Error instead of crash when generating trees referring to skolems

Previously, the valueOf inline call succeeded (because the ValueOf synthesizer
calls `tpd.ref` which calls `tpd.singleton`), leading to an invalid tree which
crashed in the backend with: "assertion failed: Cannot create ClassBType from
NoSymbol".

Fixed by throwing a TypeError from `tpd.singleton`, unfortunatley this means
tests/neg/i8623.scala gets a worse error message because the implicit search
fails early, but arguably this is more correct.

Author: smarter

compiler/src/dotty/tools/dotc/ast/tpd.scala

@@ -517,9 +517,10 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
   def singleton(tp: Type, needLoad: Boolean = true)(using Context): Tree = tp.dealias match {
     case tp: TermRef => ref(tp, needLoad)
     case tp: ThisType => This(tp.cls)
-    case tp: SkolemType => singleton(tp.narrow(), needLoad)
     case SuperType(qual, _) => singleton(qual, needLoad)
     case ConstantType(value) => Literal(value)
+    case tp: SkolemType =>
+      throw TypeError(em"cannot construct a tree referring to skolem $tp")
   }
 
   /** A tree representing a `newXYZArray` operation of the right

compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -155,7 +155,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case UnknownNamedEnclosingClassOrObjectID // errorNumber: 139
   case IllegalCyclicTypeReferenceID // errorNumber: 140
   case MissingTypeParameterInTypeAppID // errorNumber: 141
-  case SkolemInInferredID // errorNumber: 142
+  case SkolemInInferredID extends ErrorMessageID(isActive = false) // errorNumber: 142
   case ErasedTypesCanOnlyBeFunctionTypesID // errorNumber: 143
   case CaseClassMissingNonImplicitParamListID // errorNumber: 144
   case EnumerationsShouldNotBeEmptyID // errorNumber: 145

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -1353,25 +1353,6 @@ extends CyclicMsg(CyclicReferenceInvolvingImplicitID) {
         |"""
 }
 
-class SkolemInInferred(tree: tpd.Tree, pt: Type, argument: tpd.Tree)(using Context)
-extends TypeMsg(SkolemInInferredID):
-  def msg(using Context) =
-    def argStr =
-      if argument.isEmpty then ""
-      else i" from argument of type ${argument.tpe.widen}"
-    i"""Failure to generate given instance for type $pt$argStr)
-        |
-        |I found: $tree
-        |But the part corresponding to `<skolem>` is not a reference that can be generated.
-        |This might be because resolution yielded as given instance a function that is not
-        |known to be total and side-effect free."""
-  def explain(using Context) =
-    i"""The part of given resolution that corresponds to `<skolem>` produced a term that
-        |is not a stable reference. Therefore a given instance could not be generated.
-        |
-        |To trouble-shoot the problem, try to supply an explicit expression instead of
-        |relying on implicit search at this point."""
-
 class SuperQualMustBeParent(qual: untpd.Ident, cls: ClassSymbol)(using Context)
 extends ReferenceMsg(SuperQualMustBeParentID) {
   def msg(using Context) = i"""|$qual does not name a parent of $cls"""

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

@@ -1089,18 +1089,6 @@ trait Implicits:
       val res = implicitArgTree(defn.CanEqualClass.typeRef.appliedTo(ltp, rtp), span)
       implicits.println(i"CanEqual witness found for $ltp / $rtp: $res: ${res.tpe}")
 
-  object hasSkolem extends TreeAccumulator[Boolean]:
-    def apply(x: Boolean, tree: Tree)(using Context): Boolean =
-      x || {
-        tree match
-          case tree: Ident => tree.symbol.isSkolem
-          case Select(qual, _) => apply(x, qual)
-          case Apply(fn, _) => apply(x, fn)
-          case TypeApply(fn, _) => apply(x, fn)
-          case _: This => false
-          case _ => foldOver(x, tree)
-      }
-
   /** Find an implicit parameter or conversion.
    *  @param pt              The expected type of the parameter or conversion.
    *  @param argument        If an implicit conversion is searched, the argument to which
@@ -1148,8 +1136,6 @@ trait Implicits:
               result.tstate.commit()
             if result.gstate ne ctx.gadt then
               ctx.gadtState.restore(result.gstate)
-            if hasSkolem(false, result.tree) then
-              report.error(SkolemInInferred(result.tree, pt, argument), ctx.source.atSpan(span))
             implicits.println(i"success: $result")
             implicits.println(i"committing ${result.tstate.constraint} yielding ${ctx.typerState.constraint} in ${ctx.typerState}")
             result

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

@@ -828,7 +828,14 @@ object Synthesizer:
 
   /** Tuple used to store the synthesis result with a list of errors.  */
   type TreeWithErrors = (Tree, List[String])
-  private def withNoErrors(tree: Tree): TreeWithErrors = (tree, List.empty)
+
+  private inline def withNoErrors(inline tree: => Tree): TreeWithErrors =
+    try
+      (tree, List.empty)
+    catch
+      case tp: TypeError =>
+        withErrors(tp.getMessage)
+
   private def withErrors(errors: String*): TreeWithErrors = (EmptyTree, errors.toList)
 
   private val EmptyTreeNoError: TreeWithErrors = withNoErrors(EmptyTree)

tests/neg/i8623.check

@@ -1,13 +1,12 @@
--- [E142] Type Error: tests/neg/i8623.scala:11:2 -----------------------------------------------------------------------
+-- [E008] Not Found Error: tests/neg/i8623.scala:11:9 ------------------------------------------------------------------
 11 |  unseal.pos  // error
-   |  ^^^^^^
-   |  Failure to generate given instance for type ?{ pos: ? } from argument of type ?1.tasty.Tree)
+   |  ^^^^^^^^^^
+   |  value pos is not a member of ?1.tasty.Tree.
+   |  Extension methods were tried, but the search failed with:
    |
-   |  I found: <skolem>.tasty.pos(unseal(given_QC[Any]))
-   |  But the part corresponding to `<skolem>` is not a reference that can be generated.
-   |  This might be because resolution yielded as given instance a function that is not
-   |  known to be total and side-effect free.
+   |      cannot construct a tree referring to skolem (?1 : QC)
+   |      
+   |      where:    ?1 is an unknown value of type QC
+   |      
    |
    |  where:    ?1 is an unknown value of type QC
-   |
-   | longer explanation available when compiling with `-explain`

tests/neg/valueOf-skolem.scala

@@ -0,0 +1,23 @@
+case class Foo(
+  aaaa: Int
+)
+
+case class Bar(
+  foo: Foo,
+  bar: Bla[foo.aaaa.type]
+)
+
+class Bla[T](using Ev[T])
+
+class Ev[T](x: T)
+object Ev:
+  inline given ev: [T] => Ev[T] =
+    Ev(valueOf[T])
+
+object Test:
+  def test: Unit =
+    val x =
+      Bar(
+          Foo(0),
+          Bla() // error
+      )