lang: funcs: core: world: Add unification to schedule return expr

This adds a sneaky unification between the expression of the function
return value in the unification. I am not entirely sure how often this
will get used, but it could be valuable in the right instance if this
isn't already learned through other sources. I'm fairly confident that
it isn't incorrect, so in the worst case scenario it's redundant
information for the unification solver.

This is being added as a separate commit so that it's obvious how this
type of unification invariant can be applied.

lang/funcs/core/world/schedule_func.go

@@ -142,7 +142,7 @@ func (obj *SchedulePolyFunc) Unify(expr interfaces.Expr) ([]interfaces.Invariant
 			if cfavInvar.Func != expr {
 				continue
 			}
-			// cfavInvar.Expr is the ExprCall!
+			// cfavInvar.Expr is the ExprCall! (the return pointer)
 			// cfavInvar.Args are the args that ExprCall uses!
 			if len(cfavInvar.Args) == 0 {
 				return nil, fmt.Errorf("unable to build function with no args")
@@ -155,6 +155,13 @@ func (obj *SchedulePolyFunc) Unify(expr interfaces.Expr) ([]interfaces.Invariant
 			var invariants []interfaces.Invariant
 			var invar interfaces.Invariant
 
+			// add the relationship to the returned value
+			invar = &interfaces.EqualityInvariant{
+				Expr1: cfavInvar.Expr,
+				Expr2: dummyOut,
+			}
+			invariants = append(invariants, invar)
+
 			// add the relationships to the called args
 			invar = &interfaces.EqualityInvariant{
 				Expr1: cfavInvar.Args[0],
@@ -180,7 +187,7 @@ func (obj *SchedulePolyFunc) Unify(expr interfaces.Expr) ([]interfaces.Invariant
 				optsTypeKnown := false
 
 				// speculate about the type?
-				if typ, err := cfavInvar.Args[1].Type(); err == nil {
+				if typ, exists := solved[cfavInvar.Args[1]]; exists {
 					optsTypeKnown = true
 					if typ.Kind != types.KindStruct {
 						return nil, fmt.Errorf("second arg must be of kind struct")
@@ -234,6 +241,14 @@ func (obj *SchedulePolyFunc) Unify(expr interfaces.Expr) ([]interfaces.Invariant
 					}
 					invariants = append(invariants, invar)
 				}
+				// redundant?
+				if typ, err := cfavInvar.Args[1].Type(); err == nil {
+					invar := &interfaces.EqualsInvariant{
+						Expr: cfavInvar.Args[1],
+						Type: typ,
+					}
+					invariants = append(invariants, invar)
+				}
 
 				// If we're strict, require it, otherwise let
 				// in whatever, and let Build() deal with it.