Suppress follow-on errors for erroneous import qualifiers

Fixes #16653

Author: odersky

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

@@ -808,7 +808,7 @@ trait Checking {
 
   /** Check that type `tp` is stable. */
   def checkStable(tp: Type, pos: SrcPos, kind: String)(using Context): Unit =
-    if !tp.isStable then report.error(NotAPath(tp, kind), pos)
+    if !tp.isStable && !tp.isErroneous then report.error(NotAPath(tp, kind), pos)
 
   /** Check that all type members of `tp` have realizable bounds */
   def checkRealizableBounds(cls: Symbol, pos: SrcPos)(using Context): Unit = {
@@ -904,7 +904,7 @@ trait Checking {
   private def checkLegalImportOrExportPath(path: Tree, kind: String)(using Context): Unit = {
     checkStable(path.tpe, path.srcPos, kind)
     if (!ctx.isAfterTyper) Checking.checkRealizable(path.tpe, path.srcPos)
-    if !isIdempotentExpr(path) then
+    if !isIdempotentExpr(path) && !path.tpe.isErroneous then
       report.error(em"import prefix is not a pure expression", path.srcPos)
   }
 

tests/neg/i16653.check

@@ -0,0 +1,6 @@
+-- [E006] Not Found Error: tests/neg/i16653.scala:1:7 ------------------------------------------------------------------
+1 |import demo.implicits._ // error
+  |       ^^^^
+  |       Not found: demo
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/i16653.scala

@@ -0,0 +1,3 @@
+import demo.implicits._ // error
+import demo._
+object Demo {}

tests/neg/i6056.scala

@@ -2,6 +2,6 @@ object i0{
     import i0.i0   // error // error
     def i0={
         import _  // error
-        import    // error
+        import
     }             // error
 }

tests/neg/parser-stability-9.scala

@@ -1,2 +1,2 @@
-import // error
+import
 // error