fix typo causing a panic in tuple unification

cty/convert/unify.go

@@ -447,7 +447,6 @@ func unifyTupleTypes(types []cty.Type, unsafe bool, hasDynamic bool) (cty.Type,
 			conversions[i] = GetConversion(ty, retTy)
 		}
 		if conversions[i] == nil {
-			// Shouldn't be reachable, since we were able to unify
 			return unifyTupleTypesToList(types, unsafe)
 		}
 	}
@@ -483,8 +482,8 @@ func unifyTupleTypesToList(types []cty.Type, unsafe bool) (cty.Type, []Conversio
 			conversions[i] = GetConversion(ty, retTy)
 		}
 		if conversions[i] == nil {
-			// Shouldn't be reachable, since we were able to unify
-			return unifyObjectTypesToMap(types, unsafe)
+			// no conversion was found
+			return cty.NilType, nil
 		}
 	}
 	return retTy, conversions

cty/convert/unify_test.go

@@ -163,6 +163,23 @@ func TestUnify(t *testing.T) {
 			cty.List(cty.Map(cty.String)),
 			[]bool{true, true},
 		},
+		{
+			// The second tuple value could be anything, so we can't unify
+			// these as a list.
+			// FIXME: While a unification is possible, we get a NilType for
+			// now until we can handle more complex recursive unification.
+			[]cty.Type{
+				cty.Tuple([]cty.Type{
+					cty.Object(map[string]cty.Type{
+						"a": cty.String,
+					}),
+					cty.DynamicPseudoType,
+				}),
+				cty.List(cty.DynamicPseudoType),
+			},
+			cty.NilType,
+			nil,
+		},
 		{
 			// unifies to the same result as above, since the only difference
 			// is the addition of a list