We need to properly compare the C# type, not just the StackType, otherwise our implicit conversions might end up converting to the wrong type.

This also matters in the `1 => DateTime.Now, 2 => null` case -- BestCommonType infers `DateTime` here, but we need `DateTime?` instead. But both had `StackType.O` so this went wrong prior to this commit.

ICSharpCode.Decompiler.Tests/TestCases/Pretty/SwitchExpressions.cs

@@ -244,6 +244,15 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			}).ToValidated();
 		}
 
+		public static bool NullableDateTime(int i)
+		{
+			return (i switch {
+				0 => (DateTime?)DateTime.Now,
+				1 => DateTime.UtcNow,
+				_ => null,
+			}).HasValue;
+		}
+
 		public static void ThrowDifferentExceptions(int i)
 		{
 			throw i switch {

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -4377,7 +4377,9 @@ namespace ICSharpCode.Decompiler.CSharp
 			IType commonType = ti.GetBestCommonType(
 				expressionsForTypeInference.SelectArray(e => e.ResolveResult),
 				out bool success);
-			if (success && commonType.GetStackType() == inst.ResultType)
+			// Note: we need to ensure the compiler actually picked the type that we used for the
+			// implicit conversions.
+			if (success && NormalizeTypeVisitor.TypeErasure.EquivalentTypes(commonType, resultType))
 			{
 				return switchExpr.WithILInstruction(inst).WithRR(new ResolveResult(commonType));
 			}
@@ -4390,7 +4392,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				commonType = ti.GetBestCommonType(
 					expressionsForTypeInference.SelectArray(e => e.ResolveResult),
 					out success);
-				if (success && commonType.GetStackType() == inst.ResultType)
+				if (success && NormalizeTypeVisitor.TypeErasure.EquivalentTypes(commonType, resultType))
 				{
 					return switchExpr.WithILInstruction(inst).WithRR(new ResolveResult(commonType));
 				}