Update GetFirstRelatedDocumentId to not return documents with the same path within the same project (#78475)

GetFirstRelatedDocumentId had a mismatch in behavior for files within the same project that have the same path (shouldn't really happen, would cause compile errors). Previously, the code would return files from the same proejct when searching the cache, but not when doing the linear walk. Talked with Jason, and he indicated that it was better to have the code not return files from the same project, as callers wouldn't expect that.

Author: ToddGrun

src/Workspaces/Core/Portable/Workspace/Solution/SolutionState.cs

@@ -1284,7 +1284,8 @@ public SolutionState WithAnalyzerReferences(IReadOnlyList<AnalyzerReference> ana
         {
             foreach (var relatedDocumentId in relatedDocumentIds)
             {
-                if (relatedDocumentId != documentId)
+                // Match the linear search behavior below and do not return documents from the same project.
+                if (relatedDocumentId != documentId && relatedDocumentId.ProjectId != documentId.ProjectId)
                     return relatedDocumentId;
             }
 

src/Workspaces/CoreTest/SolutionTests/SolutionTests.cs

@@ -2435,6 +2435,29 @@ public void AddDocument_SourceText()
         Assert.Throws<InvalidOperationException>(() => solution.AddDocument(documentId: DocumentId.CreateNewId(ProjectId.CreateNewId()), "name", sourceText));
     }
 
+    [Fact]
+    public async Task GetFirstRelatedDocumentIdWithDuplicatedDocuments()
+    {
+        using var workspace = CreateWorkspaceWithProjectAndDocuments();
+        var origSolution = workspace.CurrentSolution;
+        var project = origSolution.Projects.Single();
+
+        var origDocumentId = project.DocumentIds.Single();
+        var newDocumentId = DocumentId.CreateNewId(project.Id);
+
+        var document = project.GetRequiredDocument(origDocumentId);
+        var sourceText = await document.GetTextAsync();
+
+        var newSolution = origSolution.AddDocument(newDocumentId, document.Name, sourceText, filePath: document.FilePath!);
+
+        // Populate the SolutionState cache for this document id
+        _ = newSolution.GetRelatedDocumentIds(origDocumentId);
+
+        // Ensure a GetFirstRelatedDocumentId call with a poulated cache doesn't return newDocumentId
+        var relatedDocument = newSolution.GetFirstRelatedDocumentId(origDocumentId, relatedProjectIdHint: null);
+        Assert.Null(relatedDocument);
+    }
+
     [Fact]
     public void AddDocument_SyntaxRoot()
     {