Merge pull request #1278 from FluxML/bc/fix-rrule-lookup

Propagate ambiguities from rrule lookup instead of failing inexplicably

Author: ToucheSir

src/compiler/chainrules.jl

@@ -73,7 +73,8 @@ matching_cr_sig(t, s) = matching_cr_sig(t.method.sig, s.method.sig)
 matching_cr_sig(::DataType, ::UnionAll) = false
 matching_cr_sig(::UnionAll, ::DataType) = false
 matching_cr_sig(t::Type, s::Type) = type_tuple_tail(t) == type_tuple_tail(s)
- 
+matching_cr_sig(::Any, ::Nothing) = false  # https://github.com/FluxML/Zygote.jl/issues/1234
+
 type_tuple_tail(d::DataType) = Tuple{d.parameters[2:end]...}
 function type_tuple_tail(d::UnionAll)
     body = Base.unwrap_unionall(d)

test/chainrules.jl

@@ -275,6 +275,15 @@ using Zygote: ZygoteRuleConfig
             @test Zygote.gradient(x -> f_notimplemented(only(x)), [0.1]) === (nothing,)
         end
     end
+
+    # https://github.com/FluxML/Zygote.jl/issues/1234
+    @testset "rrule lookup ambiguities" begin
+      f_ambig(x, y) = x + y
+      ChainRulesCore.rrule(::typeof(f_ambig), x::Int, y) = x + y, _ -> (0, 0)
+      ChainRulesCore.rrule(::typeof(f_ambig), x, y::Int) = x + y, _ -> (0, 0)
+
+      @test_throws MethodError pullback(f_ambig, 1, 2)
+    end
 end
 
 @testset "ChainRulesCore.rrule_via_ad" begin