lang: If expression was missing two invariants

I forgot to ensure that the type of the final expression matched the
type of each of the branches. It's rare, but possible for this to occur.
Luckily, this never would have caused a panic, because the func engine
would have caught the issue anyways, but it's still better we catch it
here first!

lang/structs.go

@@ -5583,6 +5583,18 @@ func (obj *ExprIf) Unify() ([]interfaces.Invariant, error) {
 	}
 	invariants = append(invariants, branchesInvar)
 
+	// the two branches must match the type of the whole expression
+	thenInvar := &unification.EqualityInvariant{
+		Expr1: obj,
+		Expr2: obj.ThenBranch,
+	}
+	invariants = append(invariants, thenInvar)
+	elseInvar := &unification.EqualityInvariant{
+		Expr1: obj,
+		Expr2: obj.ElseBranch,
+	}
+	invariants = append(invariants, elseInvar)
+
 	return invariants, nil
 }