fix: handle function output value assigned to interface var

_test/fun13.go

@@ -0,0 +1,19 @@
+package main
+
+import "fmt"
+
+type T struct{}
+
+func newT() (T, error) { return T{}, nil }
+
+func main() {
+	var (
+		i   interface{}
+		err error
+	)
+	i, err = newT()
+	fmt.Println(i, err)
+}
+
+// Output:
+// {} <nil>

interp/cfg.go

@@ -542,22 +542,22 @@ func (interp *Interpreter) cfg(root *node, pkgID string) ([]*node, error) {
 		case assignXStmt:
 			wireChild(n)
 			l := len(n.child) - 1
-			switch n.child[l].kind {
+			switch lc := n.child[l]; lc.kind {
 			case callExpr:
 				n.gen = nop
 			case indexExpr:
-				n.child[l].gen = getIndexMap2
+				lc.gen = getIndexMap2
 				n.gen = nop
 			case typeAssertExpr:
 				if n.child[0].ident == "_" {
-					n.child[l].gen = typeAssertStatus
+					lc.gen = typeAssertStatus
 				} else {
-					n.child[l].gen = typeAssert2
+					lc.gen = typeAssert2
 				}
 				n.gen = nop
 			case unaryExpr:
-				if n.child[l].action == aRecv {
-					n.child[l].gen = recv2
+				if lc.action == aRecv {
+					lc.gen = recv2
 					n.gen = nop
 				}
 			}

interp/run.go

@@ -670,7 +670,12 @@ func call(n *node) {
 	case defineXStmt, assignXStmt:
 		for i := range rvalues {
 			c := n.anc.child[i]
-			if c.ident != "_" {
+			switch {
+			case c.ident == "_":
+				// Skip assigning return value to blank var.
+			case c.typ.cat == interfaceT:
+				rvalues[i] = genValueInterfaceValue(c)
+			default:
 				rvalues[i] = genValue(c)
 			}
 		}