Add better error message for control flow in higher-order reverse

Author: Keno

src/stage1/generated.jl

@@ -22,7 +22,10 @@ function perform_optic_transform(@nospecialize(ff::Type{∂⃖recurse{N}}), @nos
     ci′ = copy(ci)
     ci′.edges = MethodInstance[mi]
 
-    transform!(ci′, mi.def, length(args) - 1, match.sparams, N)
+    r = transform!(ci′, mi.def, length(args) - 1, match.sparams, N)
+    if isa(r, Expr)
+        return r
+    end
 
     ci′.ssavaluetypes = length(ci′.code)
     ci′.method_for_inference_limit_heuristics = match.method

src/stage1/recurse.jl

@@ -704,6 +704,8 @@ function transform!(ci, meth, nargs, sparams, N)
                 error()
             elseif isa(stmt, GlobalRef)
                 fwds[i] = ZeroTangent()
+            elseif isa(stmt, Union{GotoNode, GotoIfNot})
+                return :(error("Control flow support not fully implemented yet for higher-order reverse mode (TODO)"))
             elseif !isa(stmt, Expr)
                 @show stmt
                 error()