[release/9.0-preview7] Fix throwing exception when calling RunClassConstructor on a generic type with a static constructor (#105535)

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

#99183 seems to have done away with assuming that a generic type's static constructor was
always "initialized". As a result, if you call RunClassConstructor on it, the runtime would throw an exception.

Fixes #103891

* PR feedback

* Apply suggestions from code review

* More feedback

* Shouldn't wing it

---------

Co-authored-by: Steve Pfister <stpfiste@microsoft.com>
Co-authored-by: Jan Kotas <jkotas@microsoft.com>

Author: 3 people

src/coreclr/vm/methodtable.cpp

@@ -3768,6 +3768,8 @@ void MethodTable::CheckRunClassInitThrowing()
     if (IsSharedByGenericInstantiations())
         return;
 
+    _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

@@ -6,6 +6,7 @@
 using System.IO;
 using System.Reflection;
 using System.Runtime.InteropServices;
+using System.Threading;
 using Xunit;
 
 namespace System.Runtime.CompilerServices.Tests
@@ -101,7 +102,8 @@ public static void RunClassConstructor()
             RuntimeTypeHandle t = typeof(HasCctor).TypeHandle;
             RuntimeHelpers.RunClassConstructor(t);
             Assert.Equal("Hello", HasCctorReceiver.S);
-            return;
+            // Should not throw
+            RuntimeHelpers.RunClassConstructor(typeof(GenericHasCctor<>).TypeHandle);
         }
 
         internal class HasCctor
@@ -117,6 +119,14 @@ internal class HasCctorReceiver
             public static string S;
         }
 
+        internal class GenericHasCctor<T>
+        {
+            static GenericHasCctor()
+            {
+                Thread.Yield(); // Make sure the preinitialization optimization doesn't eat this.
+            }
+        }
+
         [Fact]
         public static void PrepareMethod()
         {