From a3c4877dfd882a06d5d6eaa3af5247cc3c505194 Mon Sep 17 00:00:00 2001
From: Oscar Smith <oscardssmith@gmail.com>
Date: Fri, 21 Mar 2025 11:48:49 -0400
Subject: [PATCH 1/4] fix `mod` for mixes of `Signed` and `Unsigned`

Previously this was just overfowing producing wrong answers (both for the sign convention and just the wrong modulo class)
fixes https://github.com/JuliaLang/julia/issues/57851
---
 base/int.jl | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/base/int.jl b/base/int.jl
index 38cbe366bd7ce..e46d518183d7a 100644
--- a/base/int.jl
+++ b/base/int.jl
@@ -286,8 +286,14 @@ function mod(x::T, y::T) where T<:Integer
     y == -1 && return T(0)   # avoid potential overflow in fld
     return x - fld(x, y) * y
 end
-mod(x::BitSigned, y::Unsigned) = rem(y + unsigned(rem(x, y)), y)
-mod(x::Unsigned, y::Signed) = rem(y + signed(rem(x, y)), y)
+function mod(x::BitSigned, y::Unsigned)
+    remval = rem(x, y) # correct iff x>0 or remval==0
+    return remval + (!iszero(remval) && x<zero(x))*y
+end
+function mod(x::Unsigned, y::Signed)
+    remval =  signed(rem(x, y)) #remval>0 so correct iff y>0 or remval==0
+    return remval + (!iszero(remval) && y<zero(y))*y
+end
 mod(x::T, y::T) where {T<:Unsigned} = rem(x, y)
 
 # Don't promote integers for div/rem/mod since there is no danger of overflow,

From 55e33621028656b2beb9fb0b2ecd07ca05f7eb7e Mon Sep 17 00:00:00 2001
From: Oscar Smith <oscardssmith@gmail.com>
Date: Fri, 21 Mar 2025 12:30:56 -0400
Subject: [PATCH 2/4] add test

---
 test/int.jl | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/test/int.jl b/test/int.jl
index dda736cd7a9d0..715625cf8b6d9 100644
--- a/test/int.jl
+++ b/test/int.jl
@@ -362,6 +362,23 @@ end
             end
         end
     end
+    # exhaustive UInt8/Int8 tests for mixed signedness
+    for f in (mod, rem)
+        for i in -128:127
+            for j in 0:255
+                if iszero(i)
+                    @test_throws DivideError f(UInt8(j), Int8(i))
+                else
+                    @test f(UInt8(j), Int8(i)) == f(j, i)
+                end
+                if iszero(j)
+                    @test_throws DivideError f(Int8(i), UInt8(j))
+                else
+                    @test f(Int8(i), UInt8(j)) == f(i,j)
+                end
+            end
+        end
+    end
 end
 
 @testset "Underscores in big_str" begin

From 32713ddb4b279d7ce6234d9b558d3145f8286cdf Mon Sep 17 00:00:00 2001
From: Oscar Smith <oscardssmith@gmail.com>
Date: Fri, 21 Mar 2025 12:31:42 -0400
Subject: [PATCH 3/4] simplify slightly

---
 base/int.jl | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/base/int.jl b/base/int.jl
index e46d518183d7a..042d29d8abdbe 100644
--- a/base/int.jl
+++ b/base/int.jl
@@ -287,8 +287,8 @@ function mod(x::T, y::T) where T<:Integer
     return x - fld(x, y) * y
 end
 function mod(x::BitSigned, y::Unsigned)
-    remval = rem(x, y) # correct iff x>0 or remval==0
-    return remval + (!iszero(remval) && x<zero(x))*y
+    remval = rem(x, y) # correct iff  remval>=0
+    return remval + (remval<zero(remval))*y
 end
 function mod(x::Unsigned, y::Signed)
     remval =  signed(rem(x, y)) #remval>0 so correct iff y>0 or remval==0

From d579285a3cb2b3dc8dbb0a212f3b984699f8c489 Mon Sep 17 00:00:00 2001
From: Oscar Smith <oscardssmith@gmail.com>
Date: Sat, 22 Mar 2025 14:47:59 -0400
Subject: [PATCH 4/4] fix types

---
 base/int.jl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/base/int.jl b/base/int.jl
index 042d29d8abdbe..2d7bede4fac69 100644
--- a/base/int.jl
+++ b/base/int.jl
@@ -288,7 +288,7 @@ function mod(x::T, y::T) where T<:Integer
 end
 function mod(x::BitSigned, y::Unsigned)
     remval = rem(x, y) # correct iff  remval>=0
-    return remval + (remval<zero(remval))*y
+    return unsigned(remval + (remval<zero(remval))*y)
 end
 function mod(x::Unsigned, y::Signed)
     remval =  signed(rem(x, y)) #remval>0 so correct iff y>0 or remval==0