Fix semantic check for function references

These changes fix an issue in the semantic checks against function
references. The compiler is supposed to catch references to functions
that have not been applied (e.g., f(1) is OK but just f is not.) The
compiler was failing to catch these references when they were embedded
within composites. If these references were encountered during
evaluation, it resulted in undefined behaviour.

The fix for this issue is to simply remove the flag that gated whether
terms were checked. The flag was in place to deal with nested terms like
f(g(x)) but that has not been applicable for a while since these terms
are rewritten (and have been since the unifier was reimplemented over
year ago.)

Fixes open-policy-agent#1132

Signed-off-by: Torin Sandall <torinsandall@gmail.com>

ast/check.go

@@ -472,9 +472,8 @@ func (tc *typeChecker) err(err *Error) {
 }
 
 type refChecker struct {
-	env       *TypeEnv
-	errs      Errors
-	checkTerm bool
+	env  *TypeEnv
+	errs Errors
 }
 
 func newRefChecker(env *TypeEnv) *refChecker {
@@ -496,17 +495,13 @@ func (rc *refChecker) Visit(x interface{}) Visitor {
 			}
 			return nil
 		case *Term:
-			rc.checkTerm = true
 			Walk(rc, terms)
-			rc.checkTerm = false
 			return nil
 		}
 	case Ref:
-		if rc.checkTerm {
-			if err := rc.checkApply(rc.env, x); err != nil {
-				rc.errs = append(rc.errs, err)
-				return nil
-			}
+		if err := rc.checkApply(rc.env, x); err != nil {
+			rc.errs = append(rc.errs, err)
+			return nil
 		}
 		if err := rc.checkRef(rc.env, rc.env.tree, x, 0); err != nil {
 			rc.errs = append(rc.errs, err)

ast/check_test.go

@@ -623,7 +623,6 @@ func TestCheckBuiltinErrors(t *testing.T) {
 		{"objects-bad-input", `sum({"a": 1, "b": 2}, x)`},
 		{"sets-any", `sum({1,2,"3",4}, x)`},
 		{"virtual-ref", `plus(data.test.p, data.deabeef, 0)`},
-		{"function-ref", `data.test.f(1, data.test.f)`},
 	}
 
 	env := newTestEnv([]string{
@@ -901,6 +900,18 @@ func TestUserFunctionsTypeInference(t *testing.T) {
 			`f(x) = y { {"k": x} = y }`,
 			false,
 		},
+		{
+			`p { [data.base.foo] }`,
+			true,
+		},
+		{
+			`p { x = data.base.foo }`,
+			true,
+		},
+		{
+			`p { data.base.foo(data.base.bar) }`,
+			true,
+		},
 	}
 
 	for n, test := range tests {