Fix CA1859 false positive for anonymous types

src/NetAnalyzers/Core/Microsoft.NetCore.Analyzers/Performance/UseConcreteTypeAnalyzer.cs

@@ -324,6 +324,12 @@ void Evaluate(ISymbol affectedSymbol, ITypeSymbol fromType, PooledConcurrentSet<
                     }
                 }
 
+                if (toType.IsAnonymousType)
+                {
+                    // don't recommend upgrading to an anonymous (i.e. invalid) type
+                    return;
+                }
+
                 if (toType.TypeKind is not TypeKind.Class and not TypeKind.Array and not TypeKind.Struct)
                 {
                     // we only deal with classes, arrays, or structs

src/NetAnalyzers/UnitTests/Microsoft.NetCore.Analyzers/Performance/UseConcreteTypeTests.cs

@@ -1502,6 +1502,22 @@ private void M(ValueTuple<Action> vt)
             await TestCSAsync(Source);
         }
 
+        [Fact, WorkItem(7504, "https://github.com/dotnet/roslyn-analyzers/issues/7504")]
+        public static async Task ShouldNotSuggestChangingToAnonymousType()
+        {
+            const string Source = @"
+            class Foo
+            {
+                object Bar()
+                {
+                    var outputData = new { A = 1, B = 2 };
+                    return outputData;
+                }
+            }";
+
+            await TestCSAsync(Source);
+        }
+
         private static async Task TestCSAsync(string source, params DiagnosticResult[] diagnosticResults)
         {
             var test = new VerifyCS.Test