bugfix: Retain partial type params typying information on error

Fixes #15750

Author: tgodzik

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

@@ -1993,7 +1993,10 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       typed(tree.tpt, AnyTypeConstructorProto)
     }
     val tparams = tpt1.tpe.typeParams
-    if (tparams.isEmpty) {
+     if tpt1.tpe.isError then
+       val args1 = tree.args.mapconserve(typedType(_))
+       assignType(cpy.AppliedTypeTree(tree)(tpt1, args1), tpt1, args1)
+     else if (tparams.isEmpty) {
       report.error(TypeDoesNotTakeParameters(tpt1.tpe, tree.args), tree.srcPos)
       tpt1
     }

language-server/test/dotty/tools/languageserver/CompletionTest.scala

@@ -1518,4 +1518,14 @@ class CompletionTest {
       .noCompletions()
   }
 
+  @Test def badTypeCompletions: Unit = {
+    code"""trait Foo
+          |object Test:
+          |  def foo: ArrayBuffer[Fo${m1}] = ???
+          """
+      .completion(m1, Set(
+          ("Foo",Class,"Foo")
+        )
+      )
+  }
 }