From 2b94b63ebcc773620fa7ef40cb1ae34bac2e6348 Mon Sep 17 00:00:00 2001
From: Lucas Chollet <lucas.chollet@free.fr>
Date: Wed, 23 Sep 2026 10:25:54 +0200
Subject: [PATCH] LibC: Handle edge cases correctly in erf

---
 Tests/LibC/TestMath.cpp          | 11 +++++++++++
 Userland/Libraries/LibC/math.cpp | 12 ++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/Tests/LibC/TestMath.cpp b/Tests/LibC/TestMath.cpp
index 5953c8d4bca..aaf7e09f5ab 100644
--- a/Tests/LibC/TestMath.cpp
+++ b/Tests/LibC/TestMath.cpp
@@ -508,3 +508,14 @@ TEST_CASE(exp)
 
     EXPECT_EQ(exp(AK::Infinity<double>), AK::Infinity<double>);
 }
+
+TEST_CASE(erf)
+{
+    EXPECT(isnan(erf(AK::NaN<double>)));
+
+    EXPECT_EQ(erf(0.0), 0.0);
+    EXPECT(signbit(erf(-0.0)));
+
+    EXPECT_EQ(erf(-AK::Infinity<double>), -1.);
+    EXPECT_EQ(erf(+AK::Infinity<double>), +1.);
+}
diff --git a/Userland/Libraries/LibC/math.cpp b/Userland/Libraries/LibC/math.cpp
index 276774210c0..b47b25ae898 100644
--- a/Userland/Libraries/LibC/math.cpp
+++ b/Userland/Libraries/LibC/math.cpp
@@ -342,6 +342,18 @@ static T internal_remquo(T x, T y, int* quo)
 template<FloatingPoint T>
 static T internal_erf(T x)
 {
+    // If x is NaN, a NaN shall be returned.
+    if (isnan(x))
+        return AK::NaN<T>;
+
+    // If x is ±0, ±0 shall be returned.
+    if (x == 0)
+        return x;
+
+    // If x is ±Inf, ±1 shall be returned.
+    if (isinf(x))
+        return AK::copysign<T>(1., x);
+
     // algorithm taken from Abramowitz and Stegun (no. 26.2.17)
     T t = 1 / (1 + 0.47047l * AK::fabs(x));
     T poly = t * (0.3480242l + t * (-0.958798l + t * 0.7478556l));