Tweak the display of the type of an empty Map (#2590)

Fixes #2247
Closes #2252

go/merge/three_way_set_test.go

@@ -81,6 +81,6 @@ func (s *ThreeWaySetMergeSuite) TestThreeWayMerge_Refs() {
 }
 
 func (s *ThreeWaySetMergeSuite) TestThreeWayMerge_ImmediateConflict() {
-	s.tryThreeWayConflict(types.NewMap(), s.create(ss1b), s.create(ss1), "Cannot merge Map<, > with "+s.typeStr)
+	s.tryThreeWayConflict(types.NewMap(), s.create(ss1b), s.create(ss1), "Cannot merge Map<> with "+s.typeStr)
 	s.tryThreeWayConflict(s.create(ss1b), types.NewMap(), s.create(ss1), "Cannot merge "+s.typeStr)
 }

go/types/encode_human_readable.go

@@ -245,6 +245,11 @@ func (w *hrsWriter) writeType(t *Type, parentStructTypes []*Type) {
 		w.write(KindToString[t.Kind()])
 		w.write("<")
 		for i, et := range t.Desc.(CompoundDesc).ElemTypes {
+			if et.Kind() == UnionKind && len(et.Desc.(CompoundDesc).ElemTypes) == 0 {
+				// If one of the element types is an empty union all the other element types must
+				// also be empty union types.
+				break
+			}
 			if i != 0 {
 				w.write(", ")
 			}

go/types/encode_human_readable_test.go

@@ -406,7 +406,7 @@ func TestEmptyCollections(t *testing.T) {
 	c := MakeMapType(BlobType, NumberType)
 	assertWriteTaggedHRSEqual(t, "Type(Map<Blob, Number>)", c)
 	d := NewMap()
-	assertWriteTaggedHRSEqual(t, "Map<, >({})", d)
+	assertWriteTaggedHRSEqual(t, "Map<>({})", d)
 	e := MakeSetType(StringType)
 	assertWriteTaggedHRSEqual(t, "Type(Set<String>)", e)
 	f := NewSet()

js/noms/src/encode-human-readable-test.js

@@ -56,6 +56,8 @@ suite('Encode human readable types', () => {
     assertWriteType('', makeUnionType([]));
     assertWriteType('List<Number | String>', makeListType(makeUnionType([numberType, stringType])));
     assertWriteType('List<>', makeListType(makeUnionType([])));
+    assertWriteType('Set<>', makeSetType(makeUnionType([])));
+    assertWriteType('Map<>', makeMapType(makeUnionType([]), makeUnionType([])));
   });
 
   test('struct', () => {

js/noms/src/encode-human-readable.js

@@ -88,6 +88,11 @@ export class TypeWriter {
         this._w.write('<');
         invariant(t.desc instanceof CompoundDesc);
         t.desc.elemTypes.forEach((t, i) => {
+          if (t.kind === Kind.Union && t.desc.elemTypes.length === 0) {
+            // If one of the element types is an empty union all the other element types must also
+            // be empty union types.
+            return;
+          }
           if (i !== 0) {
             this._w.write(', ');
           }