // asked `(lhs & rhs) <= lhs` where the definition is `(lhs & rhs) == lhs`, which is true for
 // any operands, so `<=` folded true unconditionally; `<` compounded it by requiring `lhs < rhs`
 // where it needs `lhs != rhs`. Under `when` this decides which declarations exist.
+}
 
 @(test)
 bit_set_subset_folding_matches_runtime :: proc(t: ^testing.T) {
@@ -75,4 +210,4 @@ bit_set_subset_folding_selects_the_right_when_arm :: proc(t: ^testing.T) {
 	testing.expect_value(t, W2, 1)
 	testing.expect_value(t, W3, 1)
 	testing.expect_value(t, W4, 1)
-}
+}
\ No newline at end of file