bugfix: fld1 order of ops (#46938)

* bugfix: fld1 order of ops

fixes #28973

Author: JeffreySarnoff

NEWS.md

@@ -89,6 +89,7 @@ Library changes
 * `eachslice` now works over multiple dimensions; `eachslice`, `eachrow` and `eachcol` return
   a `Slices` object, which allows dispatching to provide more efficient methods ([#32310]).
 * `@kwdef` is now exported and added to the public API ([#46273])
+* An issue with order of operations in `fld1` is now fixed ([#28973]).
 
 Standard library changes
 ------------------------

base/operators.jl

@@ -842,7 +842,7 @@ julia> x == (fld1(x, y) - 1) * y + mod1(x, y)
 true
 ```
 """
-fld1(x::T, y::T) where {T<:Real} = (m = mod1(x, y); fld(x + y - m, y))
+fld1(x::T, y::T) where {T<:Real} = (m = mod1(x, y); fld((x - m) + y, y))
 function fld1(x::T, y::T) where T<:Integer
     d = div(x, y)
     return d + (!signbit(x ⊻ y) & (d * y != x))

test/operators.jl

@@ -274,6 +274,9 @@ end
     end
 
     @test fldmod1(4.0, 3) == fldmod1(4, 3)
+
+    # issue 28973
+    @test fld1(0.4, 0.9) == fld1(nextfloat(0.4), 0.9) == 1.0
 end
 
 @testset "Fix12" begin