Re-establish missing symbol from previous phase (in one specific case)

Fixes #15158

Author: odersky

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

@@ -695,7 +695,19 @@ object Erasure {
         return tree.asInstanceOf[Tree] // we are re-typing a primitive array op
 
       val owner = mapOwner(origSym)
-      val sym = if (owner eq origSym.maybeOwner) origSym else owner.info.decl(tree.name).symbol
+      var sym = if (owner eq origSym.maybeOwner) origSym else owner.info.decl(tree.name).symbol
+      if !sym.exists then
+        // We fail the sym.exists test for pos/i15158.scala, where we pass an infinitely
+        // recurring match type to an overloaded constructor. An equivalent test
+        // with regular apply methods succeeds. It's at present unclear whether
+        //  - the program should be rejected, or
+        //  - there is another fix.
+        // Therefore, we apply the fix to use the pre-erasure symbol, but only
+        // for constructors, in order not to mask other possible bugs that would
+        // trigger the assert(sym.exists, ...) below.
+        val prevSym = tree.symbol(using preErasureCtx)
+        if prevSym.isConstructor then sym = prevSym
+
       assert(sym.exists, i"no owner from $owner/${origSym.showLocated} in $tree")
 
       if owner == defn.ObjectClass then checkValue(qual1)

compiler/test/dotc/pos-test-pickling.blacklist

@@ -42,6 +42,7 @@ i9999.scala
 13491.scala
 7512.scala
 i6505.scala
+i15158.scala
 
 # Opaque type
 i5720.scala

tests/pos/i15158.scala

@@ -0,0 +1,43 @@
+class Opt[T]
+
+class Buf[A](elts: Any, sz: Int):
+  def this(n: Int) = this(???, n)
+
+object Buf:
+  def apply[A](elts: Any, sz: Int): Buf[A] = new Buf[A](elts, sz)
+  def apply[A](n: Int): Buf[A] = apply[A](???, n)
+
+inline def foo(testFun: Any): Unit = {}
+
+val x = foo {
+  type Rec[A] = A match
+    case String => Opt[Rec[String]]
+
+  val arr = new Buf[Rec[String]](8)
+  val arr2 = Buf[Rec[String]](8)
+  val arr3 = Buf.apply[Rec[String]](8)
+}
+
+import scala.collection.mutable
+
+// https://github.com/plokhotnyuk/jsoniter-scala/blob/74d6d557bf81e904d07d4b8fbead4e4cab700bea/jsoniter-scala-macros/shared/src/test/scala-3/com/github/plokhotnyuk/jsoniter_scala/macros/JsonCodeMakerNewTypeSpec.scala#L40-L148
+class Spec {
+  inline def in(testFun: => Any): Unit = {
+    val _ = testFun
+  }
+
+  in {
+    type JsonPrimitive = String | Int
+    type Rec[JA[_], A] = A match {
+      case JsonPrimitive => JsonPrimitive | JA[Rec[JA, JsonPrimitive]]
+      case _ => A | JA[Rec[JA, A]]
+    }
+
+    type Json = Rec[
+      [A] =>> Seq[A],
+      JsonPrimitive
+    ]
+
+    val arr = new mutable.ArrayBuffer[Json](8)
+  }
+}