Behavior of function depends on whether method call is defined before first use of function

[astrieanna]

I defined a function that uses a yet-to-be-defined method.
If I don't call the function until after I've defined the function it calls, everything is fine.
If I call it before I define the method, then it (as I expected) throws an error.
If I then define the missing method and run the first function again, there is no error but it also doesn't return the value that it should. I expected it to either work correctly or at least keep throwing an error.

julia> function foo(x,y)
         x + y
       end
# methods for generic function foo
foo(x,y) at none:2

julia> foo(true,"hi")
ERROR: no method +(Bool,ASCIIString)
 in foo at none:2

julia> +(b::Bool,s::String) = "$b$s"
# methods for generic function +
+(x::Bool,y::Bool) at bool.jl:38
+(x::Int64,y::Int64) at int.jl:36
+(x::Union(SubArray{Bool,N,A<:Array{T,N},I<:(Union(Int64,Range1{Int64},Range{Int64})...,)},Array{Bool,N}),y::Union(SubArray{Bool,N,A<:Array{T,N},I<:(Union(Int64,Range1{Int64},Range{Int64})...,)},Array{Bool,N})) at array.jl:1003
+{S,T}(A::Union(SubArray{S,N,A<:Array{T,N},I<:(Union(Int64,Range1{Int64},Range{Int64})...,)},Array{S,N}),B::Union(SubArray{T,N,A<:Array{T,N},I<:(Union(Int64,Range1{Int64},Range{Int64})...,)},Array{T,N})) at array.jl:947
+{T<:Union(Int8,Int64,Int16,Int32)}(x::T<:Union(Int8,Int64,Int16,Int32),y::T<:Union(Int8,Int64,Int16,Int32)) at int.jl:11
... 87 methods not shown (use methods(+) to see them all)

julia> foo(true,"hi")

julia> foo(true,"hi")

julia> function foo(x,y)
         x + y
       end
# methods for generic function foo
foo(x,y) at none:2

julia> foo(true,"hi")
"truehi"

[vtjnash]

dup of #265