interp: fix the detection of builtin calls during parsing

Avoid shortcuts, and apply regular scoping rules for symbol
resolution when checking for builtin calls.

Fixes #1173.

_test/issue-1173.go

@@ -0,0 +1,10 @@
+package main
+
+var real = func() { println("Hello") }
+
+func main() {
+	real()
+}
+
+// Output:
+// Hello

interp/cfg.go

@@ -864,7 +864,7 @@ func (interp *Interpreter) cfg(root *node, importPath string) ([]*node, error) {
 		case callExpr:
 			wireChild(n)
 			switch {
-			case interp.isBuiltinCall(n):
+			case isBuiltinCall(n, sc):
 				c0 := n.child[0]
 				bname := c0.ident
 				err = check.builtin(bname, n, n.child[1:], n.action == aCallSlice)
@@ -1260,11 +1260,10 @@ func (interp *Interpreter) cfg(root *node, importPath string) ([]*node, error) {
 				}
 			}
 			// Found symbol, populate node info
-			n.typ, n.findex, n.level = sym.typ, sym.index, level
+			n.sym, n.typ, n.findex, n.level = sym, sym.typ, sym.index, level
 			if n.findex < 0 {
 				n.val = sym.node
 			} else {
-				n.sym = sym
 				switch {
 				case sym.kind == constSym && sym.rval.IsValid():
 					n.rval = sym.rval

interp/type.go

@@ -303,7 +303,7 @@ func nodeType(interp *Interpreter, sc *scope, n *node) (*itype, error) {
 		t = dt
 
 	case callExpr:
-		if interp.isBuiltinCall(n) {
+		if isBuiltinCall(n, sc) {
 			// Builtin types are special and may depend from their input arguments.
 			t.cat = builtinT
 			switch n.child[0].ident {
@@ -675,11 +675,16 @@ func nodeType(interp *Interpreter, sc *scope, n *node) (*itype, error) {
 	return t, err
 }
 
-func (interp *Interpreter) isBuiltinCall(n *node) bool {
+func isBuiltinCall(n *node, sc *scope) bool {
 	if n.kind != callExpr {
 		return false
 	}
-	s := interp.universe.sym[n.child[0].ident]
+	s := n.child[0].sym
+	if s == nil {
+		if sym, _, found := sc.lookup(n.child[0].ident); found {
+			s = sym
+		}
+	}
 	return s != nil && s.kind == bltnSym
 }