fix estimating the notnull constraints with optimized nullable metadata of nested types

Author: smdn

src/Smdn.Reflection.ReverseGenerating/Smdn.Reflection.ReverseGenerating/Generator.cs

@@ -162,13 +162,31 @@ static bool IsNullableAttribute(CustomAttributeData attr)
     static bool IsNullableContextAttribute(CustomAttributeData attr)
       => attr.AttributeType.FullName.Equals("System.Runtime.CompilerServices.NullableContextAttribute", StringComparison.Ordinal);
 
+    static CustomAttributeData? TryGetNullableContextAttributeFromDeclaringTypeOrOuterType(Type type)
+    {
+      Type? t = type;
+
+      for (; ; ) {
+        if ((t = t!.DeclaringType) is null)
+          return null;
+
+        var attr = t!.CustomAttributes.FirstOrDefault(IsNullableContextAttribute);
+
+        if (attr is not null)
+          return attr;
+
+        // retry against to the outer type
+        continue;
+      }
+    }
+
     // ref: https://github.com/dotnet/roslyn/blob/main/docs/features/nullable-metadata.md#type-parameters
     static bool HasNotNullConstraint(Type genericParameter)
     {
       var attrNullable = genericParameter.CustomAttributes.FirstOrDefault(IsNullableAttribute);
       var attrNullableContext =
         genericParameter.DeclaringMethod?.CustomAttributes?.FirstOrDefault(IsNullableContextAttribute) ??
-        genericParameter.DeclaringType.CustomAttributes.FirstOrDefault(IsNullableContextAttribute);
+        TryGetNullableContextAttributeFromDeclaringTypeOrOuterType(genericParameter);
 
       const byte notAnnotated = 1;
 

tests/Smdn.Reflection.ReverseGenerating/Smdn.Reflection.ReverseGenerating/Generator.MemberDeclaration.cs

@@ -1124,6 +1124,30 @@ public class CBase { }
             [MemberDeclarationTestCase("public T M15_1<T>(T x) where T : unmanaged, System.IDisposable {}")] public T M15_1<T>(T x) where T : unmanaged, IDisposable => throw new NotImplementedException();
           }
 
+          public class ConstraintsNotNullWithNullableMetadata {
+#nullable enable annotations
+            class ClassWithOptimizedNullableMetadata {
+              // these members are required to reproduce optimized nullable metadata
+              public void RequiredToBeOptimized0(string p) { }
+              public void RequiredToBeOptimized1(string? p) { }
+
+              // this class will have no NullableContextAttribute after optimization
+              class NotNullConstraints {
+                [MemberDeclarationTestCase($"public void {nameof(NotNullConstraint)}<TNotNull>(TNotNull p) where TNotNull : notnull {{}}")]
+                public void NotNullConstraint<TNotNull>(TNotNull p) where TNotNull : notnull { }
+
+                // this member is required to reproduce optimized nullable metadata
+#if SYSTEM_REFLECTION_NULLABILITYINFO
+                [MemberDeclarationTestCase($"public void {nameof(NotNullConstraint_Nullable)}<TNotNull>(TNotNull? p) where TNotNull : notnull {{}}")]
+#else
+                [MemberDeclarationTestCase($"public void {nameof(NotNullConstraint_Nullable)}<TNotNull>(TNotNull p) where TNotNull : notnull {{}}")]
+#endif
+                public void NotNullConstraint_Nullable<TNotNull>(TNotNull? p) where TNotNull : notnull { }
+              }
+            }
+#nullable restore annotations
+          }
+
           class Constraints2 {
             [MemberDeclarationTestCase("public T1 M1<T1, T2>(T2 x) where T1 : new() where T2 : new() {}")]
             public T1 M1<T1, T2>(T2 x)