Fix autodetection of input size for dnn networks

modules/dnn/src/dnn.cpp

@@ -2506,7 +2506,9 @@ struct Net::Impl
     {
         std::vector<LayerPin>& inputLayerIds = layers[id].inputBlobsId;
 
-        if (inOutShapes[0].in[0].empty() && !layers[0].outputBlobs.empty())
+        if (id == 0 && inOutShapes[id].in[0].empty())
+        {
+            if (!layers[0].outputBlobs.empty())
             {
                 ShapesVec shapes;
                 for (int i = 0; i < layers[0].outputBlobs.size(); i++)
@@ -2517,6 +2519,12 @@ struct Net::Impl
                 }
                 inOutShapes[0].in = shapes;
             }
+            else
+            {
+                inOutShapes[0].out.clear();
+                return;
+            }
+        }
 
         if (inOutShapes[id].in.empty())
         {

modules/dnn/test/test_model.cpp

@@ -105,8 +105,7 @@ TEST_P(Test_Model, Classify)
 }
 
 
-// disabled: https://github.com/opencv/opencv/pull/15593
-TEST_P(Test_Model, DISABLED_DetectRegion)
+TEST_P(Test_Model, DetectRegion)
 {
     applyTestTag(CV_TEST_TAG_LONG, CV_TEST_TAG_MEMORY_1GB);