Fix widen types before checking an implicit view exists (#18719)

Fixes #18650

Author: odersky

Diff for: compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -421,6 +421,7 @@ private sealed trait YSettings:
   val YshowVarBounds: Setting[Boolean] = BooleanSetting("-Yshow-var-bounds", "Print type variables with their bounds.")
 
   val YnoDecodeStacktraces: Setting[Boolean] = BooleanSetting("-Yno-decode-stacktraces", "Show raw StackOverflow stacktraces, instead of decoding them into triggering operations.")
+  val YnoEnrichErrorMessages: Setting[Boolean] = BooleanSetting("-Yno-enrich-error-messages", "Show raw error messages, instead of enriching them with contextual information.")
 
   val Yinstrument: Setting[Boolean] = BooleanSetting("-Yinstrument", "Add instrumentation code that counts allocations and closure creations.")
   val YinstrumentDefs: Setting[Boolean] = BooleanSetting("-Yinstrument-defs", "Add instrumentation code that counts method calls; needs -Yinstrument to be set, too.")

Diff for: compiler/src/dotty/tools/dotc/report.scala

@@ -132,50 +132,26 @@ object report:
   private object messageRendering extends MessageRendering
 
   // Should only be called from Run#enrichErrorMessage.
-  def enrichErrorMessage(errorMessage: String)(using Context): String = try {
+  def enrichErrorMessage(errorMessage: String)(using Context): String =
+    if ctx.settings.YnoEnrichErrorMessages.value then errorMessage
+    else try enrichErrorMessage1(errorMessage)
+    catch case _: Throwable => errorMessage // don't introduce new errors trying to report errors, so swallow exceptions
+
+  private def enrichErrorMessage1(errorMessage: String)(using Context): String = {
+    import untpd.*, config.Settings.*
     def formatExplain(pairs: List[(String, Any)]) = pairs.map((k, v) => f"$k%20s: $v").mkString("\n")
 
     val settings = ctx.settings.userSetSettings(ctx.settingsState).sortBy(_.name)
-    val tree     = ctx.tree
-    val sym      = tree.symbol
-    val pos      = tree.sourcePos
-    val path     = pos.source.path
-    val site     = ctx.outersIterator.map(_.owner).filter(sym => !sym.exists || sym.isClass || sym.is(Method)).next()
-
-    import untpd.*
-    extension (tree: Tree) def summaryString: String = tree match
-      case Literal(const)     => s"Literal($const)"
-      case Ident(name)        => s"Ident(${name.decode})"
-      case Select(qual, name) => s"Select(${qual.summaryString}, ${name.decode})"
-      case tree: NameTree     => (if tree.isType then "type " else "") + tree.name.decode
-      case tree               => s"${tree.className}${if tree.symbol.exists then s"(${tree.symbol})" else ""}"
+    def showSetting(s: Setting[?]): String = if s.value == "" then s"${s.name} \"\"" else s"${s.name} ${s.value}"
 
     val info1 = formatExplain(List(
       "while compiling"    -> ctx.compilationUnit,
       "during phase"       -> ctx.phase.megaPhase,
       "mode"               -> ctx.mode,
       "library version"    -> scala.util.Properties.versionString,
       "compiler version"   -> dotty.tools.dotc.config.Properties.versionString,
-      "settings"           -> settings.map(s => if s.value == "" then s"${s.name} \"\"" else s"${s.name} ${s.value}").mkString(" "),
-    ))
-    val symbolInfos = if sym eq NoSymbol then List("symbol" -> sym) else List(
-      "symbol"             -> sym.showLocated,
-      "symbol definition"  -> s"${sym.showDcl} (a ${sym.className})",
-      "symbol package"     -> sym.enclosingPackageClass.fullName,
-      "symbol owners"      -> sym.showExtendedLocation,
-    )
-    val info2 = formatExplain(List(
-      "tree"               -> tree.summaryString,
-      "tree position"      -> (if pos.exists then s"$path:${pos.line + 1}:${pos.column}" else s"$path:<unknown>"),
-      "tree type"          -> tree.typeOpt.show,
-    ) ::: symbolInfos ::: List(
-      "call site"          -> s"${site.showLocated} in ${site.enclosingPackageClass}"
+      "settings"           -> settings.map(showSetting).mkString(" "),
     ))
-    val context_s = try
-      s"""  == Source file context for tree position ==
-         |
-         |${messageRendering.messageAndPos(Diagnostic.Error("", pos))}""".stripMargin
-    catch case _: Exception => "<Cannot read source file>"
     s"""
        |  $errorMessage
        |
@@ -184,9 +160,6 @@ object report:
        |  https://github.com/lampepfl/dotty/issues/new/choose
        |
        |$info1
-       |
-       |$info2
-       |
-       |$context_s""".stripMargin
-  } catch case _: Throwable => errorMessage // don't introduce new errors trying to report errors, so swallow exceptions
+       |""".stripMargin
+  }
 end report

Diff for: compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -850,7 +850,7 @@ trait Implicits:
     && !to.isError
     && !ctx.isAfterTyper
     && ctx.mode.is(Mode.ImplicitsEnabled)
-    && from.isValueType
+    && from.widen.isValueType
     && (  from.isValueSubType(to)
        || inferView(dummyTreeOfType(from), to)
             (using ctx.fresh.addMode(Mode.ImplicitExploration).setExploreTyperState()).isSuccess

Diff for: tests/neg/i18650.min.scala

@@ -0,0 +1,8 @@
+class Church[B]:
+  type Nat = Tuple1[B]
+
+class Test:
+  given makeChurch[C]: Church[C] = ??? // necessary to cause crash
+
+  def churchTest(c: Church[Int]): Unit =
+    val res1 = summon[c.Nat =:= Int] // error (not a compiler crash)

Diff for: tests/neg/i18650.min2.scala

@@ -0,0 +1,8 @@
+class Church[B]:
+  type Nat = Tuple1[B]
+
+class Test2:
+  given makeChurch2[C](using DummyImplicit): Church[C] = ???
+
+  def churchTest2(c: Church[Int]): Unit =
+    val res2 = summon[c.Nat =:= Int] // error (not a compiler crash)

Diff for: tests/neg/i18650.scala

@@ -0,0 +1,26 @@
+trait Lam:
+  type F[_]
+  extension [A, B](f: F[A => B]) def apply(arg: F[A]): F[B]
+  def lam[A, B](f: F[A] => F[B]): F[A => B]
+  final def id[A]: F[A => A] = lam(identity[F[A]])
+
+object LamInterpreter extends Lam:
+  type F[t] = t
+  def lam[A, B](f: F[A] => F[B]): F[A => B] = f
+  extension [A, B](f: F[A => B]) def apply(arg: F[A]): F[B] = f(arg)
+
+
+class Church[A](using val l: Lam):
+  import l.*
+  type Nat = F[(A => A) => (A => A)]
+  def zero: Nat = id
+  extension (n: Nat) def suc: Nat = lam(f => lam(x => f(n(f)(x))))
+
+given [A](using l: Lam): Church[A] = Church()
+
+
+@main
+def churchTest =
+  given Lam = LamInterpreter
+  val c: Church[Int] = summon
+  summon[c.Nat =:= ((Int => Int) => (Int => Int))] // error (not a compiler crash)