Fix #7883: Better error message for unapplySeq matches

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

@@ -327,4 +327,7 @@ abstract class Reporter extends interfaces.ReporterResult {
   /** Issue all error messages in this reporter to next outer one, or make sure they are written. */
   def flush()(implicit ctx: Context): Unit =
     removeBufferedMessages.foreach(ctx.reporter.report)
+
+  /** If this reporter buffers messages, all buffered messages, otherwise Nil */
+  def pendingMessages(using Context): List[MessageContainer] = Nil
 }

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

@@ -38,5 +38,7 @@ class StoreReporter(outer: Reporter) extends Reporter {
     if (infos != null) try infos.toList finally infos = null
     else Nil
 
+  override def pendingMessages(using Context): List[MessageContainer] = infos.toList
+
   override def errorsReported: Boolean = hasErrors || (outer != null && outer.errorsReported)
 }

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

@@ -1066,22 +1066,28 @@ trait Applications extends Compatibility {
       if (!tree.tpe.isError && tree.tpe.isErroneous) tree
       else errorTree(tree, NotAnExtractor(qual))
 
+    /** Does `state` contain a single "NotAMember" message as pending error message
+     *  that says `$membername is not a member of ...` ?
+     */
+    def saysNotAMember(state: TyperState, memberName: TermName): Boolean =
+      state.reporter.pendingMessages match
+        case msg :: Nil =>
+          msg.contained match
+            case NotAMember(_, name, _, _) => name == memberName
+            case _ => false
+        case _ => false
+
     /** Report errors buffered in state.
      *  @pre state has errors to report
      *  If there is a single error stating that "unapply" is not a member, print
      *  the more informative "notAnExtractor" message instead.
      */
     def reportErrors(tree: Tree, state: TyperState): Tree =
       assert(state.reporter.hasErrors)
-      val msgs = state.reporter.removeBufferedMessages
-      msgs match
-        case msg :: Nil =>
-          msg.contained match
-            case NotAMember(_, nme.unapply, _, _) => return notAnExtractor(tree)
-            case _ =>
-        case _ =>
-      msgs.foreach(ctx.reporter.report)
-      tree
+      if saysNotAMember(state, nme.unapply) then notAnExtractor(tree)
+      else
+        state.reporter.flush()
+        tree
 
     /** If this is a term ref tree, try to typecheck with its type name.
      *  If this refers to a type alias, follow the alias, and if
@@ -1145,7 +1151,12 @@ trait Applications extends Compatibility {
       tryWithName(nme.unapply) {
         (sel, state) =>
           tryWithName(nme.unapplySeq) {
-            (_, _) => fallBack(sel, state)
+            (sel2, state2) =>
+              // if both fail, return unapply error, unless that is simply a
+              // "not a member", and the unapplySeq error is more refined.
+              if saysNotAMember(state, nme.unapply) && !saysNotAMember(state2, nme.unapplySeq)
+              then fallBack(sel2, state2)
+              else fallBack(sel, state)
           }
       }
     }

tests/neg-custom-args/explicit-nulls/i7883.check

@@ -0,0 +1,20 @@
+-- [E134] Type Mismatch Error: tests/neg-custom-args/explicit-nulls/i7883.scala:6:11 -----------------------------------
+6 |      case r(hd, tl) => Some((hd, tl))  // error // error // error
+  |           ^
+  |           None of the overloaded alternatives of method unapplySeq in class Regex with types
+  |            (m: scala.util.matching.Regex.Match): Option[List[String]]
+  |            (c: Char): Option[List[Char]]
+  |            (s: CharSequence): Option[List[String]]
+  |           match arguments (String | UncheckedNull)
+-- [E006] Unbound Identifier Error: tests/neg-custom-args/explicit-nulls/i7883.scala:6:30 ------------------------------
+6 |      case r(hd, tl) => Some((hd, tl))  // error // error // error
+  |                              ^^
+  |                              Not found: hd
+
+longer explanation available when compiling with `-explain`
+-- [E006] Unbound Identifier Error: tests/neg-custom-args/explicit-nulls/i7883.scala:6:34 ------------------------------
+6 |      case r(hd, tl) => Some((hd, tl))  // error // error // error
+  |                                  ^^
+  |                                  Not found: tl
+
+longer explanation available when compiling with `-explain`

tests/neg-custom-args/explicit-nulls/i7883.scala

@@ -0,0 +1,9 @@
+import scala.util.matching.Regex
+
+object Test extends App {
+  def head(s: String, r: Regex): Option[(String, String)] =
+    s.trim match {
+      case r(hd, tl) => Some((hd, tl))  // error // error // error
+      case _ => None
+    }
+}