fix: C# compilation of newtype-char to int conversion

Source/DafnyCore/Backends/CSharp/CsharpCodeGenerator.cs

@@ -1643,7 +1643,7 @@ internal override string TypeName(Type type, ConcreteSyntaxTree wr, IOrigin tok,
     private string CharTypeDescriptorName => DafnyHelpersClass + (UnicodeCharEnabled ? ".RUNE" : ".CHAR");
 
     private void ConvertFromChar(Expression e, ConcreteSyntaxTree wr, bool inLetExprBody, ConcreteSyntaxTree wStmts) {
-      if (e.Type.IsCharType && UnicodeCharEnabled) {
+      if (GetRuntimeType(e.Type).IsCharType && UnicodeCharEnabled) {
         wr.Write("(");
         TrParenExpr(e, wr, inLetExprBody, wStmts);
         wr.Write(".Value)");

Source/IntegrationTests/TestFiles/LitTests/LitTest/comp/GeneralNewtypes.dfy

@@ -15,6 +15,7 @@ method Main() {
   AutoInit.Test();
   RefinementB.Test(2.0, 3.0);
   SimpleNewtypeWitness.Test();
+  CharToInt.Test();
 }
 
 module Numerics {
@@ -659,3 +660,15 @@ module SimpleNewtypeWitness {
     print a, " ", b, " ", c, "\n"; // 102 103 104
   }
 }
+
+module CharToInt {
+  newtype char16 = c: char | c as int < 65536
+
+  method Test() {
+    var c: char := 'c';
+    var d: char16 := 'd';
+
+    print c, " (", c as int, ", ", c as char as int, "), ";
+    print d, " (", d as int, ", ", d as char as int, ")\n";
+  }
+}

Source/IntegrationTests/TestFiles/LitTests/LitTest/comp/GeneralNewtypes.dfy.expect

@@ -66,3 +66,4 @@ false true | true
 7
 2.0 3.0
 102 103 104
+'c' (99, 99), 'd' (100, 100)

docs/dev/news/6296.fix

@@ -0,0 +1 @@
+Compilation of type conversion from a `newtype`-based on `char` to `int`