Ensure reflection property metadata is instantiated lazily.

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/ReflectionJsonTypeInfoOfT.cs

@@ -23,11 +23,6 @@ internal ReflectionJsonTypeInfo(JsonConverter converter, JsonSerializerOptions o
             PopulatePolymorphismMetadata();
             MapInterfaceTypesToCallbacks();
 
-            if (converter.ConverterStrategy == ConverterStrategy.Object)
-            {
-                AddPropertiesAndParametersUsingReflection();
-            }
-
             Func<object>? createObject = JsonSerializerOptions.MemberAccessorStrategy.CreateConstructor(typeof(T));
             SetCreateObjectIfCompatible(createObject);
             CreateObjectForExtensionDataProperty = createObject;
@@ -37,11 +32,17 @@ internal ReflectionJsonTypeInfo(JsonConverter converter, JsonSerializerOptions o
             converter.ConfigureJsonTypeInfoUsingReflection(this, options);
         }
 
-        [RequiresUnreferencedCode(JsonSerializer.SerializationUnreferencedCodeMessage)]
-        [RequiresDynamicCode(JsonSerializer.SerializationRequiresDynamicCodeMessage)]
-        private void AddPropertiesAndParametersUsingReflection()
+        [UnconditionalSuppressMessage("ReflectionAnalysis", "IL2075:RequiresUnreferencedCode",
+            Justification = "The ctor is marked RequiresUnreferencedCode.")]
+        internal override void LateAddProperties()
         {
-            Debug.Assert(Converter.ConverterStrategy == ConverterStrategy.Object);
+            Debug.Assert(!IsConfigured);
+            Debug.Assert(PropertyCache is null);
+
+            if (Kind != JsonTypeInfoKind.Object)
+            {
+                return;
+            }
 
             const BindingFlags BindingFlags =
                 BindingFlags.Instance |

src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/MetadataTests/DefaultJsonTypeInfoResolverTests.JsonPropertyInfo.cs

@@ -1700,5 +1700,69 @@ public string? Value
                 set => _value = value ?? "NULL";
             }
         }
+
+        [Fact]
+        public static void TypeWithIgnoredUnsupportedType_ShouldBeSupported()
+        {
+            // Regression test for https://github.com/dotnet/runtime/issues/76807
+
+            // Sanity check -- metadata resolution for the unsupported type is failing
+            Assert.Throws<InvalidOperationException>(() => JsonSerializerOptions.Default.GetTypeInfo(typeof(UnsupportedType)));
+
+            // Serialization works as expected
+            string json = JsonSerializer.Serialize(new PocoWithIgnoredUnsupportedType());
+            JsonTestHelper.AssertJsonEqual("{}", json);
+
+            // Metadata is reported as expected
+            JsonTypeInfo jti = JsonSerializerOptions.Default.GetTypeInfo(typeof(PocoWithIgnoredUnsupportedType));
+            Assert.Equal(1, jti.Properties.Count);
+            JsonPropertyInfo propertyInfo = jti.Properties[0];
+            Assert.Null(propertyInfo.Get);
+            Assert.Null(propertyInfo.Set);
+        }
+
+        public class PocoWithIgnoredUnsupportedType
+        {
+            [JsonIgnore]
+            public UnsupportedType UnsuportedProperty { get; set; }
+        }
+
+        [Fact]
+        public static void TypeWithUnIgnoredUnsupportedType_CanModifyUnsupportedType()
+        {
+            var options = new JsonSerializerOptions
+            {
+                TypeInfoResolver = new DefaultJsonTypeInfoResolver
+                {
+                    Modifiers =
+                    {
+                        static jti =>
+                        {
+                            if (jti.Type == typeof(PocoWithUnIgnoredUnsupportedType))
+                            {
+                                Assert.Equal(1, jti.Properties.Count);
+                                JsonPropertyInfo propertyInfo = jti.Properties[0];
+                                Assert.Equal(typeof(UnsupportedType), propertyInfo.PropertyType);
+
+                                jti.Properties.Clear();
+                            }
+                        }
+                    }
+                }
+            };
+
+            string json = JsonSerializer.Serialize(new PocoWithUnIgnoredUnsupportedType(), options);
+            JsonTestHelper.AssertJsonEqual("{}", json);
+        }
+
+        public class PocoWithUnIgnoredUnsupportedType
+        {
+            public UnsupportedType UnsuportedProperty { get; set; }
+        }
+
+        public class UnsupportedType
+        {
+            public ReadOnlySpan<byte> Span => Array.Empty<byte>();
+        }
     }
 }