Survive TypeErrors in isMatchedBy

Fixes #15673

Author: odersky

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -299,13 +299,13 @@ object Types {
      *  ThisType of `symd`'s owner, or a reference to `symd`'s owner.'
      */
     def isArgPrefixOf(symd: SymDenotation)(using Context): Boolean =
-      symd.exists && !symd.owner.is(Package) && // Early exit if possible because the next check would force SymbolLoaders
-      symd.isAllOf(ClassTypeParam) && {
-        this match {
+      symd.exists
+      && !symd.owner.is(Package) // Early exit if possible because the next check would force SymbolLoaders
+      && symd.isAllOf(ClassTypeParam)
+      && { this match
           case tp: ThisType => tp.cls ne symd.owner
           case tp: TypeRef => tp.symbol ne symd.owner
           case _ => true
-        }
       }
 
     /** Is this type a (possibly aliased) singleton type? */
@@ -2337,7 +2337,8 @@ object Types {
             i"""bad parameter reference $this at ${ctx.phase}
                |the parameter is ${param.showLocated} but the prefix $prefix
                |does not define any corresponding arguments.
-               |idx = $idx, args = $args""")
+               |idx = $idx, args = $args%, %,
+               |constraint = ${ctx.typerState.constraint}""")
         NoDenotation
       }
     }

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

@@ -189,20 +189,33 @@ object ProtoTypes {
       case _ => false
 
     override def isMatchedBy(tp1: Type, keepConstraint: Boolean)(using Context): Boolean =
-      name == nme.WILDCARD || hasUnknownMembers(tp1) ||
-      {
-        val mbr = if (privateOK) tp1.member(name) else tp1.nonPrivateMember(name)
-        def qualifies(m: SingleDenotation) =
-          val isAccessible = !m.symbol.exists || m.symbol.isAccessibleFrom(tp1, superAccess = true)
-          isAccessible
-          && (memberProto.isRef(defn.UnitClass)
-             || tp1.isValueType && compat.normalizedCompatible(NamedType(tp1, name, m), memberProto, keepConstraint))
-              // Note: can't use `m.info` here because if `m` is a method, `m.info`
-              //       loses knowledge about `m`'s default arguments.
-        mbr match { // hasAltWith inlined for performance
-          case mbr: SingleDenotation => mbr.exists && qualifies(mbr)
-          case _ => mbr hasAltWith qualifies
-        }
+      name == nme.WILDCARD
+      || hasUnknownMembers(tp1)
+      || {
+        try
+          val mbr = if privateOK then tp1.member(name) else tp1.nonPrivateMember(name)
+          def qualifies(m: SingleDenotation) =
+            val isAccessible = !m.symbol.exists || m.symbol.isAccessibleFrom(tp1, superAccess = true)
+            isAccessible
+            && (memberProto.isRef(defn.UnitClass)
+              || tp1.isValueType && compat.normalizedCompatible(NamedType(tp1, name, m), memberProto, keepConstraint))
+                // Note: can't use `m.info` here because if `m` is a method, `m.info`
+                //       loses knowledge about `m`'s default arguments.
+          mbr match // hasAltWith inlined for performance
+            case mbr: SingleDenotation => mbr.exists && qualifies(mbr)
+            case _ => mbr hasAltWith qualifies
+        catch case ex: TypeError =>
+          // A scenario where this can happen is in pos/15673.scala:
+          // We have a type `CC[A]#C` where `CC`'s upper bound is `[X] => Any`, but
+          // the current constraint stipulates CC <: SeqOps[...], where `SeqOps` defines
+          // the `C` parameter. We try to resolve this using `argDenot` but `argDenot`
+          // consults the base type of `CC`, which is not `SeqOps`, so it does not
+          // find a corresponding argument. In fact, `argDenot` is not allowed to
+          // consult short-lived things like the current constraint, so it has no other
+          // choice. The problem will be healed later, when normal selection fails
+          // and we try to instantiate type variables to compensate. But we have to make
+          // sure we do not issue a type error before we get there.
+          false
       }
 
     def underlying(using Context): Type = WildcardType

tests/pos/i15673.scala

@@ -0,0 +1,7 @@
+trait SeqOps[+A, +CC[_], +C]:   // scala.collection.SeqOps
+  def reverse: C
+
+extension[A, CC[B] <: SeqOps[B, CC, CC[B]]](ring: CC[A])
+  def startAt(i: Int): CC[A] = ???
+  def reflectAt(i: Int): CC[A] =
+    startAt(i).reverse