Fix #25555: Preserve singleton precision during scoverage lifting

compiler/src/dotty/tools/dotc/typer/EtaExpansion.scala

@@ -48,12 +48,16 @@ abstract class Lifter {
     // Mark the type of lifted definitions as inferred
     ValDef(sym, rhs, inferred = true)
 
+  /** Type assigned to a lifted temporary symbol. */
+  protected def liftedExprType(expr: Tree)(using Context): Type =
+    expr.tpe.widen.deskolemized
+
   private def lift(defs: mutable.ListBuffer[Tree], expr: Tree, prefix: TermName = EmptyTermName)(using Context): Tree =
     if (noLift(expr)) expr
     else {
       val name = UniqueName.fresh(prefix)
       // don't instantiate here, as the type params could be further constrained, see tests/pos/pickleinf.scala
-      var liftedType = expr.tpe.widen.deskolemized
+      var liftedType = liftedExprType(expr)
       if (liftedFlags.is(Method)) liftedType = ExprType(liftedType)
       val lifted = newSymbol(ctx.owner, name, liftedFlags | Synthetic, liftedType, coord = spanCoord(expr.span),
         // Lifted definitions will be added to a local block, so they need to be
@@ -213,6 +217,16 @@ object LiftCoverage extends LiftImpure {
   override def noLift(expr: tpd.Tree)(using Context) =
     if liftingArgs then noLiftArg(expr) else super.noLift(expr)
 
+  /** Preserve singleton precision for lifted coverage temps when the underlying value is a
+   *  compile-time constant (same notion ConstFold uses), so constant re-folding after lifting
+   *  still matches the original inferred singleton type. Everything else uses the base widen.
+   */
+  override protected def liftedExprType(expr: tpd.Tree)(using Context): Type =
+    val dealiased = expr.tpe.dealias.deskolemized
+    dealiased.widenTermRefExpr.normalized.simplified match
+      case _: ConstantType => dealiased
+      case _ => super.liftedExprType(expr)
+
   def liftForCoverage(defs: mutable.ListBuffer[tpd.Tree], tree: tpd.Apply)(using Context) = {
     val liftedFun = liftApp(defs, tree.fun)
     val liftedArgs = liftArgs(defs, tree.fun.tpe, tree.args)(using liftingArgsContext)

compiler/test/dotc/scoverage-ignore.excludelist

@@ -73,7 +73,6 @@ i9228.scala
 i9880.scala
 infersingle.scala
 interop-unsound-src
-java-collectors.scala
 large2.scala
 match-single-sub-case.scala
 match-sub-cases.scala

tests/coverage/pos/SingletonLiftCoverage.scala

@@ -0,0 +1,19 @@
+/** Coverage test: singleton precision preserved when lifting for coverage.
+  *
+  * Regression test for the GenBCodeOps crash (Found: Int, Required: (9 : Int)).
+  * Both examples trigger LiftCoverage to lift Apply args; the fix preserves
+  * singleton types so Ycheck passes.
+  */
+package covtest
+
+object Ops:
+  final val PUBLIC = 1
+  final val STATIC = 8
+
+object SingletonLiftMinimal:
+  // + 0 forces the arg to be an Apply (lifted by coverage), producing Ops.PUBLIC | x$1
+  final val PublicStatic = Ops.PUBLIC | (Ops.STATIC + 0)
+
+object SingletonLiftJavaReflect:
+  final val PublicStatic =
+    java.lang.reflect.Modifier.PUBLIC | java.lang.reflect.Modifier.STATIC

tests/coverage/pos/SingletonLiftCoverage.scoverage.check

@@ -0,0 +1,105 @@
+# Coverage data, format version: 3.0
+# Statement data:
+# - id
+# - source path
+# - package name
+# - class name
+# - class type (Class, Object or Trait)
+# - full class name
+# - method name
+# - start offset
+# - end offset
+# - line number
+# - symbol name
+# - tree name
+# - is branch
+# - invocations count
+# - is ignored
+# - description (can be multi-line)
+# '' sign
+# ------------------------------------------
+0
+SingletonLiftCoverage.scala
+covtest
+Ops
+Object
+covtest.Ops
+<init>
+331
+332
+10
+<none>
+Literal
+false
+0
+false
+1
+
+1
+SingletonLiftCoverage.scala
+covtest
+Ops
+Object
+covtest.Ops
+<init>
+354
+355
+11
+<none>
+Literal
+false
+0
+false
+8
+
+2
+SingletonLiftCoverage.scala
+covtest
+SingletonLiftMinimal
+Object
+covtest.SingletonLiftMinimal
+<init>
+501
+529
+15
+|
+Apply
+false
+0
+false
+Ops.PUBLIC | (Ops.STATIC + 0
+
+3
+SingletonLiftCoverage.scala
+covtest
+SingletonLiftMinimal
+Object
+covtest.SingletonLiftMinimal
+<init>
+515
+529
+15
++
+Apply
+false
+0
+false
+Ops.STATIC + 0
+
+4
+SingletonLiftCoverage.scala
+covtest
+SingletonLiftJavaReflect
+Object
+covtest.SingletonLiftJavaReflect
+<init>
+596
+665
+19
+|
+Apply
+false
+0
+false
+java.lang.reflect.Modifier.PUBLIC | java.lang.reflect.Modifier.STATIC
+