[mono][interp] Fix GetType called on ptr constrained to Nullable` (#6…

…1020) (#61305)

* [interp] Fix GetType called on ptr constrained to Nullable`

We were statically optimizing this call to return the actual constrained class type, which is incorrect for nullables, because boxing of a nullable (as part of the constrained call) actually creates an object with the type of the nullable's value (or null if there is no value).

* Add test for GetType call on ptr constrained to nullable

src/mono/mono/mini/interp/transform.c

@@ -2453,7 +2453,7 @@ interp_handle_intrinsics (TransformData *td, MonoMethod *target_method, MonoClas
 		if (!strcmp (tm, "InternalGetHashCode")) {
 			*op = MINT_INTRINS_GET_HASHCODE;
 		} else if (!strcmp (tm, "GetType")) {
-			if (constrained_class && m_class_is_valuetype (constrained_class)) {
+			if (constrained_class && m_class_is_valuetype (constrained_class) && !mono_class_is_nullable (constrained_class)) {
 				// If constrained_class is valuetype we already know its type.
 				// Resolve GetType to a constant so we can fold type comparisons
 				ERROR_DECL(error);
@@ -2470,8 +2470,16 @@ interp_handle_intrinsics (TransformData *td, MonoMethod *target_method, MonoClas
 				return TRUE;
 			} else {
 				if (constrained_class) {
-					// deref the managed pointer to get the object
-					interp_add_ins (td, MINT_LDIND_I);
+					if (mono_class_is_nullable (constrained_class)) {
+						// We can't determine the behavior here statically because we don't know if the
+						// nullable vt has a value or not. If it has a value, the result type is
+						// m_class_get_cast_class (constrained_class), otherwise GetType should throw NRE.
+						interp_add_ins (td, MINT_BOX_NULLABLE_PTR);
+						td->last_ins->data [0] = get_data_item_index (td, constrained_class);
+					} else {
+						// deref the managed pointer to get the object
+						interp_add_ins (td, MINT_LDIND_I);
+					}
 					td->sp--;
 					interp_ins_set_sreg (td->last_ins, td->sp [0].local);
 					push_simple_type (td, STACK_TYPE_O);

src/tests/JIT/Directed/nullabletypes/gettype.cs

@@ -0,0 +1,48 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Linq;
+using System.Collections.Generic;
+
+class C<T>
+{
+    public IEnumerable<T> Data { get; set; }
+
+    public C() { }
+
+    public bool Check()
+    {
+	return Data.ElementAt(0).GetType() == typeof(bool);
+    }
+}
+
+public class P
+{
+    public static int Main()
+    {
+        C<bool?> c = new();
+
+        // Try a nullable with value
+        c.Data = new List<bool?> { true };
+        if(!c.Check())
+            return 666;
+
+        // Try a nullable without value. Should throw NRE
+        c.Data = new List<bool?> { new Nullable<bool>() };
+
+        bool thrown = false;
+        try
+        {
+            c.Check();
+        }
+        catch(NullReferenceException)
+        {
+            thrown = true;
+        }
+        if(!thrown)
+            return 667;
+        return 100;
+    }
+}
+

src/tests/JIT/Directed/nullabletypes/gettype_d.csproj

@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <CLRTestPriority>1</CLRTestPriority>
+  </PropertyGroup>
+  <PropertyGroup>
+    <DebugType>Full</DebugType>
+    <Optimize>False</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="gettype.cs" />
+  </ItemGroup>
+</Project>

src/tests/JIT/Directed/nullabletypes/gettype_do.csproj

@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <CLRTestPriority>1</CLRTestPriority>
+  </PropertyGroup>
+  <PropertyGroup>
+    <DebugType>Full</DebugType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="gettype.cs" />
+  </ItemGroup>
+</Project>

src/tests/JIT/Directed/nullabletypes/gettype_r.csproj

@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <CLRTestPriority>1</CLRTestPriority>
+  </PropertyGroup>
+  <PropertyGroup>
+    <DebugType>None</DebugType>
+    <Optimize>False</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="gettype.cs" />
+  </ItemGroup>
+</Project>

src/tests/JIT/Directed/nullabletypes/gettype_ro.csproj

@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <CLRTestPriority>1</CLRTestPriority>
+  </PropertyGroup>
+  <PropertyGroup>
+    <DebugType>None</DebugType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="gettype.cs" />
+  </ItemGroup>
+</Project>