SI-6863 Fix verify error in captured var inited from expr with try/catch

If a captured var was inited from a try/catch we did something
reasonable. But if the var was inited from a more complicated expression
(if/else, a block, match/case, etc) that ended with
a try/catch then we didn't and we were generating faulty byte code.
This fix patches LambdaLift to add the missing cases.

For known simple expressions, the translation is just new *Ref(expr).

For try/catch, if/else, match/case, and blocks this recursively
walks down the internal result expressions to translate them. E.g.
if(cond) trueExpr else falseExpr becomes if(cone) translate(trueExpr)
else translate(falseExpr)

For unknown expression types, the translation is {val temp = expr; new
  *Ref(expr) }

Author: JamesIry

src/compiler/scala/tools/nsc/transform/LambdaLift.scala

@@ -451,20 +451,45 @@ abstract class LambdaLift extends InfoTransform {
                 }
               case arg => arg
             }
-            /** Wrap expr argument in new *Ref(..) constructor, but make
-             *  sure that Try expressions stay at toplevel.
+            
+            /** Wrap expr argument in new *Ref(..) constructor. But try/catch
+             * is a problem because a throw will clear the stack and post catch
+             * we would expect the partially-constructed object to be on the stack
+             * for the call to init. So we recursively
+             * search for "leaf" result expressions where we know its safe
+             * to put the new *Ref(..) constructor or, if all else fails, transform
+             * an expr to { val temp=expr; new *Ref(temp) }.
+             * The reason we narrowly look for try/catch in captured var definitions 
+             * is because other try/catch expression have already been lifted
+             * see SI-6863
              */
-            def refConstr(expr: Tree): Tree = expr match {
+            def refConstr(expr: Tree): Tree = typer.typedPos(expr.pos) {expr match {
+              // very simple expressions can be wrapped in a new *Ref(expr) because they can't have
+              // a try/catch in final expression position.
+              case Ident(_) | Apply(_, _) | Literal(_) | New(_) | Select(_, _) | Throw(_) | Assign(_, _) | ValDef(_, _, _, _) | Return(_) | EmptyTree =>
+                New(sym.tpe, expr)
               case Try(block, catches, finalizer) =>
                 Try(refConstr(block), catches map refConstrCase, finalizer)
+              case Block(stats, expr) =>
+                Block(stats, refConstr(expr))
+              case If(cond, trueBranch, falseBranch) =>
+                If(cond, refConstr(trueBranch), refConstr(falseBranch))
+              case Match(selector, cases) =>
+                Match(selector, cases map refConstrCase)
+              // if we can't figure out what else to do, turn expr into {val temp1 = expr; new *Ref(temp1)} to avoid
+              // any possibility of try/catch in the *Ref constructor. This should be a safe tranformation as a default
+              // though it potentially wastes a variable slot. In particular this case handles LabelDefs.
               case _ =>
-                New(sym.tpe, expr)
-            }
+                debuglog("assigning expr to temp: " + (expr.pos))
+                val tempSym = currentOwner.newValue(unit.freshTermName("temp"), expr.pos) setInfo expr.tpe
+                val tempDef = ValDef(tempSym, expr) setPos expr.pos
+                val tempRef = Ident(tempSym) setPos expr.pos 
+                Block(tempDef, New(sym.tpe, tempRef))
+            }}
             def refConstrCase(cdef: CaseDef): CaseDef =
               CaseDef(cdef.pat, cdef.guard, refConstr(cdef.body))
-            treeCopy.ValDef(tree, mods, name, tpt1, typer.typedPos(rhs.pos) {
-              refConstr(constructorArg)
-            })
+
+            treeCopy.ValDef(tree, mods, name, tpt1, refConstr(constructorArg))
           } else tree
         case Return(Block(stats, value)) =>
           Block(stats, treeCopy.Return(tree, value)) setType tree.tpe setPos tree.pos

src/compiler/scala/tools/nsc/transform/UnCurry.scala

@@ -603,8 +603,6 @@ abstract class UnCurry extends InfoTransform
             }
           case ValDef(_, _, _, rhs) =>
             if (sym eq NoSymbol) throw new IllegalStateException("Encountered Valdef without symbol: "+ tree + " in "+ unit)
-            // a local variable that is mutable and free somewhere later should be lifted
-            // as lambda lifting (coming later) will wrap 'rhs' in an Ref object.
             if (!sym.owner.isSourceMethod)
               withNeedLift(true) { super.transform(tree) }
             else

test/files/run/t6863.scala

@@ -0,0 +1,114 @@
+/** Make sure that when a variable is captured its initialization expression is handled properly */
+object Test {
+  def lazyVal() = {
+  	// internally lazy vals become vars which are initialized with "_", so they need to be tested just like vars do
+  	lazy val x = "42"
+    assert({ () => x }.apply == "42")    
+  }
+  def ident() = {
+    val y = "42"
+    var x = y
+    assert({ () => x }.apply == "42")    
+  }
+  def apply() = {
+    def y(x : Int) = x.toString
+    var x = y(42)
+    assert({ () => x }.apply == "42")        
+  }
+  def literal() = {
+    var x = "42"
+    assert({ () => x }.apply == "42")    
+  }
+  def `new`() = {
+    var x = new String("42")
+    assert({ () => x }.apply == "42")            
+  }
+  def select() = {
+    object Foo{val bar = "42"}
+    var x = Foo.bar
+    assert({ () => x }.apply == "42")            
+  }
+  def `throw`() = {
+    var x = if (true) "42" else throw new Exception("42")
+    assert({ () => x }.apply == "42")            
+  }
+  def assign() = {
+    var y = 1
+    var x = y = 42
+    assert({ () => x}.apply == ())
+  }
+  def valDef() = {
+    var x = {val y = 42}
+    assert({ () => x}.apply == ())
+  }
+  def `return`(): String = {
+    var x = if (true) return "42" else ()
+    assert({ () => x}.apply == ())
+    "42"
+  }
+  def tryFinally() = {
+    var x = try { "42" } finally ()
+    assert({ () => x }.apply == "42")
+  }
+  def tryCatch() = {
+    var x = try { "42" } catch { case _ => "43" }
+    assert({ () => x }.apply == "42")
+  }
+  def `if`() = {
+  	var x = if (true) ()
+    assert({ () => x }.apply == ())
+  }
+  def ifElse() = {
+    var x = if(true) "42" else "43" 
+    assert({ () => x }.apply == "42")
+  }
+  def matchCase() = {
+    var x = 100 match {
+       case 100 => "42"
+      case _ => "43"
+    }
+    assert({ () => x }.apply == "42")
+  }
+  def block() = {
+    var x = {
+      val y = 42
+      "42"
+    }
+    assert({ () => x }.apply == "42")
+  }
+  def labelDef() = {
+    var x = 100 match { 
+      case 100 => try "42" finally ()
+    }
+    assert({ () => x }.apply == "42")
+  }
+  def nested() = {
+    var x = {
+      val y = 42
+        if(true) try "42" catch {case _ => "43"}
+        else "44"       
+    }
+    assert({ () => x }.apply == "42")
+  }
+  def main(args: Array[String]) {
+  	lazyVal()
+    ident()
+    apply()
+    literal()
+    `new`()
+    select()
+    `throw`()
+    assign()
+    valDef()
+    `return`()
+    tryFinally()
+    tryCatch()
+    ifElse()
+    `if`()
+    matchCase()
+    block()
+    labelDef()
+    nested()
+  }
+}
+