Add error for illegal singleton ops

Division by 0 and modulo by 0 would otherwise crash the compiler. 
Throwing an error prints an error message positioned on the operator,
and does not prevent other type errors from being found and shown.

Author: MaximeKjaer

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -3643,8 +3643,14 @@ object Types {
             case tpnme.Plus => constantFold2(intValue, _ + _)
             case tpnme.Minus => constantFold2(intValue, _ - _)
             case tpnme.Times => constantFold2(intValue, _ * _)
-            case tpnme.Div => constantFold2(intValue, _ / _)
-            case tpnme.Mod => constantFold2(intValue, _ % _)
+            case tpnme.Div => constantFold2(intValue, {
+              case (_, 0) => throw new TypeError("Division by 0")
+              case (a, b) => a / b
+            })
+            case tpnme.Mod => constantFold2(intValue, {
+              case (_, 0) => throw new TypeError("Modulo by 0")
+              case (a, b) => a % b
+            })
             case tpnme.Lt => constantFold2(intValue, _ < _)
             case tpnme.Gt => constantFold2(intValue, _ > _)
             case tpnme.Ge => constantFold2(intValue, _ >= _)

tests/neg/singleton-ops.scala

@@ -25,14 +25,14 @@ object Test {
   val t7: -2 * -2 = 4
 
   val t8: 10 / 2 = 5
-  val t9: 11 / 2 = 5 // Integer division
+  val t9: 11 / -2 = -5 // Integer division
   val t10: 2 / 4 = 2 // error
-  val t11: -1 / -1 = 1
+  val t11: -1 / 0 = 1 // error
 
   val t12: 10 % 3 = 1
   val t13: 12 % 2 = 1 // error
   val t14: 1 % -3 = 1
-  val t15: -3 % -2 = 0 // error
+  val t15: -3 % 0 = 0 // error
 
   val t16: 1 < 0 = false
   val t17: 0 < 1 = true