make length(StepRange()) type stable (#45236)

Author: Moelf

base/range.jl

@@ -762,13 +762,13 @@ let bigints = Union{Int, UInt, Int64, UInt64, Int128, UInt128}
         # therefore still be valid (if the result is representable at all)
         # n.b. !(s isa T)
         if s isa Unsigned || -1 <= s <= 1 || s == -s
-            a = div(diff, s)
+            a = div(diff, s) % T
         elseif s < 0
-            a = div(unsigned(-diff), -s) % typeof(diff)
+            a = div(unsigned(-diff), -s) % T
         else
-            a = div(unsigned(diff), s) % typeof(diff)
+            a = div(unsigned(diff), s) % T
         end
-        return Integer(a) + oneunit(a)
+        return a + oneunit(T)
     end
     function checked_length(r::OrdinalRange{T}) where T<:bigints
         s = step(r)
@@ -786,7 +786,7 @@ let bigints = Union{Int, UInt, Int64, UInt64, Int128, UInt128}
         else
             a = div(checked_sub(start, stop), -s)
         end
-        return checked_add(a, oneunit(a))
+        return checked_add(convert(T, a), oneunit(T))
     end
 end
 

test/ranges.jl

@@ -2031,6 +2031,11 @@ end
     @test typeof(step(r)) === Int8
 end
 
+@testset "length(StepRange()) type stability" begin
+    typeof(length(StepRange(1,Int128(1),1))) == typeof(length(StepRange(1,Int128(1),0)))
+    typeof(checked_length(StepRange(1,Int128(1),1))) == typeof(checked_length(StepRange(1,Int128(1),0)))
+end
+
 @testset "LinRange eltype for element types that wrap integers" begin
     struct RealWrapper{T <: Real} <: Real
         x :: T