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https://github.com/scala/scala/pull/2072

1 files changed, 29 insertions, 2 deletions

diff --git a/src/compiler/scala/reflect/reify/codegen/GenSymbols.scala b/src/compiler/scala/reflect/reify/codegen/GenSymbols.scala
index 0d795e3783..47c966ea24 100644
--- a/src/compiler/scala/reflect/reify/codegen/GenSymbols.scala
+++ b/src/compiler/scala/reflect/reify/codegen/GenSymbols.scala
@@ -1,6 +1,8 @@
 package scala.reflect.reify
 package codegen
 
+import scala.reflect.internal.Flags._
+
 trait GenSymbols {
   self: Reifier =>
 
@@ -100,8 +102,33 @@ trait GenSymbols {
     reifyIntoSymtab(binding.symbol) { sym =>
       if (reifyDebug) println("Free term" + (if (sym.isCapturedVariable) " (captured)" else "") + ": " + sym + "(" + sym.accurateKindString + ")")
       val name = newTermName(nme.REIFY_FREE_PREFIX + sym.name + (if (sym.isType) nme.REIFY_FREE_THIS_SUFFIX else ""))
-      // Flag <stable> is set here to make reified free value pass stability test during reflective compilation
-      if (!sym.isMutable) sym setFlag reflect.internal.Flags.STABLE
+      // We need to note whether the free value being reified is stable or not to guide subsequent reflective compilation.
+      // Here's why reflection compilation needs our help.
+      //
+      // When dealing with a tree, which contain free values, toolboxes extract those and wrap the entire tree in a Function
+      // having parameters defined for every free values in the tree. For example, evaluating
+      //
+      //   Ident(setTypeSignature(newFreeTerm("x", 2), <Int>))
+      //
+      // Will generate something like
+      //
+      //   object wrapper {
+      //     def wrapper(x: () => Int) = {
+      //       x()
+      //     }
+      //   }
+      //
+      // Note that free values get transformed into, effectively, by-name parameters. This is done to make sure
+      // that evaluation order is kept intact. And indeed, we cannot just evaluate all free values at once in order
+      // to obtain arguments for wrapper.wrapper, because if some of the free values end up being unused during evaluation,
+      // we might end up doing unnecessary calculations.
+      //
+      // So far, so good - we didn't need any flags at all. However, if the code being reified contains path-dependent types,
+      // we're in trouble, because valid code like `free.T` ends up being transformed into `free.apply().T`, which won't compile.
+      //
+      // To overcome this glitch, we note whether a given free term is stable or not (because vars can also end up being free terms).
+      // Then, if a free term is stable, we tell the compiler to treat `free.apply()` specially and assume that it's stable.
+      if (!sym.isMutable) sym setFlag STABLE
       if (sym.isCapturedVariable) {
         assert(binding.isInstanceOf[Ident], showRaw(binding))
         val capturedBinding = referenceCapturedVariable(sym)