Fix findfirst(==(x::Integer), r::IdentityUnitRange) (JuliaLang#42756)

Author: N5N3

base/array.jl

@@ -2070,7 +2070,7 @@ findfirst(p::Union{Fix2{typeof(isequal),Int},Fix2{typeof(==),Int}}, r::OneTo{Int
     1 <= p.x <= r.stop ? p.x : nothing
 
 findfirst(p::Union{Fix2{typeof(isequal),T},Fix2{typeof(==),T}}, r::AbstractUnitRange) where {T<:Integer} =
-    first(r) <= p.x <= last(r) ? 1+Int(p.x - first(r)) : nothing
+    first(r) <= p.x <= last(r) ? firstindex(r) + Int(p.x - first(r)) : nothing
 
 function findfirst(p::Union{Fix2{typeof(isequal),T},Fix2{typeof(==),T}}, r::StepRange{T,S}) where {T,S}
     isempty(r) && return nothing

test/ranges.jl

@@ -348,6 +348,7 @@ end
         end
     end
     @testset "findfirst" begin
+        @test findfirst(==(1), Base.IdentityUnitRange(-1:1)) == 1
         @test findfirst(isequal(3), Base.OneTo(10)) == 3
         @test findfirst(==(0), Base.OneTo(10)) == nothing
         @test findfirst(==(11), Base.OneTo(10)) == nothing