Fix #856: Handle try/catch cases as catch cases if possible.

Previously they were all lifted into a match with the came cases.
Now the first cases are handled directly by by the catch. If one
of the cases can not be handled the old scheme is applied to to it
and all subsequent cases.

3 files changed, 103 insertions, 75 deletions

diff --git a/src/dotty/tools/dotc/Compiler.scala b/src/dotty/tools/dotc/Compiler.scala
index 3844f42a7..ce9280d82 100644
--- a/src/dotty/tools/dotc/Compiler.scala
+++ b/src/dotty/tools/dotc/Compiler.scala
@@ -57,7 +57,8 @@ class Compiler {
            new TailRec,             // Rewrite tail recursion to loops
            new LiftTry,             // Put try expressions that might execute on non-empty stacks into their own methods
            new ClassOf),            // Expand `Predef.classOf` calls.
-      List(new PatternMatcher,      // Compile pattern matches
+      List(new TryCatchPatterns,    // Compile cases in try/catch
+           new PatternMatcher,      // Compile pattern matches
            new ExplicitOuter,       // Add accessors to outer classes from nested ones.
            new ExplicitSelf,        // Make references to non-trivial self types explicit as casts
            new CrossCastAnd,        // Normalize selections involving intersection types.
diff --git a/src/dotty/tools/dotc/transform/PatternMatcher.scala b/src/dotty/tools/dotc/transform/PatternMatcher.scala
index fd89696a8..974053769 100644
--- a/src/dotty/tools/dotc/transform/PatternMatcher.scala
+++ b/src/dotty/tools/dotc/transform/PatternMatcher.scala
@@ -1,6 +1,8 @@
 package dotty.tools.dotc
 package transform
 
+import scala.language.postfixOps
+
 import TreeTransforms._
 import core.Denotations._
 import core.SymDenotations._
@@ -53,19 +55,6 @@ class PatternMatcher extends MiniPhaseTransform with DenotTransformer {thisTrans
     translated.ensureConforms(tree.tpe)
   }
 
-
-  override def transformTry(tree: tpd.Try)(implicit ctx: Context, info: TransformerInfo): tpd.Tree = {
-    val selector =
-      ctx.newSymbol(ctx.owner, ctx.freshName("ex").toTermName, Flags.Synthetic | Flags.Case, defn.ThrowableType, coord = tree.pos)
-    val sel = Ident(selector.termRef).withPos(tree.pos)
-    val rethrow = tpd.CaseDef(EmptyTree, EmptyTree, Throw(ref(selector)))
-    val newCases = tpd.CaseDef(
-      Bind(selector, Underscore(selector.info).withPos(tree.pos)),
-      EmptyTree,
-      transformMatch(tpd.Match(sel, tree.cases ::: rethrow :: Nil)))
-    cpy.Try(tree)(tree.expr, newCases :: Nil, tree.finalizer)
-  }
-
   class Translator(implicit ctx: Context) {
 
   def translator = {
@@ -1264,27 +1253,6 @@ class PatternMatcher extends MiniPhaseTransform with DenotTransformer {thisTrans
         t
     }
 
-    /** Is this pattern node a catch-all or type-test pattern? */
-    def isCatchCase(cdef: CaseDef) = cdef match {
-      case CaseDef(Typed(Ident(nme.WILDCARD), tpt), EmptyTree, _) =>
-        isSimpleThrowable(tpt.tpe)
-      case CaseDef(Bind(_, Typed(Ident(nme.WILDCARD), tpt)), EmptyTree, _) =>
-        isSimpleThrowable(tpt.tpe)
-      case _ =>
-        isDefaultCase(cdef)
-    }
-
-    private def isSimpleThrowable(tp: Type)(implicit ctx: Context): Boolean = tp match {
-      case tp @ TypeRef(pre, _) =>
-        val sym = tp.symbol
-        (pre == NoPrefix || pre.widen.typeSymbol.isStatic) &&
-          (sym.derivesFrom(defn.ThrowableClass)) &&  /* bq */ !(sym is Flags.Trait)
-      case _ =>
-        false
-    }
-
-
-
     /** Implement a pattern match by turning its cases (including the implicit failure case)
       * into the corresponding (monadic) extractors, and combining them with the `orElse` combinator.
       *
@@ -1335,46 +1303,6 @@ class PatternMatcher extends MiniPhaseTransform with DenotTransformer {thisTrans
       Block(List(ValDef(selectorSym, sel)), combined)
     }
 
