fix: do not throw when operation tag is missing matching global tag

src/Microsoft.OpenApi/Models/OpenApiOperation.cs

@@ -39,7 +39,7 @@
                 {
                     return;
                 }
                 _tags = value is HashSet<OpenApiTagReference> tags && tags.Comparer is OpenApiTagComparer ?
                         tags :
                         new HashSet<OpenApiTagReference>(value, OpenApiTagComparer.Instance);
             }
@@ -183,7 +183,7 @@
             // tags
             writer.WriteOptionalCollection(
                 OpenApiConstants.Tags,
-                VerifyTagReferences(Tags),
+                Tags,
                 callback);
 
             // summary
@@ -228,7 +228,7 @@
         /// <summary>
         /// Serialize <see cref="OpenApiOperation"/> to Open Api v2.0.
         /// </summary>
         public virtual void SerializeAsV2(IOpenApiWriter writer)
         {
             Utils.CheckArgumentNull(writer);
 
@@ -237,7 +237,7 @@
             // tags
             writer.WriteOptionalCollection(
                 OpenApiConstants.Tags,
-                VerifyTagReferences(Tags),
+                Tags,
                 (w, t) => t.SerializeAsV2(w));
 
             // summary
@@ -338,7 +338,7 @@
 
                 writer.WriteOptionalCollection(OpenApiConstants.Schemes, schemes, (w, s) => 
                 {
                     if (!string.IsNullOrEmpty(s) && s is not null)
                     {
                         w.WriteValue(s);
                     }
@@ -356,21 +356,5 @@
 
             writer.WriteEndObject();
         }
-
-        private static HashSet<OpenApiTagReference>? VerifyTagReferences(HashSet<OpenApiTagReference>? tags)
-        {
-            if (tags?.Count > 0)
-            {
-                foreach (var tag in tags)
-                {
-                    if (tag.Target is null)
-                    {
-                        throw new OpenApiException($"The OpenAPI tag reference '{tag.Reference.Id}' does not reference a valid tag.");
-                    }
-                }
-            }
-
-            return tags;
-        }
     }
 }

src/Microsoft.OpenApi/Models/References/OpenApiTagReference.cs

@@ -59,7 +59,7 @@ public string? Description
         public Dictionary<string, IOpenApiExtension>? Extensions { get => Target?.Extensions; }
 
         /// <inheritdoc/>
-        public string? Name { get => Target?.Name; }
+        public string? Name { get => Target?.Name ?? Reference?.Id; }
         /// <inheritdoc/>
         public override IOpenApiTag CopyReferenceAsTargetElementWithOverrides(IOpenApiTag source)
         {

test/Microsoft.OpenApi.Tests/Models/OpenApiOperationTests.cs

@@ -860,34 +860,5 @@ public void OpenApiOperationCopyConstructorWithAnnotationsSucceeds()
 
             Assert.NotEqual(baseOperation.Metadata["key1"], actualOperation.Metadata["key1"]);
         }
-
-        [Theory]
-        [InlineData(OpenApiSpecVersion.OpenApi2_0)]
-        [InlineData(OpenApiSpecVersion.OpenApi3_0)]
-        [InlineData(OpenApiSpecVersion.OpenApi3_1)]
-        public async Task SerializeAsJsonAsyncThrowsIfTagReferenceIsUnresolved(OpenApiSpecVersion version)
-        {
-            var document = new OpenApiDocument()
-            {
-                Tags =
-                [
-                    new() { Name = "one" },
-                    new() { Name = "three" }
-                ]
-            };
-
-            var operation = new OpenApiOperation()
-            {
-                Tags =
-                [
-                    new OpenApiTagReference("one", document),
-                    new OpenApiTagReference("two", document),
-                    new OpenApiTagReference("three", document)
-                ]
-            };
-
-            var exception = await Assert.ThrowsAsync<OpenApiException>(() => operation.SerializeAsJsonAsync(version));
-            Assert.Equal("The OpenAPI tag reference 'two' does not reference a valid tag.", exception.Message);
-        }
     }
 }