SI-6514 Avoid spurious dead code warnings

`deadCode.expr` stores the method symbol most recently encountered
in `handleMonomorphicCall`, and uses this to avoid warnings
for arguments to label jumps and `Object#synchronized` (which
sneakily acts by-name without advertising the fact in its type.)

But this scheme was insufficient if the argument itself contains
another method call, such as `matchEnd(throw e(""))`.

This commit changes the single slot to a stack, and also
grants exemption to `LabelDef` trees. They were incorrectly
flagged in the enclosed test case after I made the the first change.

1 files changed, 3 insertions, 3 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index dc5491a509..ce5d3cba7a 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -3305,8 +3305,6 @@ trait Typers extends Modes with Adaptations with Tags {
                 // but behaves as if it were (=> T) => T) we need to know what is the actual
                 // target of a call.  Since this information is no longer available from
                 // typedArg, it is recorded here.
-                checkDead.updateExpr(fun)
-
                 val args1 =
                   // no expected type when jumping to a match label -- anything goes (this is ok since we're typing the translation of well-typed code)
                   // ... except during erasure: we must take the expected type into account as it drives the insertion of casts!
@@ -3361,7 +3359,9 @@ trait Typers extends Modes with Adaptations with Tags {
                 else
                   constfold(treeCopy.Apply(tree, fun, args1) setType ifPatternSkipFormals(restpe))
               }
-              handleMonomorphicCall
+              checkDead.updateExpr(fun) {
+                handleMonomorphicCall
+              }
             } else if (needsInstantiation(tparams, formals, args)) {
               //println("needs inst "+fun+" "+tparams+"/"+(tparams map (_.info)))
               inferExprInstance(fun, tparams)