[class] Ensure 'cast_class' is initialized for the type parameters of…

… a generic class.

Fixes mono#9507

mono/metadata/class-init.c

@@ -4158,6 +4158,16 @@ mono_class_init (MonoClass *klass)
 
 		if (MONO_CLASS_IS_INTERFACE (klass))
 			mono_class_setup_interface_id (klass);
+
+		MonoGenericInst *ginstance = mono_class_get_generic_class (klass)->context.class_inst;
+		for (int x = 0; x < ginstance->type_argc; x++)
+		{
+			MonoClass *type_param = mono_class_from_mono_type (ginstance->type_argv[x]);
+			if (m_class_get_cast_class (type_param) == m_class_get_element_class (type_param))
+				break; // element_class is taken care of
+			if (!m_class_is_inited (m_class_get_cast_class(type_param)))
+				mono_class_init (m_class_get_cast_class(type_param));
+		}
 	}
 
 	if (klass->parent && !klass->parent->inited)

mono/tests/Makefile.am

@@ -690,7 +690,8 @@ TESTS_CS_SRC=		\
 	nested_type_visibility.cs	\
 	dynamic-method-churn.cs	\
 	verbose.cs \
-	generic-unmanaged-constraint.cs
+	generic-unmanaged-constraint.cs \
+	bug-gh-9507.cs
 
 # some tests fail to compile on mcs
 if CSC_IS_ROSLYN

mono/tests/bug-gh-9507.cs

@@ -0,0 +1,20 @@
+using System;
+
+public struct TestIsInst<T>
+{
+    public T[] inArray1;
+
+    public TestIsInst(int i) {
+        inArray1 = new T[16];
+
+		if (inArray1 is double[])
+			Console.WriteLine ("OK.");
+	}
+}
+
+public class Bug9507 // https://github.com/mono/mono/issues/9507
+{
+	public static void Main () {
+		var table = new TestIsInst<ulong> (1);
+	}
+}