Type unstable function because same variable name used twice

General Usage
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10

Interesting example. Learning with you. Note that the instability is not dependent on the order of the assignments:

julia> function g(x)
           g(_) = y 
           y = (1,2)
           y = [1,2]
           return g(x)
       end
g (generic function with 1 method)

julia> @code_warntype g(1)
Variables
  #self#::Core.Const(g)
  x::Int64
  y::Core.Box
  g::var"#g#15"

Body::Any
1 ─      (y = Core.Box())
│        (g = %new(Main.:(var"#g#15"), y))
│   %3 = Core.tuple(1, 2)::Core.Const((1, 2))
│        Core.setfield!(y, :contents, %3)
│   %5 = Base.vect(1, 2)::Vector{Int64}
│        Core.setfield!(y, :contents, %5)
│   %7 = (g)(x)::Any
└──      return %7

Thus, to be stable, the compiler should have noticed that wherever g is called y is constant. It is understandable that this type inference is tricky. What if g was called twice, should two specialized versions of g be compiled (but not specialized to arguments, but to captured variables)?

Of course that is solved if y is now a parameter of g (i. e. g(y) = y).

What is happening here appears to be related to the discussion in this thread. From what I understand, the possibility of changing the value of a captured variable in a closure is a feature in some sense, but can lead to these problems. Probably the good advice here is that one should never change the value of a captured variable except if, for some reason, the purpose of the closure is to modify that variable. Probably in most cases, one should consider these as a kind of “avoid non-constant globals” performance tip, but in this case the non-constant variable is local to the scope of the enclosing function.

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