Change mod(x, Inf) semantics to align with C when x is finite (#47102)

fix #46694

---------

Co-authored-by: Dilum Aluthge <dilum@aluthge.com>
Co-authored-by: Lilith Orion Hafner <lilithhafner@gmail.com>
Co-authored-by: Oscar Smith <oscardssmith@gmail.com>
Co-authored-by: Mosè Giordano <765740+giordano@users.noreply.github.com>

Author: 5 people

NEWS.md

@@ -8,6 +8,8 @@ New language features
 
 Language changes
 ----------------
+* `mod(x::AbstractFloat, -Inf)` now returns `x` (as long as `x` is finite), this aligns with C standard and
+is considered a bug fix ([#47102])
 
 Compiler/Runtime improvements
 -----------------------------

base/float.jl

@@ -603,7 +603,10 @@ function rem(x::T, y::T) where {T<:IEEEFloat}
     end
 end
 
-function mod(x::T, y::T) where {T<:AbstractFloat}
+function mod(x::T, y::T) where T<:AbstractFloat
+    if isinf(y) && isfinite(x)
+        return x
+    end
     r = rem(x,y)
     if r == 0
         copysign(r,y)

base/int.jl

@@ -250,7 +250,7 @@ end
 The reduction of `x` modulo `y`, or equivalently, the remainder of `x` after floored
 division by `y`, i.e. `x - y*fld(x,y)` if computed without intermediate rounding.
 
-The result will have the same sign as `y`, and magnitude less than `abs(y)` (with some
+The result will have the same sign as `y` if `isfinite(y)`, and magnitude less than `abs(y)` (with some
 exceptions, see note below).
 
 !!! note

test/numbers.jl

@@ -1595,36 +1595,44 @@ end
         end
     end
 
-    for x=0:5, y=1:5
-        @test div(UInt(x),UInt(y)) == div(x,y)
-        @test div(UInt(x),y) == div(x,y)
-        @test div(x,UInt(y)) == div(x,y)
-        @test div(UInt(x),-y) == reinterpret(UInt,div(x,-y))
-        @test div(-x,UInt(y)) == div(-x,y)
-
-        @test fld(UInt(x),UInt(y)) == fld(x,y)
-        @test fld(UInt(x),y) == fld(x,y)
-        @test fld(x,UInt(y)) == fld(x,y)
-        @test fld(UInt(x),-y) == reinterpret(UInt,fld(x,-y))
-        @test fld(-x,UInt(y)) == fld(-x,y)
-
-        @test cld(UInt(x),UInt(y)) == cld(x,y)
-        @test cld(UInt(x),y) == cld(x,y)
-        @test cld(x,UInt(y)) == cld(x,y)
-        @test cld(UInt(x),-y) == reinterpret(UInt,cld(x,-y))
-        @test cld(-x,UInt(y)) == cld(-x,y)
-
-        @test rem(UInt(x),UInt(y)) == rem(x,y)
-        @test rem(UInt(x),y) == rem(x,y)
-        @test rem(x,UInt(y)) == rem(x,y)
-        @test rem(UInt(x),-y) == rem(x,-y)
-        @test rem(-x,UInt(y)) == rem(-x,y)
-
-        @test mod(UInt(x),UInt(y)) == mod(x,y)
-        @test mod(UInt(x),y) == mod(x,y)
-        @test mod(x,UInt(y)) == mod(x,y)
-        @test mod(UInt(x),-y) == mod(x,-y)
-        @test mod(-x,UInt(y)) == mod(-x,y)
+    @test isnan(mod(NaN, Inf))
+    @test isnan(mod(NaN, -Inf))
+    for x=0:5
+        @test mod(x, Inf) == x
+        @test mod(x, -Inf) == x
+        @test mod(-x, Inf) == -x
+        @test mod(-x, -Inf) == -x
+        for y=1:5
+            @test div(UInt(x),UInt(y)) == div(x,y)
+            @test div(UInt(x),y) == div(x,y)
+            @test div(x,UInt(y)) == div(x,y)
+            @test div(UInt(x),-y) == reinterpret(UInt,div(x,-y))
+            @test div(-x,UInt(y)) == div(-x,y)
+
+            @test fld(UInt(x),UInt(y)) == fld(x,y)
+            @test fld(UInt(x),y) == fld(x,y)
+            @test fld(x,UInt(y)) == fld(x,y)
+            @test fld(UInt(x),-y) == reinterpret(UInt,fld(x,-y))
+            @test fld(-x,UInt(y)) == fld(-x,y)
+
+            @test cld(UInt(x),UInt(y)) == cld(x,y)
+            @test cld(UInt(x),y) == cld(x,y)
+            @test cld(x,UInt(y)) == cld(x,y)
+            @test cld(UInt(x),-y) == reinterpret(UInt,cld(x,-y))
+            @test cld(-x,UInt(y)) == cld(-x,y)
+
+            @test rem(UInt(x),UInt(y)) == rem(x,y)
+            @test rem(UInt(x),y) == rem(x,y)
+            @test rem(x,UInt(y)) == rem(x,y)
+            @test rem(UInt(x),-y) == rem(x,-y)
+            @test rem(-x,UInt(y)) == rem(-x,y)
+
+            @test mod(UInt(x),UInt(y)) == mod(x,y)
+            @test mod(UInt(x),y) == mod(x,y)
+            @test mod(x,UInt(y)) == mod(x,y)
+            @test mod(UInt(x),-y) == mod(x,-y)
+            @test mod(-x,UInt(y)) == mod(-x,y)
+        end
     end
 
     @test div(typemax(UInt64)  , 1) ==  typemax(UInt64)