Fix #5212: Fix bridge generation for implicit shortcuts

compiler/src/dotty/tools/dotc/transform/Bridges.scala

@@ -119,7 +119,7 @@ class Bridges(root: ClassSymbol, thisPhase: DenotTransformer)(implicit ctx: Cont
       if (!opc.overriding.is(Deferred)) {
         addBridgeIfNeeded(opc.overriding, opc.overridden)
 
-        if (needsImplicitShortcut(opc.overriding)(ectx))
+        if (needsImplicitShortcut(opc.overriding)(ectx) && needsImplicitShortcut(opc.overridden)(ectx))
           // implicit shortcuts do not show up in the Bridges cursor, since they
           // are created only when referenced. Therefore we need to generate a bridge
           // for them specifically, if one is needed for the original methods.

tests/run/implicit-shortcut-bridge.scala

@@ -0,0 +1,45 @@
+abstract class A[T] {
+  def foo: T
+}
+class B extends A[implicit Int => Int] {
+  // No bridge needed for foo$direct
+  def foo: implicit Int => Int = 1
+}
+
+abstract class X[T] extends A[implicit T => T] {
+  def foo: implicit T => T
+}
+
+class Y extends X[Int] {
+  def foo: implicit Int => Int = 1
+}
+
+object Test {
+  def check(expected: Set[String], cls: Class[_]): Unit = {
+    val actual = cls.getMethods.filter(_.getName.startsWith("foo")).map(_.toString).toSet
+    assert(expected == actual, s"[$cls] expected: ${expected}\nactual: $actual")
+  }
+
+  def main(args: Array[String]): Unit = {
+    val expectedB = Set(
+      "public scala.Function1 B.foo()", // user-written method
+      "public int B.foo$direct(int)", // shortcut added by ShortcutImplicits
+      "public java.lang.Object B.foo()" // bridge to A#foo
+    )
+    val expectedX = Set(
+      "public abstract scala.Function1 X.foo()", // user-written method
+      "public abstract java.lang.Object X.foo$direct(java.lang.Object)", // shortcut
+      "public abstract java.lang.Object A.foo()" // Inherited from A
+    )
+    val expectedY = Set(
+      "public scala.Function1 Y.foo()", // user-written method
+      "public java.lang.Object Y.foo()", // Bridge to A#foo
+      "public int Y.foo$direct(int)", // shortcut
+      "public java.lang.Object Y.foo$direct(java.lang.Object)", // bridge to X#foo$direct
+    )
+
+    check(expectedB, classOf[B])
+    check(expectedX, classOf[X[_]])
+    check(expectedY, classOf[Y])
+  }
+}