SI-7895 Avoid cascade of "symbol not found" in pattern matches

If we can't type check the `Foo` in `case Foo(a, b) => (a, b)`,
we should enter error symbols for `a` and `b` to avoid further
errors being reported in the case body.

Author: retronym

src/compiler/scala/tools/nsc/typechecker/Typers.scala

@@ -2392,9 +2392,16 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
       if (pat1.tpe.paramSectionCount > 0)
         pat1 modifyType (_.finalResultType)
 
-      for (bind @ Bind(name, _) <- cdef.pat)
-        if (name.toTermName != nme.WILDCARD && bind.symbol != null && bind.symbol != NoSymbol)
-          namer.enterIfNotThere(bind.symbol)
+      for (bind @ Bind(name, _) <- cdef.pat) {
+        val sym = bind.symbol
+        if (name.toTermName != nme.WILDCARD && sym != null) {
+          if (sym == NoSymbol) {
+            if (context.scope.lookup(name) == NoSymbol)
+              namer.enterInScope(context.owner.newErrorSymbol(name))
+          } else
+            namer.enterIfNotThere(sym)
+        }
+      }
 
       val guard1: Tree = if (cdef.guard == EmptyTree) EmptyTree
                          else typed(cdef.guard, BooleanTpe)

test/files/neg/t0418.check

@@ -1,7 +1,4 @@
 t0418.scala:2: error: not found: value Foo12340771
   null match { case Foo12340771.Bar(x) => x }
                     ^
-t0418.scala:2: error: not found: value x
-  null match { case Foo12340771.Bar(x) => x }
-                                          ^
-two errors found
+one error found

test/files/neg/t418.check

@@ -1,7 +1,4 @@
 t418.scala:2: error: not found: value Foo12340771
   null match { case Foo12340771.Bar(x) => x }
                     ^
-t418.scala:2: error: not found: value x
-  null match { case Foo12340771.Bar(x) => x }
-                                          ^
-two errors found
+one error found

test/files/neg/t545.check

@@ -1,7 +1,4 @@
 t545.scala:4: error: value blah is not a member of Test.Foo
   val x = foo.blah match {
               ^
-t545.scala:5: error: recursive value x needs type
-    case List(x) => x
-                    ^
-two errors found
+one error found

test/files/neg/t5903a.check

@@ -1,7 +1,4 @@
 Test_2.scala:4: error: wrong number of patterns for <$anon: AnyRef> offering (SomeTree.type, SomeTree.type): expected 2, found 3
     case nq"$x + $y + $z" => println((x, y))
          ^
-Test_2.scala:4: error: not found: value x
-    case nq"$x + $y + $z" => println((x, y))
-                                      ^
-two errors found
+one error found

test/files/neg/t5903b.check

@@ -3,7 +3,4 @@ Test_2.scala:4: error: type mismatch;
  required: String
     case t"$x" => println(x)
          ^
-Test_2.scala:4: error: not found: value x
-    case t"$x" => println(x)
-                          ^
-two errors found
+one error found

test/files/neg/t5903c.check

@@ -1,7 +1,4 @@
 Test_2.scala:4: error: String is not supported
     case t"$x" => println(x)
          ^
-Test_2.scala:4: error: not found: value x
-    case t"$x" => println(x)
-                          ^
-two errors found
+one error found

test/files/neg/t5903d.check

@@ -1,7 +1,4 @@
 Test_2.scala:4: error: extractor macros can only expand into extractor calls
     case t"$x" => println(x)
          ^
-Test_2.scala:4: error: not found: value x
-    case t"$x" => println(x)
-                          ^
-two errors found
+one error found

test/files/neg/t7895.check

@@ -0,0 +1,4 @@
+t7895.scala:4: error: not found: value Goop
+    case Goop(a, b, c) => Tuple2(a, b)
+         ^
+one error found

test/files/neg/t7895.scala

@@ -0,0 +1,6 @@
+class A {
+  (null: Any) match {
+    // We don't want "symbol not found errors" for `a` and `b` in the case body.
+    case Goop(a, b, c) => Tuple2(a, b)
+  }
+}