Fix deadlock in t5375 and similar tests.

See t5375.scala for details.

Author: DarkDimius

src/dotty/tools/dotc/transform/LambdaLift.scala

@@ -369,8 +369,9 @@ class LambdaLift extends MiniPhase with IdentityDenotTransformer { thisTransform
                // though the second condition seems weird, it's not true for symbols which are defined in some
                // weird combinations of super calls.
               (encClass, EmptyFlags)
-            } else
-              (topClass, JavaStatic)
+            } else if (encClass.is(ModuleClass, butNot = Package) && encClass.isStatic) // needed to not cause deadlocks in classloader. see t5375.scala
+              (encClass, EmptyFlags)
+            else (topClass, JavaStatic)
           }
           else (lOwner, EmptyFlags)
         local.copySymDenotation(

tests/run/t5375.scala

@@ -1,3 +1,14 @@
+/** Hello fellow compiler developer.
+    if you are wondering why does test suite hang on this test
+    then it's likely that the lambda inside map has been compiled into static method
+    unfotrunatelly, as it is executed inside static object initializer,
+    it is executed inside class-loader, in a synchronized block that is not source defined.
+    
+    If the lambda will be static Test$#foo, calling it through a different thread would require grabbing the
+    lock inside classloader. Unlike if it not static and is called through This(Test).foo, no lock is grabbed.
+    
+    @DarkDimius  
+*/  
 object Test extends dotty.runtime.LegacyApp {
   val foos = (1 to 1000).toSeq
   try