From 482a6a0e932ee7f68a8da73f17f6d5aadf42f3ea Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Mon, 29 Jan 2024 18:51:05 +0100
Subject: [PATCH] Don't search for implicit conversions to NoType

---
 .../src/dotty/tools/dotc/typer/Implicits.scala    |  2 +-
 tests/neg/i19320.scala                            | 15 +++++++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)
 create mode 100644 tests/neg/i19320.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Implicits.scala b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
index 389669beff01..c3bf2dd822c9 100644
--- a/compiler/src/dotty/tools/dotc/typer/Implicits.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
@@ -1074,7 +1074,7 @@ trait Implicits:
         if (argument.isEmpty) i"missing implicit parameter of type $pt after typer at phase ${ctx.phase.phaseName}"
         else i"type error: ${argument.tpe} does not conform to $pt${err.whyNoMatchStr(argument.tpe, pt)}")
 
-      val usableForInference = !pt.unusableForInference
+      val usableForInference = pt.exists && !pt.unusableForInference
         && (argument.isEmpty || !argument.tpe.unusableForInference)
 
       val result0 = if usableForInference then
diff --git a/tests/neg/i19320.scala b/tests/neg/i19320.scala
new file mode 100644
index 000000000000..e802215a1f10
--- /dev/null
+++ b/tests/neg/i19320.scala
@@ -0,0 +1,15 @@
+//> using scala "3.3.1"
+//> using dep org.http4s::http4s-ember-client:1.0.0-M40
+//> using dep org.http4s::http4s-ember-server:1.0.0-M40
+//> using dep org.http4s::http4s-dsl:1.0.0-M40
+
+//import cats.effect.*
+//import cats.implicits.*
+
+class Concurrent[F[_]]
+
+class Test[F[_]: Concurren]: // error
+    def hello = ???
+
+object Test:
+    def apply[F[_]: Concurrent] = new Test[F]