Ensure TypeSymbolWithAnnotations.IsNullable != null for value type (#28298)

src/Compilers/CSharp/Portable/Symbols/SymbolWithAnnotations.cs

@@ -566,7 +566,7 @@ public bool ApplyNullableTransforms(ImmutableArray<bool> transforms, bool useNon
                 result = result.DoUpdate(newTypeSymbol, result.CustomModifiers);
             }
 
-            //if (!result.IsValueType) // PROTOTYPE(NullableReferenceTypes): this is causing cycle 6 in NullableAttribute_01
+            if (!result.IsValueType)
             {
                 if (isNullable)
                 {

src/Compilers/CSharp/Test/Semantic/Semantics/StaticNullChecking.cs

@@ -1863,6 +1863,21 @@ public class Oblivious { }
             VerifyNonNullTypes(obliviousComp.GetMember("Oblivious"), true); // PROTOTYPE(NullableReferenceTypes): should be false
         }
 
+        [Fact]
+        public void NonNullTypes_ValueTypeArgument()
+        {
+            var source =
+@"using System.Runtime.CompilerServices;
+[module: NonNullTypes(false)]
+class A<T> { }
+class B
+{
+    A<byte> P { get; }
+}";
+            var comp = CreateCompilation(new[] { source, NonNullTypesAttributesDefinition }, parseOptions: TestOptions.Regular8);
+            comp.VerifyDiagnostics();
+        }
+
         [Fact]
         public void AssignObliviousIntoLocals()
         {