SI-7433 Fix spurious warning about catching control throwable

In the same vein as SI-6994, we have to be careful not to warn
about synthetic code. In that case, the spurious warnings came
because we warned in the typechecker, which was also called in
erasure. In this case, we are issuing the warning in Uncurry,
so we must be mindful of the pattern matchers translations of
non-trivial catch patterns, which look like:

    case (ex8 @ _) => {
    <synthetic> val x5: Throwable = ex8;
    case11(){
      if ({
        case14(){
          if (x5.$isInstanceOf[NullPointerException]())
            matchEnd13(true)
          else
            case15()
        };
        case15(){
          if (x5.$isInstanceOf[RuntimeException]())
            matchEnd13(true)
          else
            case16()
        };
        case16(){
          matchEnd13(false)
        };
        matchEnd13(x: Boolean){
          x
        }
      })

This commit detects that `ex8` is synthetic and disables the warning.

4 files changed, 24 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/UnCurry.scala b/src/compiler/scala/tools/nsc/transform/UnCurry.scala
index 64f4e579e1..e2ce2743f7 100644
--- a/src/compiler/scala/tools/nsc/transform/UnCurry.scala
+++ b/src/compiler/scala/tools/nsc/transform/UnCurry.scala
@@ -187,8 +187,14 @@ abstract class UnCurry extends InfoTransform
         val keyDef   = ValDef(key, New(ObjectTpe))
         val tryCatch = Try(body, pat -> rhs)
 
-        for (Try(t, catches, _) <- body ; cdef <- catches ; if treeInfo catchesThrowable cdef)
+        import treeInfo.{catchesThrowable, isSyntheticCase}
+        for {
+          Try(t, catches, _) <- body
+          cdef <- catches
+          if catchesThrowable(cdef) && !isSyntheticCase(cdef)
+        } {
           unit.warning(body.pos, "catch block may intercept non-local return from " + meth)
+        }
 
         Block(List(keyDef), tryCatch)
       }
diff --git a/src/reflect/scala/reflect/internal/TreeInfo.scala b/src/reflect/scala/reflect/internal/TreeInfo.scala
index 3a8d3fd460..30ec555bc5 100644
--- a/src/reflect/scala/reflect/internal/TreeInfo.scala
+++ b/src/reflect/scala/reflect/internal/TreeInfo.scala
@@ -553,6 +553,12 @@ abstract class TreeInfo {
     })
   )
 
+  /** Is this CaseDef synthetically generated, e.g. by `MatchTranslation.translateTry`? */
+  def isSyntheticCase(cdef: CaseDef) = cdef.pat.exists {
+    case dt: DefTree => dt.symbol.isSynthetic
+    case _           => false
+  }
+
   /** 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, _) =>
diff --git a/test/files/pos/t7433.flags b/test/files/pos/t7433.flags
new file mode 100644
index 0000000000..e8fb65d50c
--- /dev/null
+++ b/test/files/pos/t7433.flags
@@ -0,0 +1 @@
+-Xfatal-warnings
\ No newline at end of file
diff --git a/test/files/pos/t7433.scala b/test/files/pos/t7433.scala
new file mode 100644
index 0000000000..f2109f4afa
--- /dev/null
+++ b/test/files/pos/t7433.scala
@@ -0,0 +1,10 @@
+object Test {
+  def foo() {
+    try {
+      for (i <- 1 until 5) return
+    } catch {
+      case _: NullPointerException | _: RuntimeException =>
+        // was: "catch block may intercept non-local return from method check"
+    }
+  }
+}