[nomaster] SI-8114 Binary compat. workaround for erasure bug SI-7120

We can't backport SI-7120 to 2.10.x as it changes erased signatures,
which can lead to interop problems between 2.10.3 and 2.10.4.

But, we can detect one of the nasty symptoms -- a bridge method
with the same signature as its target -- and treat that.

This commit detects duplicate bridges in the ASM (only) backend
and removes them.

Author: retronym

src/compiler/scala/tools/nsc/backend/jvm/GenASM.scala

@@ -1397,13 +1397,38 @@ abstract class GenASM extends SubComponent with BytecodeWriters with GenJVMASM {
       }
 
       clasz.fields  foreach genField
-      clasz.methods foreach { im => genMethod(im, c.symbol.isInterface) }
+      clasz.methods foreach { im =>
+        if (im.symbol.isBridge && isRedundantBridge(im, clasz))
+          // We can't backport the erasure fix of SI-7120 to 2.10.x, but we can detect and delete
+          // bridge methods with identical signatures to their targets.
+          //
+          // NOTE: this backstop only implemented here in the ASM backend, and is not implemented in the FJBG backend.
+          debugwarn(s"Discarding redundant bridge method: ${im.symbol.debugLocationString}. See SI-8114.")
+        else 
+          genMethod(im, c.symbol.isInterface)
+      }
 
       addInnerClasses(clasz.symbol, jclass)
       jclass.visitEnd()
       writeIfNotTooBig("" + c.symbol.name, thisName, jclass, c.symbol)
     }
 
+    private def isRedundantBridge(bridge: IMethod, owner: IClass): Boolean = {
+      def lastCalledMethod: Option[Symbol] = bridge.code.instructions.reverseIterator.collectFirst {
+        case CALL_METHOD(meth, _) => meth
+      }
+      def hasSameSignatureAsBridge(targetMethod: Symbol): Boolean = {
+        val targetIMethod = clasz.methods find (m => m.symbol == targetMethod)
+        // Important to compare the IMethod#paramss, rather then the erased MethodTypes, as
+        // due to the bug SI-7120, these are out of sync. For example, in the `applyOrElse`
+        // method in run/t8114.scala, the method symbol info has a parameter of type `Long`,
+        // but the IMethod parameter has type `Object`. The latter comes from the info of the
+        // symbol representing the parameter ValDef in the tree, which is incorrectly erased.
+        targetIMethod exists (m => bridge.matchesSignature(m))
+      }
+      lastCalledMethod exists hasSameSignatureAsBridge
+    }
+
     /**
      * @param owner internal name of the enclosing class of the class.
      *

test/files/run/t8114.scala

@@ -0,0 +1,15 @@
+class AbstractTable[T] { type TableElementType }
+class Table[T] extends AbstractTable[T] { type TableElementType = T }
+ 
+class Query[E, U]
+class TableQuery[E <: AbstractTable[_]] extends Query[E, E#TableElementType]
+ 
+object Test extends App {
+  object MyTable extends TableQuery[Table[Long]]
+ 
+  def list[R](q: Query[_, R]): List[R] = Nil
+  list/*[Long]*/(MyTable) collect { case x => x }
+
+  // Generates a redundant bridge method (double definition error)
+  // in 2.10.x due to (at least) the bug in erasure fixed in SI-7120
+}