TransformExpressionTrees: keep a constant's declared type

A constant narrower than its stack type builds as a plain ldc.i4, which
infers as int, so a conditional whose other branch really is a char saw
two different types and the whole tree was left as the Expression calls
that built it - EF Core's StringCharConverter.ToChar is one. Bool and
enum constants were already wrapped for this reason; the wrap now
applies wherever the built value does not infer as the declared type.

Assisted-by: Claude:claude-opus-5:Claude Code

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

@@ -416,6 +416,13 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			ToCode(X(), () => "abc"[1] == 'b');
 		}
 
+		public void ConditionalWithNarrowConstant(string s)
+		{
+			ToCode(X(), () => (s.Length < 1) ? '\0' : s[0]);
+			ToCode(X(), () => (s.Length < 1) ? ((byte)0) : ((byte)s[0]));
+			ToCode(X(), () => (s.Length < 1) ? ((short)0) : ((short)s[0]));
+		}
+
 		public void StringsImplicitCast()
 		{
 			int i = 1;

ICSharpCode.Decompiler/IL/Transforms/TransformExpressionTrees.cs

@@ -1156,9 +1156,16 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				return null;
 			if (value.MatchBox(out var arg, out var boxType))
 			{
-				if (boxType.Kind == TypeKind.Enum || boxType.IsKnownType(KnownTypeCode.Boolean))
-					return () => new ExpressionTreeCast(boxType, ConvertValue(arg, invocation), false);
-				return () => ConvertValue(arg, invocation);
+				return () => {
+					// A constant narrower than its stack type - a bool, a char, an enum, one of
+					// the small integers - builds as a plain ldc.i4 that infers as int, and
+					// consumers compare inferred types. The cast keeps the type the tree
+					// declared for it.
+					var constantValue = ConvertValue(arg, invocation);
+					if (!NormalizeTypeVisitor.TypeErasure.EquivalentTypes(constantValue.InferType(context.TypeSystem), boxType))
+						return new ExpressionTreeCast(boxType, constantValue, false);
+					return constantValue;
+				};
 			}
 			return () => ConvertValue(value, invocation);