SI-6387 Clones accessor before name expansion

When a symbol's name is expanded due to a conflict during
composition (e.g. multiple traits with same-named members, but
which are not both visible at the language level in the concrete
class) the compiler renames some symbols with expanded names which
embed the full name of the declaring class to avoid clashes.

In the rare cases when the accessor overrides the member in base
class, such expansion either results in AbstractMethodError when
the base method is abstract, or, even worse, can change the
semantics of the program.

To avoid such issues, we clone the accessor symbol, clear its
ACCESSOR flag and enter the symbol with an unchanged name.

1 files changed, 24 insertions, 11 deletions

diff --git a/src/reflect/scala/reflect/internal/Symbols.scala b/src/reflect/scala/reflect/internal/Symbols.scala
index 45c16b7302..7274eeafe0 100644
--- a/src/reflect/scala/reflect/internal/Symbols.scala
+++ b/src/reflect/scala/reflect/internal/Symbols.scala
@@ -2538,20 +2538,32 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
     }
 
     /** change name by appending $$<fully-qualified-name-of-class `base`>
-     *  Do the same for any accessed symbols or setters/getters
+     *  Do the same for any accessed symbols or setters/getters.
+     *  If the accessor to be renamed is overriding a base symbol, enter
+     *  a cloned symbol with the original name but without ACCESSOR flag.
      */
     override def expandName(base: Symbol) {
-      if (!hasFlag(EXPANDEDNAME)) {
-        setFlag(EXPANDEDNAME)
-        if (hasAccessorFlag && !isDeferred) {
-          accessed.expandName(base)
-        }
-        else if (hasGetter) {
-          getter(owner).expandName(base)
-          setter(owner).expandName(base)
-        }
-        name = nme.expandedName(name.toTermName, base)
+      def expand(sym: Symbol) {
+        if ((sym eq NoSymbol) || (sym hasFlag EXPANDEDNAME)) () // skip
+        else sym setFlag EXPANDEDNAME setName nme.expandedName(sym.name.toTermName, base)
+      }
+      def cloneAndExpand(accessor: Symbol) {
+        val clone = accessor.cloneSymbol(accessor.owner, (accessor.flags | ARTIFACT) & ~ACCESSOR)
+        expand(accessor)
+        log(s"Expanded overriding accessor to $accessor, but cloned $clone to preserve override")
+        accessor.owner.info.decls enter clone
+      }
+      def expandAccessor(accessor: Symbol) {
+        if (accessor.isOverridingSymbol) cloneAndExpand(accessor) else expand(accessor)
+      }
+      if (hasAccessorFlag && !isDeferred) {
+        expand(accessed)
+      }
+      else if (hasGetter) {
+        expandAccessor(getter(owner))
+        expandAccessor(setter(owner))
       }
+      expand(this)
     }
 
     protected def doCookJavaRawInfo() {
@@ -3223,6 +3235,7 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
     override def companionModule = NoSymbol
     override def companionSymbol = NoSymbol
     override def isSubClass(that: Symbol) = false
+    override def isOverridingSymbol = false
     override def filter(cond: Symbol => Boolean) = this
     override def defString: String = toString
     override def locationString: String = ""