Change 'mixed left- and right-associative operators' to Message

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -404,14 +404,13 @@ object Parsers {
 
     var opStack: List[OpInfo] = Nil
 
-    def checkAssoc(offset: Int, op: Name, leftAssoc: Boolean) =
+    def checkAssoc(offset: Int, op: Name, leftAssoc: Boolean, op2: Name) =
       if (isLeftAssoc(op) != leftAssoc)
-        syntaxError(
-          "left- and right-associative operators with same precedence may not be mixed", offset)
+        syntaxError(MixedLeftAndRightAssociativeOps(op, op2, leftAssoc), offset)
 
-    def reduceStack(base: List[OpInfo], top: Tree, prec: Int, leftAssoc: Boolean): Tree = {
+    def reduceStack(base: List[OpInfo], top: Tree, prec: Int, leftAssoc: Boolean, op2: Name): Tree = {
       if (opStack != base && precedence(opStack.head.operator.name) == prec)
-        checkAssoc(opStack.head.offset, opStack.head.operator.name, leftAssoc)
+        checkAssoc(opStack.head.offset, opStack.head.operator.name, leftAssoc, op2)
       def recur(top: Tree): Tree = {
         if (opStack == base) top
         else {
@@ -445,20 +444,20 @@ object Parsers {
       var top = first
       while (isIdent && in.name != notAnOperator) {
         val op = if (isType) typeIdent() else termIdent()
-        top = reduceStack(base, top, precedence(op.name), isLeftAssoc(op.name))
+        top = reduceStack(base, top, precedence(op.name), isLeftAssoc(op.name), op.name)
         opStack = OpInfo(top, op, in.offset) :: opStack
         newLineOptWhenFollowing(canStartOperand)
         if (maybePostfix && !canStartOperand(in.token)) {
           val topInfo = opStack.head
           opStack = opStack.tail
-          val od = reduceStack(base, topInfo.operand, 0, true)
+          val od = reduceStack(base, topInfo.operand, 0, true, in.name)
           return atPos(startOffset(od), topInfo.offset) {
             PostfixOp(od, topInfo.operator)
           }
         }
         top = operand()
       }
-      reduceStack(base, top, 0, true)
+      reduceStack(base, top, 0, true, in.name)
     }
 
 /* -------- IDENTIFIERS AND LITERALS ------------------------------------------- */

compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -1086,4 +1086,37 @@ object messages {
           |""".stripMargin
   }
 
+  case class MixedLeftAndRightAssociativeOps(op1: Name, op2: Name, op2LeftAssoc: Boolean)(implicit ctx: Context)
+    extends Message(41) {
+    val kind = "Syntax"
+    val op1Asso = if (op2LeftAssoc) "which is right-associative" else "which is left-associative"
+    val op2Asso = if (op2LeftAssoc) "which is left-associative" else "which is right-associative"
+    val msg = s"${hl"`${op1}`"} (${op1Asso}) and ${hl"`${op2}`"} ($op2Asso) have same precedence and may not be mixed"
+    val explanation =
+      s"""|The operators ${hl"${op1}"} and ${hl"${op2}"} are used as infix operators in the same expression,
+          |but they bind to different sides:
+          |${hl"${op1}"} is applied to the operand to its ${if (op2LeftAssoc) "right" else "left"}
+          |${hl"${op2}"} is applied to the operand to its ${if (op2LeftAssoc) "left" else "right"}
+          |As both have the same precedence the compiler can't decide which to apply first.
+          |
+          |You may use parenthesis to make the application order explicit,
+          |or use method application syntax ${hl"`operand1.${op1}(operand2)`"}.
+          |
+          |Operators ending in a colon `:` are right-associative. All other operators are left-associative.
+          |
+          |Infix operator precedence is determined by the operator's first character:
+          |  (all letters)
+          |  |
+          |  ^
+          |  &
+          |  = !
+          |  < >
+          |  :
+          |  + -
+          |  * / %
+          |  (all other special characters)
+          |Operators starting with a letter have lowest precedence, followed by operators starting with `|', etc.
+          |""".stripMargin
+  }
+
 }

compiler/test/dotty/tools/dotc/reporting/ErrorMessagesTests.scala

@@ -136,4 +136,27 @@ class ErrorMessagesTests extends ErrorMessagesTest {
       """.stripMargin
     }
     .expectNoErrors
+
+  @Test def leftAndRightAssociative =
+    checkMessagesAfter("frontend") {
+      """
+        |object Ops {
+        |  case class I(j: Int) {
+        |    def +-(i: Int) = i
+        |    def +:(i: Int) = i
+        |  }
+        |  val v = I(1) +- I(4) +: I(4)
+        |}
+      """.stripMargin
+    }
+    .expect { (ictx, messages) =>
+      implicit val ctx: Context = ictx
+      val defn = ictx.definitions
+
+      assertMessageCount(1, messages)
+      val MixedLeftAndRightAssociativeOps(op1, op2, op2LeftAssoc) :: Nil = messages
+      assertEquals("+-", op1.show)
+      assertEquals("+:", op2.show)
+      assertFalse(op2LeftAssoc)
+    }
 }