closes #4692: unification in type constructor i...

closes #4692: unification in type constructor inference now widens *and*
dealiases when necessary

in 2.8.1 implicit conversion search started with a widened type, so that
combo never came up

no review

2 files changed, 30 insertions, 2 deletions

diff --git a/src/compiler/scala/reflect/internal/Types.scala b/src/compiler/scala/reflect/internal/Types.scala
index 3264c34d4b..cc5c2fa5c1 100644
--- a/src/compiler/scala/reflect/internal/Types.scala
+++ b/src/compiler/scala/reflect/internal/Types.scala
@@ -2484,8 +2484,9 @@ A type's typeSymbol should never be inspected directly.
       // this <: tp.baseType(sym)
       if (suspended) checkSubtype(tp, origin)
       else if (constr.instValid) checkSubtype(tp, constr.inst)  // type var is already set
-      else isRelatable(tp) && {
-        unifySimple || unifyFull(tp) || unifyFull(tp.dealias) || unifyFull(tp.widen) || unifyParents
+      else isRelatable(tp) && { // gradually let go of some type precision in hopes of finding a type that has the same shape as the type variable
+        // okay, this just screams "CLEAN ME UP" -- I think we could use tp.widen instead of tp straight from the get-go in registerBound, since we don't infer singleton types anyway (but maybe that'll change?)
+        unifySimple || unifyFull(tp) || unifyFull(tp.dealias) || unifyFull(tp.widen) || unifyFull(tp.widen.dealias) || unifyParents
       }
     }
 
diff --git a/test/files/pos/t4692.scala b/test/files/pos/t4692.scala
new file mode 100644
index 0000000000..409daf2257
--- /dev/null
+++ b/test/files/pos/t4692.scala
@@ -0,0 +1,27 @@
+class TypeAliasVsImplicitTest {
+
+    class For[m[_], a](x: m[a]) {
+       def map[b](y: a => b): m[b] = throw new Error
+    }
+    implicit def toFor[m[_], a](x: m[a]): For[m, a] = new For[m, a](x)
+
+    trait MyList[A]
+
+    def foo(xs: MyList[Int]) = xs.map(x => x) // compiles fine.
+
+    type MyListOfInt = MyList[Int]
+    def bar(xs: MyListOfInt) = xs.map(x => x) // doesn't compile: value map is not a member of TypeAliasVsImplicitTest.this.MyListOfInt
+}
+
+// minimal case -- the bug was in type constructor inference where `xs.type` needed to be widened *and* dealiased
+// in 2.8.1 implicit conversion search started with a widened type, so that combo never came up
+// object Test {
+//   class For[m[_], a](x: m[a])
+//   def toFor[m[_], a](x: m[a]): For[m, a] = new For[m, a](x)
+//
+//   trait MyList[A]
+//   type MyListOfInt = MyList[Int]
+//
+//   val xs: MyListOfInt = error("")
+//   toFor(xs : xs.type)
+// }
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