Add an assert to catch the missing places

src/coreclr/jit/compiler.h

@@ -502,6 +502,9 @@ class LclVarDsc
     unsigned char lvUnusedStruct : 1; // All references to this promoted struct are through its field locals.
                                       // I.e. there is no longer any reference to the struct directly.
                                       // In this case we can simply remove this struct local.
+
+    unsigned char lvUndoneStructPromotion : 1; // The struct promotion was undone and hence there should be no
+                                               // reference to the fields of this struct.
 #endif
 
     unsigned char lvLRACandidate : 1; // Tracked for linear scan register allocation purposes

src/coreclr/jit/lclvars.cpp

@@ -4075,6 +4075,16 @@ void Compiler::lvaMarkLclRefs(GenTree* tree, BasicBlock* block, Statement* stmt,
 
     varDsc->incRefCnts(weight, this);
 
+#ifdef DEBUG
+    if (varDsc->lvIsStructField)
+    {
+        // If ref count was increased for struct field, ensure that the
+        // parent struct is still promoted.
+        LclVarDsc* parentStruct = &lvaTable[varDsc->lvParentLcl];
+        assert(!parentStruct->lvUndoneStructPromotion);
+    }
+#endif
+
     if (!isRecompute)
     {
         if (lvaVarAddrExposed(lclNum))

src/coreclr/jit/morph.cpp

@@ -17535,6 +17535,8 @@ void Compiler::fgRetypeImplicitByRefArgs()
 
 void Compiler::fgMarkDemotedImplicitByRefArgs()
 {
+    JITDUMP("\n*************** In fgMarkDemotedImplicitByRefArgs()\n");
+
 #if (defined(TARGET_AMD64) && !defined(UNIX_AMD64_ABI)) || defined(TARGET_ARM64)
 
     for (unsigned lclNum = 0; lclNum < info.compArgsCount; lclNum++)
@@ -17543,6 +17545,8 @@ void Compiler::fgMarkDemotedImplicitByRefArgs()
 
         if (lvaIsImplicitByRefLocal(lclNum))
         {
+            JITDUMP("Clearing annotation for V%02d\n", lclNum);
+
             if (varDsc->lvPromoted)
             {
                 // The parameter is simply a pointer now, so clear lvPromoted.  It was left set
@@ -17569,7 +17573,8 @@ void Compiler::fgMarkDemotedImplicitByRefArgs()
                 LclVarDsc* structVarDsc     = &lvaTable[structLclNum];
                 structVarDsc->lvAddrExposed = false;
 #ifdef DEBUG
-                structVarDsc->lvUnusedStruct = true;
+                structVarDsc->lvUnusedStruct          = true;
+                structVarDsc->lvUndoneStructPromotion = true;
 #endif // DEBUG
 
                 unsigned fieldLclStart = structVarDsc->lvFieldLclStart;
@@ -17578,6 +17583,8 @@ void Compiler::fgMarkDemotedImplicitByRefArgs()
 
                 for (unsigned fieldLclNum = fieldLclStart; fieldLclNum < fieldLclStop; ++fieldLclNum)
                 {
+                    JITDUMP("Fixing pointer for field V%02d from V%02d to V%02d\n", fieldLclNum, lclNum, structLclNum);
+
                     // Fix the pointer to the parent local.
                     LclVarDsc* fieldVarDsc = &lvaTable[fieldLclNum];
                     assert(fieldVarDsc->lvParentLcl == lclNum);