[mono] Fix the emission of EnumEqualityComparer instances into the corlib AOT image.

Add a few dummy enums to the Mono namespace in corlib, and use them
to create valid EnumEqualityComparer instances. Also make sure the
instances are actually emitted and not replaced by gsharedvt instances.

Fixes #49229.

Author: vargaz

src/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml

@@ -618,6 +618,15 @@
 		<type fullname="Mono.ValueTuple`3" preserve="fields"/>
 		<type fullname="Mono.ValueTuple`4" preserve="fields"/>
 		<type fullname="Mono.ValueTuple`5" preserve="fields"/>
+		<!-- aot-compiler.c -->
+		<type fullname="Mono.I8Enum" preserve="fields"/>
+		<type fullname="Mono.UI8Enum" preserve="fields"/>
+		<type fullname="Mono.I16Enum" preserve="fields"/>
+		<type fullname="Mono.UI16Enum" preserve="fields"/>
+		<type fullname="Mono.I32Enum" preserve="fields"/>
+		<type fullname="Mono.UI32Enum" preserve="fields"/>
+		<type fullname="Mono.I64Enum" preserve="fields"/>
+		<type fullname="Mono.UI64Enum" preserve="fields"/>
 
 		<type fullname="System.AppContext">
 			<!-- appdomain.c: mono_runtime_install_appctx_properties -->

src/mono/System.Private.CoreLib/src/Mono/RuntimeStructs.cs

@@ -118,6 +118,38 @@ internal struct ValueTuple<T1, T2, T3, T4, T5>
         public T5 Item5;
     }
 
+    internal enum I8Enum : byte
+    {
+    }
+
+    internal enum UI8Enum : sbyte
+    {
+    }
+
+    internal enum I16Enum : short
+    {
+    }
+
+    internal enum UI16Enum : ushort
+    {
+    }
+
+    internal enum I32Enum : int
+    {
+    }
+
+    internal enum UI32Enum : uint
+    {
+    }
+
+    internal enum I64Enum : long
+    {
+    }
+
+    internal enum UI64Enum : ulong
+    {
+    }
+
     internal class NullByRefReturnException : Exception
     {
         public NullByRefReturnException()

src/mono/mono/mini/aot-compiler.c

@@ -5185,6 +5185,19 @@ is_vt_inst (MonoGenericInst *inst)
 	return FALSE;
 }
 
+static gboolean
+is_vt_inst_no_enum (MonoGenericInst *inst)
+{
+	int i;
+
+	for (i = 0; i < inst->type_argc; ++i) {
+		MonoType *t = inst->type_argv [i];
+		if (MONO_TYPE_ISSTRUCT (t))
+			return TRUE;
+	}
+	return FALSE;
+}
+
 static gboolean
 method_has_type_vars (MonoMethod *method)
 {
@@ -5338,7 +5351,7 @@ add_generic_class_with_depth (MonoAotCompile *acfg, MonoClass *klass, int depth,
 	 * WASM only since other platforms depend on the
 	 * previous behavior.
 	 */
-	if ((acfg->jit_opts & MONO_OPT_GSHAREDVT) && mono_class_is_ginst (klass) && mono_class_get_generic_class (klass)->context.class_inst && is_vt_inst (mono_class_get_generic_class (klass)->context.class_inst)) {
+	if ((acfg->jit_opts & MONO_OPT_GSHAREDVT) && mono_class_is_ginst (klass) && mono_class_get_generic_class (klass)->context.class_inst && is_vt_inst_no_enum (mono_class_get_generic_class (klass)->context.class_inst)) {
 		use_gsharedvt = TRUE;
 		use_gsharedvt_for_array = TRUE;
 	}
@@ -5758,17 +5771,19 @@ add_generic_instances (MonoAotCompile *acfg)
 
 		/* Add instances of EnumEqualityComparer which are created by EqualityComparer<T> for enums */
 		{
-			MonoClass *enum_comparer;
+			MonoClass *k, *enum_comparer;
 			MonoType *insts [16];
 			int ninsts;
+			const char *enum_names [] = { "I8Enum", "I16Enum", "I32Enum", "I64Enum", "UI8Enum", "UI16Enum", "UI32Enum", "UI64Enum" };
 
 			ninsts = 0;
-			insts [ninsts ++] = int32_type;
-			insts [ninsts ++] = uint32_type;
-			insts [ninsts ++] = uint16_type;
-			insts [ninsts ++] = byte_type;
+			for (int i = 0; i < G_N_ELEMENTS (enum_names); ++i) {
+				k = mono_class_try_load_from_name (acfg->image, "Mono", enum_names [i]);
+				g_assert (k);
+				insts [ninsts ++] = m_class_get_byval_arg (k);
+			}
 			enum_comparer = mono_class_load_from_name (mono_defaults.corlib, "System.Collections.Generic", "EnumEqualityComparer`1");
-			add_instances_of (acfg, enum_comparer, insts, ninsts, FALSE);
+			add_instances_of (acfg, enum_comparer, insts, ninsts, TRUE);
 		}
 
 		/* Add instances of the array generic interfaces for primitive types */