Do not devirtualize shared default interface methods (#15979)

The result of the devirtualization for a method on a generic type should be an instantiating stub, but this doesn't seem to work right.

Fixing that is a perf issue (that can be triaged/punted - #15977). This commit is a correctness fix to avoid bad codegen for that case.

Resolves #15591.

1 files changed, 8 insertions, 0 deletions

diff --git a/src/vm/jitinterface.cpp b/src/vm/jitinterface.cpp
index 874beae85b..d08ba0cbe2 100644
--- a/src/vm/jitinterface.cpp
+++ b/src/vm/jitinterface.cpp
@@ -8870,6 +8870,14 @@ CORINFO_METHOD_HANDLE CEEInfo::resolveVirtualMethodHelper(CORINFO_METHOD_HANDLE
         {
             return nullptr;
         }
+
+        // If we devirtualized into a default interface method on a generic type, we should actually return an
+        // instantiating stub but this is not happening.
+        // Making this work is tracked by https://github.com/dotnet/coreclr/issues/15977
+        if (pDevirtMD->GetMethodTable()->IsInterface() && pDevirtMD->HasClassInstantiation())
+        {
+            return nullptr;
+        }
     }
     else
     {