Throw on non-promotion in fallback UnitRange constructor (JuliaLang…

…#50292)

* Throw on promotion failure in UnitRange

* rename variables

base/range.jl

@@ -404,7 +404,11 @@ struct UnitRange{T<:Real} <: AbstractUnitRange{T}
 end
 UnitRange{T}(start, stop) where {T<:Real} = UnitRange{T}(convert(T, start), convert(T, stop))
 UnitRange(start::T, stop::T) where {T<:Real} = UnitRange{T}(start, stop)
-UnitRange(start, stop) = UnitRange(promote(start, stop)...)
+function UnitRange(start, stop)
+    startstop_promoted = promote(start, stop)
+    not_sametype((start, stop), startstop_promoted)
+    UnitRange(startstop_promoted...)
+end
 
 # if stop and start are integral, we know that their difference is a multiple of 1
 unitrange_last(start::Integer, stop::Integer) =

test/ranges.jl

@@ -59,6 +59,9 @@ using InteractiveUtils: code_llvm
     @test last(10:0.2:3) === 9.8
     @test step(10:0.2:3) === 0.2
     @test isempty(10:0.2:3)
+
+    unitrangeerrstr = "promotion of types Char and Char failed to change any arguments"
+    @test_throws unitrangeerrstr UnitRange('a', 'b')
 end
 
 using Dates, Random