fix #672

Author: mcabbott

src/rulesets/Base/mapreduce.jl

@@ -420,11 +420,13 @@ end
 ##### `mapfoldl(f, g, ::Tuple)`
 #####
 
+using Base: mapfoldl_impl
+
 # For tuples there should be no harm in handling `map` first.
 # This will also catch `mapreduce`.
 
 function rrule(
-        cfg::RuleConfig{>:HasReverseMode}, ::typeof(Base.mapfoldl_impl), f::F, op::G, init, x::Tuple;
+        cfg::RuleConfig{>:HasReverseMode}, ::typeof(mapfoldl_impl), f::F, op::G, init, x::Tuple;
     ) where {F,G}
     y, backmap = rrule(cfg, map, f, x)
     z, backred = rrule(cfg, Base.mapfoldl_impl, identity, op, init, y)
@@ -436,6 +438,11 @@ function rrule(
     return z, mapfoldl_pullback_tuple
 end
 
+function rrule(::RuleConfig{>:HasReverseMode}, ::typeof(mapfoldl_impl), f, op, init, x::Tuple{})
+    foldl_pullback_empty(dy) = (NoTangent(), NoTangent(), NoTangent(), dy, NoTangent())
+    return init, foldl_pullback_empty
+end
+
 #####
 ##### `foldl(f, ::Tuple)`
 #####
@@ -495,6 +502,12 @@ function rrule(
     return y, foldl_pullback_tuple_init
 end
 
+# Base.tail doesn't work on (), trivial case:
+function rrule(::RuleConfig{>:HasReverseMode}, ::typeof(mapfoldl_impl), ::typeof(identity), op, init, x::Tuple{})
+    foldl_pullback_empty(dy) = (NoTangent(), NoTangent(), NoTangent(), dy, NoTangent())
+    return init, foldl_pullback_empty
+end
+
 #####
 ##### `foldl(f, ::Array)`
 #####

test/rulesets/Base/mapreduce.jl

@@ -303,6 +303,10 @@ const _INIT = Base._InitialValue()
         # Finite differencing
         test_rrule(mapfoldl_impl, identity, /, _INIT, Tuple(1 .+ rand(5)))
         test_rrule(mapfoldl_impl, identity, *, 1+rand(), Tuple(rand(ComplexF64, 5)))
+        
+        # Trivial case
+        test_rrule(mapfoldl_impl, identity, /, 2pi, ())
+        test_rrule(mapfoldl_impl, sqrt, /, 2pi, ())
     end
     @testset "mapfoldl(f, g, ::Tuple)" begin
         test_rrule(mapfoldl_impl, cbrt, /, _INIT, Tuple(1 .+ rand(5)), check_inferred=false)