Fix handling of retargeted fixed size fields (#47429)

src/Compilers/CSharp/Portable/Symbols/Wrapped/WrappedFieldSymbol.cs

@@ -118,6 +118,14 @@ internal override ImmutableArray<byte> MarshallingDescriptor
             }
         }
 
+        public override bool IsFixedSizeBuffer
+        {
+            get
+            {
+                return _underlyingField.IsFixedSizeBuffer;
+            }
+        }
+
         internal override int? TypeLayoutOffset
         {
             get

src/Compilers/CSharp/Test/Emit/CodeGen/FixedSizeBufferTests.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using Microsoft.CodeAnalysis.CSharp.Symbols;
+using Microsoft.CodeAnalysis.CSharp.Symbols.Retargeting;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Microsoft.CodeAnalysis.CSharp.Test.Utilities;
 using Microsoft.CodeAnalysis.CSharp.UnitTests.Emit;
@@ -1136,5 +1137,22 @@ .maxstack  1
   IL_001f:  ret
 }");
         }
+
+        [Fact]
+        [WorkItem(1141012, "https://dev.azure.com/devdiv/DevDiv/_workitems/edit/1141012")]
+        public void FixedSizeBufferRetargeting()
+        {
+            var source = @"
+public unsafe struct FixedBuffer
+{
+    public fixed byte buffer[256];
+}";
+            var comp = CreateCompilation(source, options: TestOptions.UnsafeReleaseDll, targetFramework: TargetFramework.Mscorlib40, assemblyName: "fixedBuffer");
+            comp.VerifyDiagnostics();
+
+            var comp3 = CreateCompilation("", references: new[] { comp.ToMetadataReference() }, targetFramework: TargetFramework.Mscorlib46);
+            var retargetingField = comp3.GlobalNamespace.GetMember<NamedTypeSymbol>("FixedBuffer").GetMember<RetargetingFieldSymbol>("buffer");
+            Assert.True(retargetingField.IsFixedSizeBuffer);
+        }
     }
 }