diff --git a/src/Analyzers/CSharp/Analyzers/UsePatternCombinators/CSharpUsePatternCombinatorsAnalyzer.cs b/src/Analyzers/CSharp/Analyzers/UsePatternCombinators/CSharpUsePatternCombinatorsAnalyzer.cs
index 0fc5ec3a5e484..e8afa1654b21b 100644
--- a/src/Analyzers/CSharp/Analyzers/UsePatternCombinators/CSharpUsePatternCombinatorsAnalyzer.cs
+++ b/src/Analyzers/CSharp/Analyzers/UsePatternCombinators/CSharpUsePatternCombinatorsAnalyzer.cs
@@ -159,7 +159,7 @@ public static BinaryOperatorKind Flip(BinaryOperatorKind operatorKind)
         private static bool IsConstant(IOperation operation)
         {
             // By-design, constants will not propagate to conversions.
-            return operation is IConversionOperation op
+            return operation is IConversionOperation { Conversion.IsUserDefined: false } op
                 ? IsConstant(op.Operand)
                 : operation.ConstantValue.HasValue;
         }
diff --git a/src/Features/CSharpTest/UsePatternCombinators/CSharpUsePatternCombinatorsDiagnosticAnalyzerTests.cs b/src/Features/CSharpTest/UsePatternCombinators/CSharpUsePatternCombinatorsDiagnosticAnalyzerTests.cs
index e415d3e041c98..10aca277a0933 100644
--- a/src/Features/CSharpTest/UsePatternCombinators/CSharpUsePatternCombinatorsDiagnosticAnalyzerTests.cs
+++ b/src/Features/CSharpTest/UsePatternCombinators/CSharpUsePatternCombinatorsDiagnosticAnalyzerTests.cs
@@ -258,6 +258,30 @@ bool M1(int v)
                 """);
         }
 
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/66787")]
+        public async Task TestConvertedConstants()
+        {
+            await TestAllAsync(
+                """
+                class C
+                {
+                    bool M(long l)
+                    {
+                        return {|FixAllInDocument:(l > int.MaxValue || l < int.MinValue)|};
+                    }
+                }
+                """,
+                """
+                class C
+                {
+                    bool M(long l)
+                    {
+                        return (l is > int.MaxValue or < int.MinValue);
+                    }
+                }
+                """);
+        }
+
         [Fact]
         public async Task TestMissingInExpressionTree()
         {
@@ -397,6 +421,42 @@ private static bool IsS(char[] ch, int count)
 }}");
         }
 
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/66787")]
+        public async Task TestMissingForImplicitUserDefinedCasts1()
+        {
+            await TestMissingAsync(
+                """
+                using System;
+                class C
+                {
+                    void M0(Int128 i)
+                    {
+                        if (i == int.MaxValue [||] i == int.MinValue)
+                        {
+                        }
+                    }
+                }
+                """);
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/66787")]
+        public async Task TestMissingForImplicitUserDefinedCasts2()
+        {
+            await TestMissingAsync(
+                """
+                using System;
+                class C
+                {
+                    void M0(Int128 i)
+                    {
+                        if (i > int.MaxValue [||] i < int.MinValue)
+                        {
+                        }
+                    }
+                }
+                """);
+        }
+
         [Fact]
         public async Task TestOnSideEffects1()
         {
diff --git a/src/Features/Core/Portable/ConvertIfToSwitch/AbstractConvertIfToSwitchCodeRefactoringProvider.Analyzer.cs b/src/Features/Core/Portable/ConvertIfToSwitch/AbstractConvertIfToSwitchCodeRefactoringProvider.Analyzer.cs
index 96f06ca0432d4..2f738b5682ccd 100644
--- a/src/Features/Core/Portable/ConvertIfToSwitch/AbstractConvertIfToSwitchCodeRefactoringProvider.Analyzer.cs
+++ b/src/Features/Core/Portable/ConvertIfToSwitch/AbstractConvertIfToSwitchCodeRefactoringProvider.Analyzer.cs
@@ -434,7 +434,7 @@ private static bool IsRelationalOperator(BinaryOperatorKind operatorKind)
             private static bool IsConstant(IOperation operation)
             {
                 // Constants do not propagate to conversions
-                return operation is IConversionOperation op
+                return operation is IConversionOperation { Conversion.IsUserDefined: false } op
                     ? IsConstant(op.Operand)
                     : operation.ConstantValue.HasValue;
             }