UndefVarError not raised if the variable is not used

[yuyichao]

Having a unused undefined variable in a function doesn't seems to cause an error...

julia> function f(a)
           println(1)
           aaaaaa
           println(2)
       end
f (generic function with 1 method)

julia> f(1)
1
2

julia> @code_lowered f(1)
1-element Array{Any,1}:
 :($(Expr(:lambda, Any[:a], Any[Any[Any[:a,:Any,0]],Any[],0,Any[]], :(begin  # none, line 2:
        (Main.println)(1) # none, line 3: # none, line 4:
        return (Main.println)(2)
    end))))

[mbauman]

Dup of the likely unsearchable "if cond then works" issue: #6846