Fix KeyValuePair nullability detection on Mono

Summary

Fixed a bug in the Mono runtime where NullableAttribute metadata was being lost for generic parameters. This was specifically causing KeyValuePair<TKey, TValue> to incorrectly report NotNull nullability in ASP.NET Core DataAnnotations tests.

Problem

On Mono, the reflection API failed to detect nullability attributes on generic parameters (like TKey and TValue). While CoreCLR correctly identified these as Nullable, Mono defaulted to NotNull

Failing Tests

• Microsoft.AspNetCore.Mvc.DataAnnotations.DataAnnotationsMetadataProviderTest.IsNullableReferenceType_ReturnsFalse_ForKeyValuePairWithoutNullableConstraints
• Microsoft.AspNetCore.Mvc.DataAnnotations.DataAnnotationsMetadataProviderTest.IsNullableReferenceType_ReturnsTrue_ForKeyValuePairWithNullableConstraints

Root Cause

The issue was located in the Mono native metadata initialization (src/mono/mono/metadata/class-init.c). In the MonoClass structure, MonoClassSizes is a union where class_size and generic_param_token share the same memory location.

During the execution of mono_class_layout_fields(), the runtime was explicitly assigning class_size = 0. Because of the union, this assignment was overwriting and zeroing out the generic_param_token that had been correctly initialized earlier. When the reflection system later tried to look up custom attributes using that token, it failed because the token was now 0x0.

Solution

The fix modifies mono_class_layout_fields to skip the class_size assignment for types that are generic parameters (MONO_TYPE_VAR and MONO_TYPE_MVAR). This preserves the metadata token, allowing mono_custom_attrs_from_class_checked() to correctly retrieve the NullableAttribute.

Reproduction Case

using System;
using System.Collections.Generic;
using System.Reflection;

var kvpType = typeof(KeyValuePair<string, object>);
var keyProperty = kvpType.GetProperty("Key");
var nullabilityContext = new NullabilityInfoContext();
var result = nullabilityContext.Create(keyProperty);

Console.WriteLine($"ReadState: {result.ReadState}");
// Expected: Nullable
// Mono (before fix): NotNull  
// Mono (after fix): Nullable 

Test Results

• Before fix: 361 pass, 3 fail (including 2 KeyValuePair tests)
• After fix: 363 pass, 1 fail (KeyValuePair tests now pass)
• Net improvement: +2 tests fixed, 0 regressions

Files Changed

• src/mono/mono/metadata/class-init.c

cc: @giritrivedi