1 files changed, 117 insertions, 0 deletions
diff --git a/src/dotty/tools/dotc/reporting/ErrorMessages.scala b/src/dotty/tools/dotc/reporting/ErrorMessages.scala
new file mode 100644
index 000000000..7186ccbe2
--- /dev/null
+++ b/src/dotty/tools/dotc/reporting/ErrorMessages.scala
@@ -0,0 +1,117 @@
+package dotty.tools
+package dotc
+package reporting
+
+import dotc.core._
+import Contexts.Context, Decorators._, Symbols._
+import dotc.printing.SyntaxHighlighting._
+import util.{SourcePosition, NoSourcePosition}
+
+object ErrorMessages {
+  import dotc.ast.Trees._
+  import dotc.ast.untpd
+
+  implicit class ShouldExplainCtx(val c: Context) extends AnyVal {
+    def shouldExplain(expl: ErrorMessage): Boolean = {
+      implicit val ctx = c
+      expl match {
+        case _: NoExplanation => false
+        case expl if ctx.settings.explainerrors.value => true
+        case _ => false
+      }
+    }
+  }
+
+  trait ErrorMessage {
+    def kind: String
+    def msg: String
+    def explanation: String
+  }
+
+  case class NoExplanation(msg: String)(implicit val kind: String) extends ErrorMessage {
+    val explanation = ""
+  }
+
+  object Syntax {
+    implicit val kind: String = "Syntax"
+    implicit def stringToErrorMessage(s: String): ErrorMessage = NoExplanation(s)
+
+    abstract class EmptyCatchOrFinallyBlock(tryBody: untpd.Tree)(implicit ctx: Context, val kind: String) extends ErrorMessage {
+      val explanation = {
+        val tryString = tryBody match {
+          case Block(Nil, untpd.EmptyTree) => "{}"
+          case _ => tryBody.show
+        }
+
+        val code1 =
+          s"""|try $tryString catch {
+              |  case t: Throwable => ???
+              |}""".stripMargin
+
+        val code2 =
+          s"""|try $tryString finally {
+              |  // perform your cleanup here!
+              |}""".stripMargin
+
+        hl"""|Explanation:
+             |============
+             |A ${"try"} expression should be followed by some mechanism to handle any exceptions
+             |thrown. Typically a ${"catch"} expression follows the ${"try"} and pattern matches
+             |on any expected exceptions. For example:
+             |
+             |$code1
+             |
+             |It is also possible to follow a ${"try"} immediately by a ${"finally"} - letting the
+             |exception propagate - but still allowing for some clean up in ${"finally"}:
+             |
+             |$code2
+             """.stripMargin
+      }
+    }
+
+    case class EmptyCatchBlock(tryBody: untpd.Tree)(implicit ctx: Context, override val kind: String)
+    extends EmptyCatchOrFinallyBlock(tryBody) {
+      val msg =
+        hl"""The ${"catch"} block does not contain a valid expression, try adding a case like - `${"case e: Exception =>"}` to the block"""
+    }
+
+    case class EmptyCatchAndFinallyBlock(tryBody: untpd.Tree)(implicit ctx: Context, override val kind: String)
+    extends EmptyCatchOrFinallyBlock(tryBody) {
+      val msg =
+        hl"""A ${"try"} without ${"catch"} or ${"finally"} is equivalent to putting its body in a block; no exceptions are handled."""
+    }
+  }
+
+  object Type {
+    implicit val kind: String = "Type"
+    implicit def stringToErrorMessage(s: String): ErrorMessage = NoExplanation(s)
+
+    case class DuplicateBind(bind: untpd.Bind, tree: untpd.CaseDef)(implicit ctx: Context, val kind: String) extends ErrorMessage {
+      val msg =
+        em"duplicate pattern variable: `${bind.name}`"
+
+      val explanation = {
+        val pat = tree.pat.show
+        val guard = tree.guard match {
+          case untpd.EmptyTree => ""
+          case guard => s"if ${guard.show}"
+        }
+
+        val body = tree.body match {
+          case Block(Nil, untpd.EmptyTree) => ""
+          case body => s" ${body.show}"
+        }
+
+        val caseDef = s"case $pat$guard => $body"
+
+        hl"""|Explanation
+             |===========
+             |For each ${"case"} bound variable names  have to be unique. In:
+             |
+             |$caseDef
+             |
+             |`${bind.name}` is not unique. Rename one of the binds!""".stripMargin
+      }
+    }
+  }
+}