From bc6eb79f74a30aef2eb874eb7ba3c443c49e7554 Mon Sep 17 00:00:00 2001
From: Li Haoyi <haoyi.sg@gmail.com>
Date: Sat, 4 Nov 2017 13:57:01 -0700
Subject: Add some basic compile-time checks to enforce usage of `T{...}`
 within traits

---
 core/src/main/scala/forge/Target.scala        | 16 +++++++++++----
 core/src/main/scala/forge/util/LocalDef.scala | 28 +++++++++++++++++++++++++++
 core/src/test/scala/forge/CacherTests.scala   | 10 ++++++++++
 core/src/test/scala/forge/TestGraphs.scala    |  1 +
 4 files changed, 51 insertions(+), 4 deletions(-)
 create mode 100644 core/src/main/scala/forge/util/LocalDef.scala

diff --git a/core/src/main/scala/forge/Target.scala b/core/src/main/scala/forge/Target.scala
index a6e977ae..ae5b488b 100644
--- a/core/src/main/scala/forge/Target.scala
+++ b/core/src/main/scala/forge/Target.scala
@@ -2,7 +2,7 @@ package forge
 
 
 import ammonite.ops.{ls, mkdir}
-import forge.util.{Args, PathRef}
+import forge.util.{Args, LocalDef, PathRef}
 import play.api.libs.json.{Format, JsValue, Json}
 
 import scala.annotation.compileTimeOnly
@@ -34,8 +34,10 @@ abstract class Target[T] extends Target.Ops[T]{
 object Target{
   trait Cacher{
     private[this] val cacherLazyMap = mutable.Map.empty[sourcecode.Enclosing, Target[_]]
-    protected[this] def T[T](t: T): Target[T] = macro impl[T]
-    protected[this] def T[T](t: => Target[T])(implicit c: sourcecode.Enclosing): Target[T] = {
+    protected[this] def T[T](t: T)
+                            (implicit l: LocalDef): Target[T] = macro localDefImpl[T]
+    protected[this] def T[T](t: => Target[T])
+                            (implicit c: sourcecode.Enclosing, l: LocalDef): Target[T] = {
       cacherLazyMap.getOrElseUpdate(c, t).asInstanceOf[Target[T]]
     }
   }
@@ -46,7 +48,13 @@ object Target{
   }
   def apply[T](t: Target[T]): Target[T] = t
   def apply[T](t: T): Target[T] = macro impl[T]
-  def impl[T: c.WeakTypeTag](c: Context)(t: c.Expr[T]): c.Expr[Target[T]] = {
+  def localDefImpl[T: c.WeakTypeTag](c: Context)
+                                    (t: c.Expr[T])
+                                    (l: c.Expr[LocalDef]): c.Expr[Target[T]] = {
+    impl(c)(t)
+  }
+  def impl[T: c.WeakTypeTag](c: Context)
+                            (t: c.Expr[T]): c.Expr[Target[T]] = {
     import c.universe._
     val bound = collection.mutable.Buffer.empty[(c.Tree, Symbol)]
     val OptionGet = c.universe.typeOf[Target[_]].member(TermName("apply"))
diff --git a/core/src/main/scala/forge/util/LocalDef.scala b/core/src/main/scala/forge/util/LocalDef.scala
new file mode 100644
index 00000000..2a58bbd2
--- /dev/null
+++ b/core/src/main/scala/forge/util/LocalDef.scala
@@ -0,0 +1,28 @@
+package forge.util
+
+import scala.reflect.macros.blackbox
+import language.experimental.macros
+class LocalDef
+object LocalDef {
+  implicit def default: LocalDef = macro enclosing
+  def enclosing(c: blackbox.Context): c.Expr[LocalDef] = {
+
+    import c.universe._
+    val current = c.internal.enclosingOwner
+
+    if (
+      !current.isMethod ||
+      // We can't do this right now because it causes recursive method errors
+      // current.asMethod.paramLists.nonEmpty ||
+      !(current.owner.isClass || current.owner.isModuleClass)
+    ) {
+      c.abort(
+        c.enclosingPosition,
+        "T{} can only be used directly within a zero-arg method defined in a class body"
+      )
+    }else{
+
+      c.Expr[LocalDef](q"""new forge.util.LocalDef()""")
+    }
+  }
+}
diff --git a/core/src/test/scala/forge/CacherTests.scala b/core/src/test/scala/forge/CacherTests.scala
index 4c346e5e..c6157a4a 100644
--- a/core/src/test/scala/forge/CacherTests.scala
+++ b/core/src/test/scala/forge/CacherTests.scala
@@ -46,5 +46,15 @@ object CacherTests extends TestSuite{
       eval(Terminal, Terminal.value) == 7,
       eval(Terminal, Terminal.overriden) == 1
     )
+    'errors{
+      val expectedMsg =
+        "T{} can only be used directly within a zero-arg method defined in a class body"
+
+      val err1 = compileError("object Foo extends Target.Cacher{ val x = T{1} }")
+      assert(err1.msg == expectedMsg)
+
+      val err2 = compileError("object Foo extends Target.Cacher{ def x = {def y = T{1}} }")
+      assert(err2.msg == expectedMsg)
+    }
   }
 }
diff --git a/core/src/test/scala/forge/TestGraphs.scala b/core/src/test/scala/forge/TestGraphs.scala
index 5960ed4c..0d6040eb 100644
--- a/core/src/test/scala/forge/TestGraphs.scala
+++ b/core/src/test/scala/forge/TestGraphs.scala
@@ -100,3 +100,4 @@ class TestGraphs(){
     val j = test(test(i), test(i, f), test(f))
   }
 }
+
-- 
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