[mono] Use underlying type in RuntimeHelpers.GetSpanDataFrom (#87019)

* [mono] Use underlying type in RuntimeHelpers.GetSpanDataFrom

Make it work correctly for spans of enums

Fixes #86865

Note that in net8 RuntimeHelpers.CreateSpan<T> is an intrinsic, so
GetSpanDataFrom is never called directly.

But in net7 CreateSpan is not intrinsified on Mono, so the underlying
method really does get called.

* test: Print all hidden field names if we can't find the right one

src/mono/mono/metadata/icall.c

@@ -985,7 +985,7 @@ ves_icall_System_Runtime_CompilerServices_RuntimeHelpers_GetSpanDataFrom (MonoCl
 		return NULL;
 	}
 
-	MonoType *type = targetTypeHandle;
+	MonoType *type = mono_type_get_underlying_type (targetTypeHandle);
 	if (MONO_TYPE_IS_REFERENCE (type) || type->type == MONO_TYPE_VALUETYPE) {
 		mono_error_set_argument (error, "array", "Cannot initialize array of non-primitive type");
 		return NULL;

src/tests/Regressions/coreclr/GitHub_86865/test86865.cs

@@ -0,0 +1,70 @@
+using System;
+using System.Reflection;
+
+namespace test86865;
+
+public class test86865
+{
+    public static int Main()
+    {
+
+        // Regression test for https://github.com/dotnet/runtime/issues/86865
+        // Verify that the RuntimeHelpers.GetSpanDataFrom method underlying RuntimeHelpers.CreateSpan<T>
+        // works correctly with enums.
+
+        ReadOnlySpan<MyEnum> myEnums = new[]
+        {
+            MyEnum.A,
+            MyEnum.B,
+            MyEnum.C,
+            MyEnum.B,
+            MyEnum.C,
+        };
+
+        if (string.Join(", ", myEnums.ToArray()) != "A, B, C, B, C")
+            return 1;
+
+        var types = new Type[] {
+            typeof(RuntimeFieldHandle),
+            typeof(RuntimeTypeHandle),
+            typeof(int).MakeByRefType(),
+        };
+        var mi = typeof(System.Runtime.CompilerServices.RuntimeHelpers).GetMethod("GetSpanDataFrom", BindingFlags.Static | BindingFlags.NonPublic, types);
+        if (mi == null)
+            return 2;
+
+        var pid = typeof(MyEnum).Assembly.GetType("<PrivateImplementationDetails>");
+        if (pid == null)
+            return 3;
+
+        var fi = pid.GetField("0B77DC554B4A81403D62BE25FB5404020AD451151D4203D544BF60E3FEDBD8AE4", BindingFlags.Static | BindingFlags.NonPublic);
+        if (fi == null)
+        {
+            Console.WriteLine("Could not find the expected array data in <PrivateImplementationDetails>. The available static non-public fields are:");
+            foreach (var f in pid.GetFields(BindingFlags.Static | BindingFlags.NonPublic)) {
+                Console.WriteLine($" - '{f}'");
+            }
+            return 4;
+        }
+
+        var parms = new object[] {
+            fi.FieldHandle,
+            typeof(MyEnum).TypeHandle,
+            new int()
+        };
+        var result = mi.Invoke(null, parms);
+        if (result == null)
+            return 6;
+        if ((int)parms[2] != myEnums.Length)
+            return 7;
+
+        return 100;
+    }
+}
+
+enum MyEnum
+{
+    A,
+    B,
+    C
+}

src/tests/Regressions/coreclr/GitHub_86865/test86865.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="test86865.cs" />
+  </ItemGroup>
+</Project>