Fix a bug in conversion classification that improperly hides a switch...

Fix a bug in conversion classification that improperly hides a switch expression conversion when there is a built-in explicit conversion (#42339)

This bug results in the compiler crashing or producing bad code in some scenarios.
Fixes #42331

src/Compilers/CSharp/Portable/Binder/Semantics/Conversions/ConversionsBase.cs

@@ -100,7 +100,10 @@ public Conversion ClassifyImplicitConversionFromExpression(BoundExpression sourc
                 Conversion fastConversion = FastClassifyConversion(sourceType, destination);
                 if (fastConversion.Exists)
                 {
-                    return fastConversion.IsImplicit ? fastConversion : Conversion.NoConversion;
+                    if (fastConversion.IsImplicit)
+                    {
+                        return fastConversion;
+                    }
                 }
                 else
                 {

src/Compilers/CSharp/Test/Semantic/Semantics/PatternMatchingTests3.cs

@@ -688,7 +688,7 @@ class Source2
         }
 
         [Fact]
-        public void TargetTypedSwitch_Overload()
+        public void TargetTypedSwitch_Overload_01()
         {
             var source = @"
 using System;
@@ -730,6 +730,53 @@ class Source2
             var comp = CompileAndVerify(compilation, expectedOutput: expectedOutput);
         }
 
+        [Fact]
+        public void TargetTypedSwitch_Overload_02()
+        {
+            var source = @"
+using System;
+
+class Program
+{
+    static void Main(string[] args)
+    {
+        bool b = true;
+        M(b switch { false => 1, true => 2 });
+    }
+    static void M(Int16 s) => Console.Write(nameof(Int16));
+    static void M(Int64 l) => Console.Write(nameof(Int64));
+}
+";
+            var expectedOutput = "Int16";
+            var compilation = CreateCompilation(source, options: TestOptions.DebugExe.WithNullableContextOptions(NullableContextOptions.Disable));
+            compilation.VerifyDiagnostics(
+                );
+            var comp = CompileAndVerify(compilation, expectedOutput: expectedOutput);
+        }
+
+        [Fact]
+        public void TargetTypedSwitch_Overload_03()
+        {
+            var source = @"
+using System;
+
+class Program
+{
+    static void Main(string[] args)
+    {
+        bool b = true;
+        M(b switch { false => 1, true => 2 });
+    }
+    static void M(Int16 s) => Console.Write(nameof(Int16));
+}
+";
+            var expectedOutput = "Int16";
+            var compilation = CreateCompilation(source, options: TestOptions.DebugExe.WithNullableContextOptions(NullableContextOptions.Disable));
+            compilation.VerifyDiagnostics(
+                );
+            var comp = CompileAndVerify(compilation, expectedOutput: expectedOutput);
+        }
+
         [Fact]
         public void TargetTypedSwitch_Lambda_01()
         {