2 files changed, 6 insertions(+), 4 deletions(-)

M lib/dhall/ast.rb
M lib/dhall/typecheck.rb

@@ 613,6 613,10 @@ module Dhall
 			self == other
 		end
 
+		def merge(other)
+			with(alternatives: alternatives.merge(other.alternatives))
+		end
+
 		def fetch(k, default=nil)
 			if (default || block_given?) && !alternatives.key?(k)
 				return (default || yield)

@@ 620,10 620,8 @@ module Dhall
 				annotated_value = @value.annotate(context)
 
 				type = Dhall::UnionType.new(
-					alternatives: { @union.tag => annotated_value.type }.merge(
-						@union.alternatives.alternatives
-					)
-				)
+					alternatives: { @union.tag => annotated_value.type }
+				).merge(@union.alternatives)
 
 				# Annotate to sanity check
 				TypeChecker.for(type).annotate(context)