-    // return list of typed CaseDefs that are supported by the backend (typed/bind/wildcard)
-    // we don't have a global scrutinee -- the caught exception must be bound in each of the casedefs
-    // there's no need to check the scrutinee for null -- "throw null" becomes "throw new NullPointerException"
-    // try to simplify to a type-based switch, or fall back to a catch-all case that runs a normal pattern match
-    // unlike translateMatch, we type our result before returning it
-    /*def translateTry(caseDefs: List[CaseDef], pt: Type, pos: Position): List[CaseDef] =
-    // if they're already simple enough to be handled by the back-end, we're done
-      if (caseDefs forall isCatchCase) caseDefs
-      else {
-        val swatches = { // switch-catches
-        val bindersAndCases = caseDefs map { caseDef =>
-            // generate a fresh symbol for each case, hoping we'll end up emitting a type-switch (we don't have a global scrut there)
-            // if we fail to emit a fine-grained switch, have to do translateCase again with a single scrutSym (TODO: uniformize substitution on treemakers so we can avoid this)
-            val caseScrutSym = freshSym(pos, pureType(defn.ThrowableType))
-            (caseScrutSym, propagateSubstitution(translateCase(caseScrutSym, pt)(caseDef), EmptySubstitution))
-          }
-
-          for(cases <- emitTypeSwitch(bindersAndCases, pt).toList
-              if cases forall isCatchCase; // must check again, since it's not guaranteed -- TODO: can we eliminate this? e.g., a type test could test for a trait or a non-trivial prefix, which are not handled by the back-end
-              cse <- cases) yield /*fixerUpper(matchOwner, pos)*/(cse).asInstanceOf[CaseDef]
-        }
-
-        val catches = if (swatches.nonEmpty) swatches else {
-          val scrutSym = freshSym(pos, pureType(defn.ThrowableType))
-          val casesNoSubstOnly = caseDefs map { caseDef => (propagateSubstitution(translateCase(scrutSym, pt)(caseDef), EmptySubstitution))}
-
-          val exSym = freshSym(pos, pureType(defn.ThrowableType), "ex")
-
-          List(
-              CaseDef(
-                Bind(exSym, Ident(??? /*nme.WILDCARD*/)), // TODO: does this need fixing upping?
-                EmptyTree,
-                combineCasesNoSubstOnly(ref(exSym), scrutSym, casesNoSubstOnly, pt, matchOwner, Some((scrut: Symbol) => Throw(ref(exSym))))
-              )
-            )
-        }
-
-        /*typer.typedCases(*/catches/*, defn.ThrowableType, WildcardType)*/
-      }*/
-
     /**  The translation of `pat if guard => body` has two aspects:
       *     1) the substitution due to the variables bound by patterns
       *     2) the combination of the extractor calls using `flatMap`.
diff --git a/src/dotty/tools/dotc/transform/TryCatchPatterns.scala b/src/dotty/tools/dotc/transform/TryCatchPatterns.scala
new file mode 100644
index 000000000..9a6ecef51
--- /dev/null
+++ b/src/dotty/tools/dotc/transform/TryCatchPatterns.scala
@@ -0,0 +1,99 @@
+package dotty.tools.dotc
+package transform
+
+import core.Symbols._
+import core.StdNames._
+import ast.Trees._
+import core.Types._
+import dotty.tools.dotc.core.Decorators._
+import dotty.tools.dotc.core.Flags
+import dotty.tools.dotc.core.Contexts.Context
+import dotty.tools.dotc.transform.TreeTransforms.{MiniPhaseTransform, TransformerInfo}
