fix: correct control flow graph for range init expression

The range init AST execution was skipped, and range could work
only over variables or direct function calls. By setting the
start node to the start of init and not init itself, we ensure
that the init AST is always taken into account.

Fixes #775.

_test/issue-775.go

@@ -0,0 +1,18 @@
+package main
+
+import (
+	"fmt"
+	"net/http/httptest"
+)
+
+func main() {
+	recorder := httptest.NewRecorder()
+	recorder.Header().Add("Foo", "Bar")
+
+	for key, value := range recorder.Header() {
+		fmt.Println(key, value)
+	}
+}
+
+// Output:
+// Foo [Bar]

interp/cfg.go

@@ -1334,7 +1334,7 @@ func (interp *Interpreter) cfg(root *node, pkgID string) ([]*node, error) {
 
 		case rangeStmt:
 			if sc.rangeChanType(n) != nil {
-				n.start = n.child[1]       // Get chan
+				n.start = n.child[1].start // Get chan
 				n.child[1].tnext = n       // then go to range function
 				n.tnext = n.child[2].start // then go to range body
 				n.child[2].tnext = n       // then body go to range function (loop)
@@ -1346,7 +1346,7 @@ func (interp *Interpreter) cfg(root *node, pkgID string) ([]*node, error) {
 				} else {
 					k, o, body = n.child[0], n.child[1], n.child[2]
 				}
-				n.start = o          // Get array or map object
+				n.start = o.start    // Get array or map object
 				o.tnext = k.start    // then go to iterator init
 				k.tnext = n          // then go to range function
 				n.tnext = body.start // then go to range body