bugfix: fld1 order of ops (#46938)

* bugfix: fld1 order of ops

fixes #28973

(cherry picked from commit fcdc5bc)

NEWS.md

@@ -111,6 +111,7 @@ New library features
 * `extrema` now accepts an `init` keyword argument ([#36265], [#43604]).
 * `Iterators.countfrom` now accepts any type that defines `+` ([#37747]).
 * `@time` now separates out % time spent recompiling invalidated methods ([#45015]).
+* An issue with order of operations in `fld1` is now fixed ([#28973]).
 
 Standard library changes
 ------------------------

base/operators.jl

@@ -855,7 +855,7 @@ 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

@@ -261,6 +261,9 @@ 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