From d0c27ff8f3eef58a18bd173d0bce47a0ccc9865f Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Sun, 15 Nov 2020 18:00:00 +0100
Subject: [PATCH] Disallow by-name trait parameters

---
 compiler/src/dotty/tools/dotc/ast/Desugar.scala | 5 +++++
 tests/neg/i9886.scala                           | 2 ++
 2 files changed, 7 insertions(+)
 create mode 100644 tests/neg/i9886.scala

diff --git a/compiler/src/dotty/tools/dotc/ast/Desugar.scala b/compiler/src/dotty/tools/dotc/ast/Desugar.scala
index 402e0d6d7e14..032cc6bb4417 100644
--- a/compiler/src/dotty/tools/dotc/ast/Desugar.scala
+++ b/compiler/src/dotty/tools/dotc/ast/Desugar.scala
@@ -428,6 +428,11 @@ object desugar {
       }
       else originalTparams
 
+    if mods.is(Trait) then
+      for vparams <- originalVparamss; vparam <- vparams do
+        if vparam.tpt.isInstanceOf[ByNameTypeTree] then
+          report.error(em"implementation restriction: traits cannot have by name parameters", vparam.srcPos)
+
     // Annotations on class _type_ parameters are set on the derived parameters
     // but not on the constructor parameters. The reverse is true for
     // annotations on class _value_ parameters.
diff --git a/tests/neg/i9886.scala b/tests/neg/i9886.scala
new file mode 100644
index 000000000000..765bd389826c
--- /dev/null
+++ b/tests/neg/i9886.scala
@@ -0,0 +1,2 @@
+trait D(l: => Any):  // error
+  def f = l