SI-9630 Fix spurious warning related to same-named case accessors [backport]

Hash consing of trees within pattern match analysis was broken, and
considered `x1.foo#1` to be the same tree as `x1.foo#2`, even though
the two `foo`-s referred to different symbols.

The hash consing was based on `Tree#correspondsStructure`, but the
predicate in that function cannot veto correspondance, it can only
supplement the default structural comparison.

I've instead created a custom tree comparison method for use in
the pattern matcher that handles the tree shapes that we use.

(cherry picked from commit 79a52e6807d2797dee12bab1730765441a0e222d)

2 files changed, 11 insertions, 7 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/patmat/Logic.scala b/src/compiler/scala/tools/nsc/transform/patmat/Logic.scala
index 91777b34d5..40fcceb0bf 100644
--- a/src/compiler/scala/tools/nsc/transform/patmat/Logic.scala
+++ b/src/compiler/scala/tools/nsc/transform/patmat/Logic.scala
@@ -691,7 +691,7 @@ trait ScalaLogic extends Interface with Logic with TreeAndTypeAnalysis {
           // if X is mutable.
           freshExistentialSubtype(t.tpe)
         }
-        else trees find (a => a.correspondsStructure(t)(sameValue)) match {
+        else trees find (a => equivalentTree(a, t)) match {
           case Some(orig) =>
             debug.patmat("unique tp for tree: " + ((orig, orig.tpe)))
             orig.tpe
diff --git a/src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala b/src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala
index 1331eb6993..c71299b893 100644
--- a/src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala
+++ b/src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala
@@ -85,11 +85,15 @@ trait TreeAndTypeAnalysis extends Debugging {
     tp <:< tpImpliedNormalizedToAny
   }
 
-  // TODO: improve, e.g., for constants
-  def sameValue(a: Tree, b: Tree): Boolean = (a eq b) || ((a, b) match {
-    case (_ : Ident, _ : Ident) => a.symbol eq b.symbol
-    case _                      => false
-  })
+  def equivalentTree(a: Tree, b: Tree): Boolean = (a, b) match {
+    case (Select(qual1, _), Select(qual2, _)) => equivalentTree(qual1, qual2) && a.symbol == b.symbol
+    case (Ident(_), Ident(_)) => a.symbol == b.symbol
+    case (Literal(c1), Literal(c2)) => c1 == c2
+    case (This(_), This(_)) => a.symbol == b.symbol
+    case (Apply(fun1, args1), Apply(fun2, args2)) => equivalentTree(fun1, fun2) && args1.corresponds(args2)(equivalentTree)
+    // Those are the only cases we need to handle in the pattern matcher
+    case _ => false
+  }
 
   trait CheckableTreeAndTypeAnalysis {
     val typer: Typer
@@ -277,7 +281,7 @@ trait MatchApproximation extends TreeAndTypeAnalysis with ScalaLogic with MatchT
 
       // hashconsing trees (modulo value-equality)
       def unique(t: Tree, tpOverride: Type = NoType): Tree =
-        trees find (a => a.correspondsStructure(t)(sameValue)) match {
+        trees find (a => equivalentTree(a, t)) match {
           case Some(orig) =>
             // debug.patmat("unique: "+ (t eq orig, orig))
             orig