SI-10207 Error before update conversion

Gaze deeper for errors before committing to conversion
of assignment to update. The error buried in the transformed
tree escapes notice of retypechecking and leaks to backend.

Author: som-snytt

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

@@ -4666,19 +4666,20 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
             val qual1 = typedQualifier(qual)
             if (treeInfo.isVariableOrGetter(qual1)) {
               if (Statistics.canEnable) Statistics.stopTimer(failedOpEqNanos, opeqStart)
-              val erred = qual1.isErroneous || args.exists(_.isErroneous)
+              val erred = qual1.exists(_.isErroneous) || args.exists(_.isErroneous)
               if (erred) reportError(error) else {
                 val convo = convertToAssignment(fun, qual1, name, args)
                 silent(op = _.typed1(convo, mode, pt)) match {
                   case SilentResultValue(t) => t
-                  case err: SilentTypeError => reportError(SilentTypeError(advice1(convo, error.errors, err), error.warnings))
+                  case err: SilentTypeError => reportError(
+                    SilentTypeError(advice1(convo, error.errors, err), error.warnings)
+                  )
                 }
               }
-            }
-            else {
+            } else {
               if (Statistics.canEnable) Statistics.stopTimer(failedApplyNanos, appStart)
               val Apply(Select(qual2, _), args2) = tree
-              val erred = qual2.isErroneous || args2.exists(_.isErroneous)
+              val erred = qual2.exists(_.isErroneous) || args2.exists(_.isErroneous)
               reportError {
                 if (erred) error else SilentTypeError(advice2(error.errors), error.warnings)
               }

test/files/neg/t10207.check

@@ -0,0 +1,4 @@
+t10207.scala:14: error: too many arguments (2) for method apply: (key: Int)scala.collection.mutable.ArrayBuffer[String] in trait MapLike
+  m(1, (_ => empty)) ++= AB("eins", "uno")
+          ^
+one error found

test/files/neg/t10207.scala

@@ -0,0 +1,16 @@
+
+// Was:
+// warning: an unexpected type representation reached the compiler backend
+// Now:
+// error: too many arguments (2) for method apply: (key: Int)scala.collection.mutable.ArrayBuffer[String] in trait MapLike
+
+trait Test {
+  import collection.mutable.{Map=>MMap, ArrayBuffer=>AB}
+
+  val m = MMap((1 -> AB("one")))
+
+  val empty = AB[String]()
+
+  m(1, (_ => empty)) ++= AB("eins", "uno")
+}
+