From 5c6ecefa08bc573944998f6328e764607d99f92f Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Wed, 6 Jan 2021 19:26:32 +0100
Subject: [PATCH] Disallow type lambdas as F-bounds

Given parameter
```
C <: [Y] => ... C[T] ...
```
change error message for `C[T]` to explain that type lambdas are not supported
as F-bounds
---
 .../src/dotty/tools/dotc/reporting/messages.scala     |  6 +++++-
 tests/neg/i8752.check                                 | 11 +++++++++++
 tests/neg/i8752.scala                                 |  3 +++
 3 files changed, 19 insertions(+), 1 deletion(-)
 create mode 100644 tests/neg/i8752.check
 create mode 100644 tests/neg/i8752.scala

diff --git a/compiler/src/dotty/tools/dotc/reporting/messages.scala b/compiler/src/dotty/tools/dotc/reporting/messages.scala
index 2c3689749334..2431fc969675 100644
--- a/compiler/src/dotty/tools/dotc/reporting/messages.scala
+++ b/compiler/src/dotty/tools/dotc/reporting/messages.scala
@@ -1386,7 +1386,11 @@ import transform.SymUtils._
 
   class TypeDoesNotTakeParameters(tpe: Type, params: List[Trees.Tree[Trees.Untyped]])(using Context)
     extends TypeMsg(TypeDoesNotTakeParametersID) {
-    def msg = em"$tpe does not take type parameters"
+    private def fboundsAddendum =
+      if tpe.typeSymbol.isAllOf(Provisional | TypeParam) then
+        "\n(Note that F-bounds of type parameters may not be type lambdas)"
+      else ""
+    def msg = em"$tpe does not take type parameters$fboundsAddendum"
     def explain =
       val ps =
         if (params.size == 1) s"a type parameter ${params.head}"
diff --git a/tests/neg/i8752.check b/tests/neg/i8752.check
new file mode 100644
index 000000000000..6d1cf47aa2fd
--- /dev/null
+++ b/tests/neg/i8752.check
@@ -0,0 +1,11 @@
+-- [E053] Type Error: tests/neg/i8752.scala:3:41 -----------------------------------------------------------------------
+3 |trait Arround1[C <:[X]=>>IterableOps[X,C,C[X]] ] // error // error
+  |                                         ^^^^
+  |                                         C does not take type parameters
+  |                                         (Note that F-bounds of type parameters may not be type lambdas)
+
+longer explanation available when compiling with `-explain`
+-- Error: tests/neg/i8752.scala:3:39 -----------------------------------------------------------------------------------
+3 |trait Arround1[C <:[X]=>>IterableOps[X,C,C[X]] ] // error // error
+  |                                       ^
+  |                                       Type argument C does not have the same kind as its bound [_]
diff --git a/tests/neg/i8752.scala b/tests/neg/i8752.scala
new file mode 100644
index 000000000000..e28f3f069675
--- /dev/null
+++ b/tests/neg/i8752.scala
@@ -0,0 +1,3 @@
+import scala.collection.IterableOps
+
+trait Arround1[C <:[X]=>>IterableOps[X,C,C[X]] ] // error // error