[backport] SI-7756 Uncripple refchecks in case bodies

In 65340ed, parts of RefChecks were disabled when
we traversed into the results of the new pattern matcher.
Similar logic existed for the old pattern matcher, but in
that case the Match / CaseDef nodes still existed in the tree.

The new approach was too broad: important checks no longer
scrutinized the body of cases.

This commit turns the checks back on when it finds the remnants
of a case body, which appears as an application to a label def.

Conflicts:
	src/compiler/scala/tools/nsc/typechecker/RefChecks.scala

Cherry pick of 3df1d77

Author: retronym

src/compiler/scala/tools/nsc/typechecker/RefChecks.scala

@@ -98,6 +98,11 @@ abstract class RefChecks extends InfoTransform with scala.reflect.internal.trans
     var localTyper: analyzer.Typer = typer;
     var currentApplication: Tree = EmptyTree
     var inPattern: Boolean = false
+    @inline final def savingInPattern[A](body: => A): A = {
+      val saved = inPattern
+      try body finally inPattern = saved
+    }
+
     var checkedCombinations = Set[List[Type]]()
 
     // only one overloaded alternative is allowed to define default arguments
@@ -1822,19 +1827,33 @@ abstract class RefChecks extends InfoTransform with scala.reflect.internal.trans
 
           case _ => tree
         }
+
         // skip refchecks in patterns....
         result = result match {
           case CaseDef(pat, guard, body) =>
-            inPattern = true
-            val pat1 = transform(pat)
-            inPattern = false
+            val pat1 = savingInPattern {
+              inPattern = true
+              transform(pat)
+            }
             treeCopy.CaseDef(tree, pat1, transform(guard), transform(body))
           case LabelDef(_, _, _) if treeInfo.hasSynthCaseSymbol(result) =>
-            val old = inPattern
-            inPattern = true
-            val res = deriveLabelDef(result)(transform) // TODO SI-7756 Too broad! The code from the original case body should be fully refchecked!
-            inPattern = old
-            res
+            savingInPattern {
+              inPattern = true
+              deriveLabelDef(result)(transform)
+            }
+          case Apply(fun, args) if fun.symbol.isLabel && treeInfo.isSynthCaseSymbol(fun.symbol) =>
+            savingInPattern {
+              // SI-7756 If we were in a translated pattern, we can now switch out of pattern mode, as the label apply signals
+              //         that we are in the user-supplied code in the case body.
+              //
+              //         Relies on the translation of:
+              //            (null: Any) match { case x: List[_] => x; x.reverse; case _ => }'
+              //         to:
+              //            <synthetic> val x2: List[_] = (x1.asInstanceOf[List[_]]: List[_]);
+              //                  matchEnd4({ x2; x2.reverse}) // case body is an argument to a label apply.
+              if (settings.future.value) inPattern = false // By default, bug compatibility with 2.10.[0-4]
+              super.transform(result)
+            }
           case ValDef(_, _, _, _) if treeInfo.hasSynthCaseSymbol(result) =>
             deriveValDef(result)(transform) // SI-7716 Don't refcheck the tpt of the synthetic val that holds the selector.
           case _ =>

test/files/neg/t7756a.check

@@ -0,0 +1,7 @@
+t7756a.scala:7: error: type arguments [Object] do not conform to trait TA's type parameter bounds [X <: CharSequence]
+        locally(null: TA[Object])
+        ^
+t7756a.scala:7: error: type arguments [Object] do not conform to trait TA's type parameter bounds [X <: CharSequence]
+        locally(null: TA[Object])
+                      ^
+two errors found

test/files/neg/t7756a.flags

@@ -0,0 +1 @@
+-Xfuture

test/files/neg/t7756a.scala

@@ -0,0 +1,11 @@
+object Test {
+  def test: Unit = {
+    trait TA[X <: CharSequence]
+    0 match {
+      case _ =>
+        // the bounds violation isn't reported. RefChecks seems to be too broadly disabled under virtpatmat: see 65340ed4ad2e
+        locally(null: TA[Object])
+        ()
+    }
+  }
+}

test/files/neg/t7756b.check

@@ -0,0 +1,4 @@
+t7756b.scala:3: error: comparing values of types Int and String using `==' will always yield false
+    case _ => 0 == ""
+                ^
+one error found

test/files/neg/t7756b.flags

@@ -0,0 +1 @@
+-Xfuture -Xfatal-warnings

test/files/neg/t7756b.scala

@@ -0,0 +1,5 @@
+object Test {
+  0 match {
+    case _ => 0 == ""
+  }
+}

test/files/pos/t7756b.scala

@@ -0,0 +1,13 @@
+// This is a pos test to show that the backported bug fix for SI-7756 is
+// only enabled under -Xfuture.
+object Test {
+  def test: Unit = {
+    trait TA[X <: CharSequence]
+    0 match {
+      case _ =>
+        // the bounds violation isn't reported. RefChecks seems to be too broadly disabled under virtpatmat: see 65340ed4ad2e
+        locally(null: TA[Object])
+        ()
+    }
+  }
+}