Fix throwing exception when calling RunClassConstructor on a generic type with a static constructor

src/coreclr/vm/methodtable.cpp

@@ -3768,6 +3768,9 @@ void MethodTable::CheckRunClassInitThrowing()
     if (IsSharedByGenericInstantiations())
         return;
 
+    // For back-compat reasons, act like it has already been initialized
+    _ASSERTE(!ContainsGenericVariables());
+
     EnsureStaticDataAllocated();
     DoRunClassInitThrowing();
 }

src/coreclr/vm/reflectioninvocation.cpp

@@ -1306,7 +1306,8 @@ extern "C" void QCALLTYPE ReflectionInvocation_RunClassConstructor(QCall::TypeHa
         return;
 
     MethodTable *pMT = typeHnd.AsMethodTable();
-    if (pMT->IsClassInited())
+    // The ContainsGenericVariables check is to preserve back-compat where we assume the generic type is already initialized
+    if (pMT->IsClassInited() || pMT->ContainsGenericVariables())
         return;
 
     BEGIN_QCALL;

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Runtime/CompilerServices/RuntimeHelpersTests.cs

@@ -101,6 +101,8 @@ public static void RunClassConstructor()
             RuntimeTypeHandle t = typeof(HasCctor).TypeHandle;
             RuntimeHelpers.RunClassConstructor(t);
             Assert.Equal("Hello", HasCctorReceiver.S);
+            // Should not throw
+            RuntimeHelpers.RunClassConstructor(typeof(GenericHasCctor<>).TypeHandle);
             return;
         }