+import dotty.tools.dotc.util.Positions.Position
+
+/** Compiles the cases that can not be handled by primitive catch cases as a common pattern match.
+ *
+ *  The following code:
+ *    ```
+ *    try { <code> }
+ *    catch {
+ *      <tryCases> // Cases that can be handled by catch
+ *      <patternMatchCases> // Cases starting with first one that can't be handled by catch
+ *    }
+ *    ```
+ *  will become:
+ *    ```
+ *    try { <code> }
+ *    catch {
+ *      <tryCases>
+ *      case e => e match {
+ *        <patternMatchCases>
+ *      }
+ *    }
+ *    ```
+ *
+ *  Cases that are not supported include:
+ *   - Applies and unapplies
+ *   - Idents
+ *   - Alternatives
+ *   - `case _: T =>` where `T` is not `Throwable`
+ *
+ */
+class TryCatchPatterns extends MiniPhaseTransform {
+  import dotty.tools.dotc.ast.tpd._
+
+  def phaseName: String = "tryCatchPatterns"
+
+  override def runsAfter = Set(classOf[ElimRepeated])
+
+  override def checkPostCondition(tree: Tree)(implicit ctx: Context): Unit = tree match {
+    case Try(_, cases, _) =>
+      cases.foreach {
+        case CaseDef(Typed(_, _), guard, _) => assert(guard.isEmpty, "Try case should not contain a guard.")
+        case CaseDef(Bind(_, _), guard, _) => assert(guard.isEmpty, "Try case should not contain a guard.")
+        case c =>
+          assert(isDefaultCase(c), "Pattern in Try should be Bind, Typed or default case.")
+      }
+    case _ =>
+  }
+
+  override def transformTry(tree: Try)(implicit ctx: Context, info: TransformerInfo): Tree = {
+    val (tryCases, patternMatchCases) = tree.cases.span(isCatchCase)
+    val fallbackCase = mkFallbackPatterMatchCase(patternMatchCases, tree.pos)
+    cpy.Try(tree)(cases = tryCases ++ fallbackCase)
+  }
+
+  /** Is this pattern node a catch-all or type-test pattern? */
+  private def isCatchCase(cdef: CaseDef)(implicit ctx: Context): Boolean = cdef match {
+    case CaseDef(Typed(Ident(nme.WILDCARD), tpt), EmptyTree, _)          => isSimpleThrowable(tpt.tpe)
+    case CaseDef(Bind(_, Typed(Ident(nme.WILDCARD), tpt)), EmptyTree, _) => isSimpleThrowable(tpt.tpe)
+    case _                                                               => isDefaultCase(cdef)
+  }
+
+  private def isSimpleThrowable(tp: Type)(implicit ctx: Context): Boolean = tp match {
+    case tp @ TypeRef(pre, _) =>
+      (pre == NoPrefix || pre.widen.typeSymbol.isStatic) && // Does not require outer class check
+      !tp.symbol.is(Flags.Trait) && // Traits not supported by JVM
+      tp.derivesFrom(defn.ThrowableClass)
+    case _ =>
+      false
+  }
+
+  private def mkFallbackPatterMatchCase(patternMatchCases: List[CaseDef], pos: Position)(
+      implicit ctx: Context, info: TransformerInfo): Option[CaseDef] = {
+    if (patternMatchCases.isEmpty) None
+    else {
+      val exName = ctx.freshName("ex").toTermName
+      val fallbackSelector =
+        ctx.newSymbol(ctx.owner, exName, Flags.Synthetic | Flags.Case, defn.ThrowableType, coord = pos)
+      val sel = Ident(fallbackSelector.termRef).withPos(pos)
+      val rethrow = CaseDef(EmptyTree, EmptyTree, Throw(ref(fallbackSelector)))
+      Some(CaseDef(
+          Bind(fallbackSelector, Underscore(fallbackSelector.info).withPos(pos)),
+          EmptyTree,
+          transformFollowing(Match(sel, patternMatchCases ::: rethrow :: Nil)))
+      )
+    }
+  }
+
+}