[RuntimeAsync] Do not fail EnC on ordinary Task-returning methods when async feature is turned on.

src/coreclr/vm/encee.cpp

@@ -344,13 +344,6 @@ HRESULT EditAndContinueModule::UpdateMethod(MethodDesc *pMethod)
     }
     CONTRACTL_END;
 
-    if (pMethod->HasAsyncMethodData())
-    {
-        // TODO: (async) revisit and examine if this can be supported
-        LOG((LF_ENC, LL_INFO100, "**Error** EnC for Async methods is NYI"));
-        return E_FAIL;
-    }
-
     // Notify the debugger of the update
     if (CORDebuggerAttached())
     {

src/coreclr/vm/method.cpp

@@ -3220,6 +3220,17 @@ void MethodDesc::ResetCodeEntryPointForEnC()
     _ASSERTE(!IsVersionableWithPrecode());
     _ASSERTE(!MayHaveEntryPointSlotsToBackpatch());
 
+    // Updates are expressed via metadata diff and a methoddef of a runtime async method
+    // would be resolved to the thunk.
+    // If we see a thunk here, fetch the other variant that owns the IL and reset that.
+    if (IsAsyncThunkMethod())
+    {
+        MethodDesc *otherVariant = GetAsyncOtherVariantNoCreate();
+        _ASSERTE(otherVariant != NULL);
+        otherVariant->ResetCodeEntryPointForEnC();
+        return;
+    }
+
     LOG((LF_ENC, LL_INFO100000, "MD::RCEPFENC: this:%p - %s::%s - HasPrecode():%s, HasNativeCodeSlot():%s\n",
         this, m_pszDebugClassName, m_pszDebugMethodName, (HasPrecode() ? "true" : "false"), (HasNativeCodeSlot() ? "true" : "false")));
     if (HasPrecode())

src/coreclr/vm/method.hpp

@@ -1657,6 +1657,13 @@ class MethodDesc
         return FindOrCreateAssociatedMethodDesc(this, GetMethodTable(), FALSE, GetMethodInstantiation(), allowInstParam, FALSE, TRUE, AsyncVariantLookup::AsyncOtherVariant);
     }
 
+    // same as above, but with allowCreate = FALSE
+    // for rare cases where we cannot allow GC, but we know that the other variant is already created.
+    MethodDesc* GetAsyncOtherVariantNoCreate(BOOL allowInstParam = TRUE)
+    {
+        return FindOrCreateAssociatedMethodDesc(this, GetMethodTable(), FALSE, GetMethodInstantiation(), allowInstParam, FALSE, FALSE, AsyncVariantLookup::AsyncOtherVariant);
+    }
+
     // True if a MD is an funny BoxedEntryPointStub (not from the method table) or
     // an MD for a generic instantiation...In other words the MethodDescs and the
     // MethodTable are guaranteed to be "tightly-knit", i.e. if one is present in