Add a test observing lack of an issue.

src/Compilers/CSharp/Test/Emit3/RefStructInterfacesTests.cs

@@ -28916,5 +28916,81 @@ interface UsePia5 : ITest29
 
             CompileAndVerify(compilation2, symbolValidator: metadataValidator, verify: Verification.Skipped).VerifyDiagnostics();
         }
+
+        [Fact]
+        [WorkItem("https://github.com/dotnet/roslyn/issues/74785")]
+        public void Issue74785()
+        {
+            var src = @"
+using System;
+using System.Diagnostics.CodeAnalysis;
+using System.Runtime.CompilerServices;
+
+#pragma warning disable CS0649
+
+[InlineArray(256)]
+public struct BufferStruct : IBufferInterface
+{
+    private byte _data;
+
+    [UnscopedRef]
+    public ReadOnlySpan<byte> Data => this;
+}
+
+
+interface IBufferInterface
+{
+    [UnscopedRef]
+    public ReadOnlySpan<byte> Data { get; }
+}
+
+struct TestStruct<T> 
+    where T : struct, IBufferInterface
+{
+    T genericBuffer;
+    BufferStruct directBuffer;
+    IBufferInterface interfaceBuffer;
+
+    [UnscopedRef]
+    public ReadOnlySpan<byte> GetGenericBuffer1()
+    {
+        return genericBuffer.Data;
+    }
+
+    [UnscopedRef]
+    public ReadOnlySpan<byte> GetDirectBuffer1()
+    {
+        return directBuffer.Data;
+    }
+
+    public ReadOnlySpan<byte> GetInterfaceBuffer()
+    {
+        return interfaceBuffer.Data;
+    }
+
+    public ReadOnlySpan<byte> GetGenericBuffer2()
+    {
+#line 1000
+        return genericBuffer.Data;
+    }
+
+    public ReadOnlySpan<byte> GetDirectBuffer2()
+    {
+#line 2000
+        return directBuffer.Data;
+    }
+}
+";
+            var comp = CreateCompilation(src, targetFramework: TargetFramework.Net90);
+
+            comp.VerifyEmitDiagnostics(
+                // (1000,16): error CS8170: Struct members cannot return 'this' or other instance members by reference
+                //         return genericBuffer.Data;
+                Diagnostic(ErrorCode.ERR_RefReturnStructThis, "genericBuffer").WithLocation(1000, 16),
+                // (2000,16): error CS8170: Struct members cannot return 'this' or other instance members by reference
+                //         return directBuffer.Data;
+                Diagnostic(ErrorCode.ERR_RefReturnStructThis, "directBuffer").WithLocation(2000, 16)
+                );
+        }
     }
 }