style: improvements

src/irm/libraries/ErrorsLib.sol

@@ -4,6 +4,7 @@ pragma solidity ^0.8.0;
 library ErrorsLib {
     string internal constant MAX_INT128_EXCEEDED = "max int128 exceeded";
     string internal constant INPUT_TOO_LARGE = "input too large";
+    string internal constant WEXP_UNDERFLOW = "wExp underflow";
     string internal constant WEXP_OVERFLOW = "wExp overflow";
     string internal constant NOT_MORPHO = "not Morpho";
 }

src/irm/libraries/MathLib.sol

@@ -20,9 +20,14 @@ library MathLib {
         unchecked {
             // Revert if x > ln(2^256-1) ~ 177.
             require(x <= 177.44567822334599921 ether, ErrorsLib.WEXP_OVERFLOW);
+            // Revert if x > min / 1e18.
+            require(x >= type(int256).min + 1 ether, ErrorsLib.WEXP_UNDERFLOW);
 
             // Decompose x as x = q * ln(2) + r with q an integer and -ln(2)/2 < r <= ln(2)/2.
-            int256 q = (x + (LN2_INT / 2)) / LN2_INT;
+            // q = x / ln(2) rounded half toward zero.
+            int256 offset = (x < 0) ? - (LN2_INT / 2) : (LN2_INT / 2);
+            // Safe unchecked because x is bounded.
+            int256 q = (x + offset) / LN2_INT;
             // Safe unchecked because |q * LN2_INT| <= x.
             int256 r = x - q * LN2_INT;
 

test/irm/MathLibTest.sol

@@ -11,13 +11,12 @@ contract MathLibTest is Test {
     using MathLib for uint256;
 
     function testWExp(int256 x) public {
-        vm.assume(x > -176 ether);
-        vm.assume(x < 135305999368893231589);
-        assertApproxEqRel(int256(MathLib.wExp(x)), wadExp(x), 0.05 ether);
+        x = bound(x, - 27 ether, 94 ether);
+        assertApproxEqRel(MathLib.wExp(x), uint256(wadExp(x)), 0.01 ether);
     }
 
     function testWExpSmall(int256 x) public {
-        vm.assume(x <= -178 ether);
+        x = bound(x, type(int256).min + 1 ether, -178 ether);
         assertEq(MathLib.wExp(x), 0);
     }