Backport "Preserve singleton types for final val aliases to Java enum constants" to 3.8.2 (#25190)

Backports #24980 to the 3.8.2-RC2.

PR submitted by the release tooling.
[skip ci]

Author: WojciechMazur

compiler/src/dotty/tools/backend/jvm/BCodeHelpers.scala

@@ -297,6 +297,17 @@ trait BCodeHelpers extends BCodeIdiomatic {
           val edesc = innerClasesStore.typeDescriptor(t.tpe) // the class descriptor of the enumeration class.
           val evalue = t.symbol.javaSimpleName // value the actual enumeration value.
           av.visitEnum(name, edesc, evalue)
+        // Handle final val aliases to Java enum values.
+        // Check if the symbol's pre-erasure type was a singleton of a Java enum value.
+        case t: tpd.RefTree if atPhase(erasurePhase) {
+          t.symbol.info.finalResultType match
+            case tr: TermRef => tr.termSymbol.owner.linkedClass.isAllOf(JavaEnum)
+            case _ => false
+        } =>
+          val enumRef = atPhase(erasurePhase)(t.symbol.info.finalResultType.asInstanceOf[TermRef])
+          val edesc = innerClasesStore.typeDescriptor(enumRef)
+          val evalue = enumRef.termSymbol.javaSimpleName
+          av.visitEnum(name, edesc, evalue)
         case t: SeqLiteral =>
           val arrAnnotV: AnnotationVisitor = av.visitArray(name)
           for (arg <- t.elems) { emitArgument(arrAnnotV, null, arg, bcodeStore)(innerClasesStore) }

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

@@ -2221,10 +2221,15 @@ class Namer { typer: Typer =>
         // `default-getter-variance.scala`.
         AnnotatedType(defaultTp, Annotation(defn.UncheckedVarianceAnnot, sym.span))
       else
+        inline def isJavaEnumValue = tp match
+          case tp: TermRef => tp.termSymbol.isAllOf(JavaDefined | Enum)
+          case _ => false
         // don't strip @uncheckedVariance annot for default getters
         TypeOps.simplify(tp.widenTermRefExpr,
             if defaultTp.exists then TypeOps.SimplifyKeepUnchecked() else null)
         match
+          // For final members pointing to Java enum values, preserve the singleton type. See i24750.
+          case _ if sym.is(Final) && isJavaEnumValue => tp
           case ctp: ConstantType if sym.isInlineVal => ctp
           case tp if isTracked => tp
           case tp => TypeComparer.widenInferred(tp, pt, Widen.Unions)

tests/pos/i24750.scala

@@ -0,0 +1,14 @@
+//> using options -Werror
+import scala.concurrent.duration.*
+
+object i24750:
+
+  def f(duration: FiniteDuration): Unit =
+    duration.unit match
+      case NANOSECONDS =>
+      case MICROSECONDS =>
+      case MILLISECONDS =>
+      case SECONDS =>
+      case MINUTES =>
+      case HOURS =>
+      case DAYS =>

tests/semanticdb/metac.expect

@@ -5650,7 +5650,7 @@ types/Test.Literal.clazzOf. => final val method clazzOf Option[Int]
 types/Test.Literal.double. => final val method double 2.0
 types/Test.Literal.float. => final val method float 1.0f
 types/Test.Literal.int. => final val method int 1
-types/Test.Literal.javaEnum. => final val method javaEnum LinkOption
+types/Test.Literal.javaEnum. => final val method javaEnum NOFOLLOW_LINKS.type
 types/Test.Literal.long. => final val method long 1L
 types/Test.Literal.nil. => final val method nil Null
 types/Test.Literal.string. => final val method string "a"

tests/warn/i24750.check

@@ -0,0 +1,8 @@
+-- [E029] Pattern Match Exhaustivity Warning: tests/warn/i24750.scala:6:13 ---------------------------------------------
+6 |    duration.unit match // warn
+  |    ^^^^^^^^^^^^^
+  |    match may not be exhaustive.
+  |
+  |    It would fail on pattern case: DAYS
+  |
+  | longer explanation available when compiling with `-explain`

tests/warn/i24750.scala

@@ -0,0 +1,12 @@
+import scala.concurrent.duration.*
+
+object i24750:
+
+  def f(duration: FiniteDuration): Unit =
+    duration.unit match // warn
+      case NANOSECONDS =>
+      case MICROSECONDS =>
+      case MILLISECONDS =>
+      case SECONDS =>
+      case MINUTES =>
+      case HOURS =>