From fac74a618ae4666490cd8c7fd3f9604d877562d9 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Sat, 11 Mar 2017 20:56:02 +0100
Subject: Fix #1569: Improve avoidance algorithm

The essential change is that we do not throw away more
precise info of the avoided type if the expected type
is fully defined.
---
 compiler/src/dotty/tools/dotc/typer/Typer.scala | 14 ++++++++++----
 tests/neg/t1569-failedAvoid.scala               |  9 ---------
 tests/pos/t1569.scala                           |  6 ++++++
 3 files changed, 16 insertions(+), 13 deletions(-)
 delete mode 100644 tests/neg/t1569-failedAvoid.scala
 create mode 100644 tests/pos/t1569.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index ba14b7498..c43a8adcd 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -642,12 +642,18 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
     }
     val leaks = escapingRefs(tree, localSyms)
     if (leaks.isEmpty) tree
-    else if (isFullyDefined(pt, ForceDegree.none)) ascribeType(tree, pt)
     else if (!forcedDefined) {
       fullyDefinedType(tree.tpe, "block", tree.pos)
-      val tree1 = ascribeType(tree, avoid(tree.tpe, localSyms))
-      ensureNoLocalRefs(tree1, pt, localSyms, forcedDefined = true)
-    } else
+      val avoidingType = avoid(tree.tpe, localSyms)
+      if (isFullyDefined(pt, ForceDegree.none) && !(avoidingType <:< pt))
+        ascribeType(tree, pt)
+      else {
+        val tree1 = ascribeType(tree, avoidingType)
+        ensureNoLocalRefs(tree1, pt, localSyms, forcedDefined = true)
+      }
+    } else if (isFullyDefined(pt, ForceDegree.none))
+      ascribeType(tree, pt)
+    else
       errorTree(tree,
           em"local definition of ${leaks.head.name} escapes as part of expression's type ${tree.tpe}"/*; full type: ${result.tpe.toString}"*/)
   }
diff --git a/tests/neg/t1569-failedAvoid.scala b/tests/neg/t1569-failedAvoid.scala
deleted file mode 100644
index 45bb96f36..000000000
--- a/tests/neg/t1569-failedAvoid.scala
+++ /dev/null
@@ -1,9 +0,0 @@
-// This was t1569.scala.
-// It fails in dotty because the expected type of the anonymous function in the last line
-// is fully determined (C). So that type is taken as the type of the anonymous function.
-// See pos/t1569a.scala for related examples that work.
-object Bug {
-  class C { type T }
-  def foo(x: Int)(y: C)(z: y.T): Unit = {}
-  foo(3)(new C { type T = String })("hello")          // error
-}
diff --git a/tests/pos/t1569.scala b/tests/pos/t1569.scala
new file mode 100644
index 000000000..5e48c03b1
--- /dev/null
+++ b/tests/pos/t1569.scala
@@ -0,0 +1,6 @@
+// See pos/t1569a.scala for related examples that work.
+object Bug {
+  class C { type T }
+  def foo(x: Int)(y: C)(z: y.T): Unit = {}
+  foo(3)(new C { type T = String })("hello")
+}
-- 
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