From 40c4fe3d8cd26a57a709c0f2bc8dcbdc5a1eaf94 Mon Sep 17 00:00:00 2001
From: Tomasz Godzik <tgodzik@virtuslab.com>
Date: Fri, 5 Aug 2022 13:48:12 +0200
Subject: [PATCH] bugfix: Retain partial type params typying information on
 error

Fixes https://github.com/lampepfl/dotty/issues/15750
---
 compiler/src/dotty/tools/dotc/typer/Typer.scala        |  5 ++++-
 .../dotty/tools/languageserver/CompletionTest.scala    | 10 ++++++++++
 tests/neg/closure-args.scala                           |  2 +-
 tests/neg/i15750.scala                                 |  4 ++++
 4 files changed, 19 insertions(+), 2 deletions(-)
 create mode 100644 tests/neg/i15750.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 5eded211657b..ee36dd7cd55c 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/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
     }
diff --git a/language-server/test/dotty/tools/languageserver/CompletionTest.scala b/language-server/test/dotty/tools/languageserver/CompletionTest.scala
index b4eeb4b01d71..1657549f9a7a 100644
--- a/language-server/test/dotty/tools/languageserver/CompletionTest.scala
+++ b/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")
+        )
+      )
+  }
 }
diff --git a/tests/neg/closure-args.scala b/tests/neg/closure-args.scala
index def85f5924a1..3b166c81c61c 100644
--- a/tests/neg/closure-args.scala
+++ b/tests/neg/closure-args.scala
@@ -14,7 +14,7 @@ val d = xs   // error
   .map: x => x.toString + xs.dropWhile:
     y => y > 0
 
-val c = List(xs.map: y => y + y)  // error // error
+val c = List(xs.map: y => y + y)  // error // error // error // error
 val d2: String = xs    // error
   .map: x => x.toString + xs.dropWhile: y => y > 0  // error // error
   .filter: z => !z.isEmpty // error
diff --git a/tests/neg/i15750.scala b/tests/neg/i15750.scala
new file mode 100644
index 000000000000..9cf414a0ca15
--- /dev/null
+++ b/tests/neg/i15750.scala
@@ -0,0 +1,4 @@
+
+val foo = A[B] // error // error
+
+val bar = A[Int] // error