fix bug fusing option-type fields (#6900)

With the new design for option types, the fuser needs to fuse
optional fields such that the option type of the field is pushed
into the field's fusion type so we have fusion(...|none) instead of
none|fusion(...).  This problem was causing upcast to fail on
such patterns.  We've added test coverage to catch this.

Author: mccanne

runtime/sam/expr/agg/fuser.go

@@ -60,7 +60,7 @@ func (f *Fuser) fuse(a, b super.Type) super.Type {
 			// First change all fields to optional that are in "a" but not in "b".
 			for k, field := range fields {
 				if _, ok := indexOfField(b.Fields, field.Name); !ok {
-					fields[k].Type = f.sctx.Option(fields[k].Type)
+					fields[k].Type = f.makeOption(fields[k].Type)
 				}
 			}
 			// Now fuse all the fields in "b" that are also in "a" and add the fields
@@ -70,7 +70,7 @@ func (f *Fuser) fuse(a, b super.Type) super.Type {
 				if ok {
 					fields[i].Type = f.fuse(fields[i].Type, field.Type)
 				} else {
-					typ := f.sctx.Option(field.Type)
+					typ := f.makeOption(field.Type)
 					fields = append(fields, super.NewField(field.Name, typ))
 				}
 			}
@@ -144,6 +144,13 @@ func (f *Fuser) fuse(a, b super.Type) super.Type {
 	return f.fusion(union)
 }
 
+func (f *Fuser) makeOption(t super.Type) super.Type {
+	if fusion, ok := t.(*super.TypeFusion); ok {
+		return f.sctx.LookupTypeFusion(f.makeOption(fusion.Type))
+	}
+	return f.sctx.Option(t)
+}
+
 func isAll(t super.Type) bool {
 	_, ok := t.(*super.TypeOfAll)
 	return ok

runtime/ztests/expr/fuser.yaml

@@ -59,3 +59,14 @@ input: &input |
   [{id:1,s:"foo"},{id:2,s:null}]
 
 output: *input
+
+---
+
+spq: fuse | defuse(this)
+
+input: &input |
+  {x:1,y:{z:"foo"}}
+  {x:2,y:{z:"bar",w:true}}
+  {x:3}
+
+output: *input