[mcs] Check type of index used in pointer array aritmetic. Fixes #17145

mcs/errors/cs0029-35.cs

@@ -0,0 +1,12 @@
+// CS0029: Cannot implicitly convert type `string' to `int'
+// Line: 10
+// Compiler options: -unsafe
+
+class TestClass
+{
+	public unsafe static void Main ()
+	{
+		int* arr = null;
+		var i = arr["c"];
+	}
+}

mcs/errors/cs0220-4.cs

@@ -0,0 +1,12 @@
+// CS0220: The operation overflows at compile time in checked mode
+// Line: 10
+// Compiler options: -unsafe
+
+class TestClass
+{
+	public unsafe static void Main ()
+	{
+		int* arr = null;
+		var i = arr[long.MaxValue];
+	}
+}

mcs/mcs/ecore.cs

@@ -1058,7 +1058,7 @@ public static void UnsafeError (Report Report, Location loc)
 		//
 		// Converts `source' to an int, uint, long or ulong.
 		//
-		protected Expression ConvertExpressionToArrayIndex (ResolveContext ec, Expression source)
+		protected Expression ConvertExpressionToArrayIndex (ResolveContext ec, Expression source, bool pointerArray = false)
 		{
 			var btypes = ec.BuiltinTypes;
 
@@ -1085,6 +1085,9 @@ protected Expression ConvertExpressionToArrayIndex (ResolveContext ec, Expressio
 				}
 			}
 
+			if (pointerArray)
+				return converted;
+
 			//
 			// Only positive constants are allowed at compile time
 			//

mcs/mcs/expression.cs

@@ -9306,17 +9306,24 @@ public override Expression CreateExpressionTree (ResolveContext ec)
 			return CreateExpressionFactoryCall (ec, "ArrayIndex", args);
 		}
 
-		Expression MakePointerAccess (ResolveContext ec, TypeSpec type)
+		Expression MakePointerAccess (ResolveContext rc, TypeSpec type)
 		{
 			if (Arguments.Count != 1){
-				ec.Report.Error (196, loc, "A pointer must be indexed by only one value");
+				rc.Report.Error (196, loc, "A pointer must be indexed by only one value");
 				return null;
 			}
 
-			if (Arguments [0] is NamedArgument)
-				Error_NamedArgument ((NamedArgument) Arguments[0], ec.Report);
+			var arg = Arguments[0];
+			if (arg is NamedArgument)
+				Error_NamedArgument ((NamedArgument) arg, rc.Report);
 
-			Expression p = new PointerArithmetic (Binary.Operator.Addition, Expr, Arguments [0].Expr.Resolve (ec), type, loc);
+			var index = arg.Expr.Resolve (rc);
+			if (index == null)
+				return null;
+
+			index = ConvertExpressionToArrayIndex (rc, index, true);
+
+			Expression p = new PointerArithmetic (Binary.Operator.Addition, Expr, index, type, loc);
 			return new Indirection (p, loc);
 		}