fix some 1.5 compatibility issues (#35965)

- restore some `copyto!` methods that Knet assumed existed
- allow `findfirst` to work on InfiniteArrays again
- allow constructing a LineNumberNode from a String

base/array.jl

@@ -320,6 +320,15 @@ Copy `N` elements from collection `src` starting at offset `so`, to array `dest`
 offset `do`. Return `dest`.
 """
 function copyto!(dest::Array, doffs::Integer, src::Array, soffs::Integer, n::Integer)
+    return _copyto_impl!(dest, doffs, src, soffs, n)
+end
+
+# this is only needed to avoid possible ambiguities with methods added in some packages
+function copyto!(dest::Array{T}, doffs::Integer, src::Array{T}, soffs::Integer, n::Integer) where T
+    return _copyto_impl!(dest, doffs, src, soffs, n)
+end
+
+function _copyto_impl!(dest::Array, doffs::Integer, src::Array, soffs::Integer, n::Integer)
     n == 0 && return dest
     n > 0 || _throw_argerror()
     if soffs < 1 || doffs < 1 || soffs+n-1 > length(src) || doffs+n-1 > length(dest)
@@ -339,6 +348,9 @@ end
 
 copyto!(dest::Array, src::Array) = copyto!(dest, 1, src, 1, length(src))
 
+# also to avoid ambiguities in packages
+copyto!(dest::Array{T}, src::Array{T}) where {T} = copyto!(dest, 1, src, 1, length(src))
+
 # N.B: The generic definition in multidimensional.jl covers, this, this is just here
 # for bootstrapping purposes.
 function fill!(dest::Array{T}, x) where T
@@ -1762,8 +1774,8 @@ CartesianIndex(1, 1)
 ```
 """
 function findnext(testf::Function, A, start)
+    i = oftype(first(keys(A)), start)
     l = last(keys(A))
-    i = oftype(l, start)
     i > l && return nothing
     while true
         testf(A[i]) && return i

base/boot.jl

@@ -370,6 +370,7 @@ _new(:QuoteNode, :Any)
 _new(:SSAValue, :Int)
 eval(Core, :(LineNumberNode(l::Int) = $(Expr(:new, :LineNumberNode, :l, nothing))))
 eval(Core, :(LineNumberNode(l::Int, @nospecialize(f)) = $(Expr(:new, :LineNumberNode, :l, :f))))
+LineNumberNode(l::Int, f::String) = LineNumberNode(l, Symbol(f))
 eval(Core, :(GlobalRef(m::Module, s::Symbol) = $(Expr(:new, :GlobalRef, :m, :s))))
 eval(Core, :(SlotNumber(n::Int) = $(Expr(:new, :SlotNumber, :n))))
 eval(Core, :(TypedSlot(n::Int, @nospecialize(t)) = $(Expr(:new, :TypedSlot, :n, :t))))