Fix handling VAR data type (#3169)

***NO_CI***

Author: josesimoes

src/CLR/Core/Execution.cpp

@@ -1971,10 +1971,26 @@ HRESULT CLR_RT_ExecutionEngine::InitializeLocals(
 
                 case DATATYPE_VAR:
                 {
+                    // type-level generic parameter in a locals signature (e.g. 'T' inside a generic type)
                     CLR_INT8 genericParamPosition = *sig++;
 
-                    methodDefInstance.assembly
-                        ->FindGenericParamAtTypeSpec(methodDefInstance, genericParamPosition, cls, dt);
+                    // parse the locals-signature to extract that T
+                    CLR_RT_SignatureParser parser;
+                    parser.Initialize_MethodLocals(assembly, methodDef);
+                    CLR_RT_SignatureParser::Element element;
+
+                    // advance into the VAR entry
+                    parser.Advance(element);
+
+                    // walk forward to the Nth generic-parameter
+                    for (int i = 0; i < genericParamPosition; i++)
+                    {
+                        parser.Advance(element);
+                    }
+
+                    // element.Class and element.DataType represent the T
+                    cls = element.Class;
+                    dt = element.DataType;
 
                     goto done;
                 }

src/CLR/Core/TypeSystem.cpp

@@ -440,8 +440,6 @@ HRESULT CLR_RT_SignatureParser::Advance(Element &res)
 
     NANOCLR_HEADER();
 
-    _ASSERTE(ParamCount > 0);
-
     ParamCount--;
 
     res.IsByRef = false;
@@ -988,13 +986,36 @@ bool CLR_RT_TypeDef_Instance::ResolveToken(
                     case DATATYPE_VAR:
                     {
                         CLR_RT_TypeDef_Index typeDef;
-                        NanoCLRDataType dataType;
+                        CLR_RT_SignatureParser::Element genericElement;
+
+                        CLR_RT_SignatureParser varParser;
+                        varParser.Initialize_TypeSpec(assm, ts);
 
-                        caller->assembly->FindGenericParamAtTypeSpec(*caller, genericParamPosition, typeDef, dataType);
+                        // advance once to consume the GENERICINST or VAR entry
+                        varParser.Advance(genericElement);
 
+                        // now walk forward genericParameterPosition steps to land on the actual
+                        // argument in the signature stream.
+                        for (CLR_INT8 i = 0; i <= genericParamPosition; i++)
+                        {
+                            if (FAILED(varParser.Advance(genericElement)))
+                            {
+                                return false;
+                            }
+                        }
+
+                        // genericElement.Class now holds the TypeDef_Index of the T or U, etc.
+                        typeDef = genericElement.Class;
+
+                        // populate this instance from the resolved TypeDef
                         data = typeDef.data;
                         assembly = g_CLR_RT_TypeSystem.m_assemblies[typeDef.Assembly() - 1];
                         target = assembly->GetTypeDef(typeDef.Type());
+
+#if defined(NANOCLR_INSTANCE_NAMES)
+                        name = assembly->GetString(target->name);
+#endif
+                        return true;
                     }
                     break;
 
@@ -4455,36 +4476,6 @@ bool CLR_RT_Assembly::FindGenericParam(CLR_INDEX typeSpecIndex, CLR_RT_GenericPa
     return false;
 }
 
-bool CLR_RT_Assembly::FindGenericParamAtTypeSpec(
-    CLR_RT_MethodDef_Instance md,
-    CLR_UINT32 genericParameterPosition,
-    CLR_RT_TypeDef_Index &index,
-    NanoCLRDataType &dataType)
-{
-    NATIVE_PROFILE_CLR_CORE();
-
-    CLR_RT_SignatureParser parser;
-    parser.Initialize_TypeSpec(md.assembly, md.assembly->GetTypeSpec(md.genericType->TypeSpec()));
-
-    CLR_RT_SignatureParser::Element element;
-
-    // get type
-    parser.Advance(element);
-
-    for (uint32_t i = 0; i <= genericParameterPosition; i++)
-    {
-        if (FAILED(parser.Advance(element)))
-        {
-            return false;
-        }
-    }
-
-    index = element.Class;
-    dataType = element.DataType;
-
-    return true;
-}
-
 bool CLR_RT_Assembly::FindGenericParamAtMethodDef(
     CLR_RT_MethodDef_Instance md,
     CLR_UINT32 genericParameterPosition,

src/CLR/Debugger/Debugger.cpp

@@ -1,4 +1,4 @@
-//
+//
 // Copyright (c) .NET Foundation and Contributors
 // Portions Copyright (c) Microsoft Corporation.  All rights reserved.
 // See LICENSE file in the project root for full license information.
@@ -2821,10 +2821,51 @@ bool CLR_DBG_Debugger::Debugging_Value_GetStack(WP_Message *msg)
             // handle generic parameters
             if (res.DataType == DATATYPE_VAR)
             {
-                // Generic parameter in a generic TypeDef
-                md.assembly->FindGenericParamAtTypeSpec(md, res.GenericParamPosition, targetClass, targetDataType);
+                // type‐level generic parameter in a generic TypeDef: re‐parse the signature
+                CLR_RT_SignatureParser typeParser;
 
-                // isGenericInstance = true;
+                // for arguments, initialize over the method signature and skip the return slot
+                if (cmd->m_kind == CLR_DBG_Commands::Debugging_Value_GetStack::c_Argument)
+                {
+                    typeParser.Initialize_MethodSignature(&md);
+
+                    // skip the return value entry
+                    iElement++;
+
+                    // if there's an implicit 'this', adjust index
+                    if (typeParser.Flags & PIMAGE_CEE_CS_CALLCONV_HASTHIS)
+                    {
+                        if (iElement == 0)
+                        {
+                            // requested the 'this' argument – invalid for a primitive
+                            return false;
+                        }
+                        iElement--;
+                    }
+                }
+                else
+                {
+                    // for locals or eval‐stack, initialize over the locals signature
+                    typeParser.Initialize_MethodLocals(md.assembly, md.target);
+                }
+
+                // advance to the requested argument/local
+                CLR_RT_SignatureParser::Element elem;
+                do
+                {
+                    typeParser.Advance(elem);
+                } while (iElement-- > 0);
+
+                // now we've landed on a DATATYPE_VAR; walk to the Nth generic‐type parameter
+                typeParser.Advance(elem);
+                for (int i = 0; i < res.GenericParamPosition; i++)
+                {
+                    parser.Advance(elem);
+                }
+
+                // this element.Class is the TypeDef_Index for 'T'
+                targetClass = elem.Class;
+                targetDataType = elem.DataType;
             }
             else if (res.DataType == DATATYPE_MVAR)
             {

src/CLR/Include/nanoCLR_Runtime.h

@@ -1423,11 +1423,6 @@ struct CLR_RT_Assembly : public CLR_RT_HeapBlock_Node // EVENT HEAP - NO RELOCAT
 
     bool FindTypeSpec(const CLR_PMETADATA sig, CLR_RT_TypeSpec_Index &index);
 
-    bool FindGenericParamAtTypeSpec(
-        CLR_RT_MethodDef_Instance md,
-        CLR_UINT32 genericParameterPosition,
-        CLR_RT_TypeDef_Index &index,
-        NanoCLRDataType &dataType);
     bool FindGenericParamAtMethodDef(
         CLR_RT_MethodDef_Instance md,
         CLR_UINT32 genericParameterPosition,