From c3bcad807db47ee4ab27ac4a725ba5f402667b4d Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Thu, 18 Jun 2015 15:15:42 +0200
Subject: Do not check for ambiguous implicits in viewExists

Previously `viewExists(X, Y)` failed if there were ambiguous
implicit conversions from X to Y. This is too fragile, as
demonstrated by test case run/array-addition.scala. Here,
the `genericArrayOps` implicit was not inserted because its
result type `Array[?T]` was deemed to be incompatible with
`? { +: : ? }`. It was incompatible because there were multiple
implicits that added :+ to arrays of various element types.
But once `genericArrayOps` gets applied, the type parameter
`?T` of the array result is fixed, and the ambuity goes away.

The scenario shows that we should not test for ambiguous implicits
in viewExists. Such a test is fragile because it depends on the
progress of type inference when the test is made. It's preferable
to just test for any implicit conversion to exist and to check
for ambiguities later, when the implicit conversion is actually
applied. This has also the potential of speeding up implicit search
in situations where `viewExists` is called often (e.g. when coupled
with overloading resolution).
---
 tests/pending/run/array-addition.check |  4 ----
 tests/pending/run/array-addition.scala | 11 -----------
 tests/run/array-addition.check         |  4 ++++
 tests/run/array-addition.scala         | 11 +++++++++++
 4 files changed, 15 insertions(+), 15 deletions(-)
 delete mode 100644 tests/pending/run/array-addition.check
 delete mode 100644 tests/pending/run/array-addition.scala
 create mode 100644 tests/run/array-addition.check
 create mode 100644 tests/run/array-addition.scala

(limited to 'tests')

diff --git a/tests/pending/run/array-addition.check b/tests/pending/run/array-addition.check
deleted file mode 100644
index 7bfbd9c71..000000000
--- a/tests/pending/run/array-addition.check
+++ /dev/null
@@ -1,4 +0,0 @@
-Array(1, 2, 3, 4)
-Array(1, 2, 3, 4)
-Array(1)
-Array(1)
diff --git a/tests/pending/run/array-addition.scala b/tests/pending/run/array-addition.scala
deleted file mode 100644
index 8def48e85..000000000
--- a/tests/pending/run/array-addition.scala
+++ /dev/null
@@ -1,11 +0,0 @@
-object Test {
-  def prettyPrintArray(x: Array[_]) = println("Array(" + x.mkString(", ") + ")")
-
-  def main(args: Array[String]): Unit = {
-    prettyPrintArray(Array(1,2,3) :+ 4)
-    prettyPrintArray(1 +: Array(2,3,4))
-    prettyPrintArray(Array() :+ 1)
-    prettyPrintArray(1 +: Array())
-  }
-}
-
diff --git a/tests/run/array-addition.check b/tests/run/array-addition.check
new file mode 100644
index 000000000..7bfbd9c71
--- /dev/null
+++ b/tests/run/array-addition.check
@@ -0,0 +1,4 @@
+Array(1, 2, 3, 4)
+Array(1, 2, 3, 4)
+Array(1)
+Array(1)
diff --git a/tests/run/array-addition.scala b/tests/run/array-addition.scala
new file mode 100644
index 000000000..8def48e85
--- /dev/null
+++ b/tests/run/array-addition.scala
@@ -0,0 +1,11 @@
+object Test {
+  def prettyPrintArray(x: Array[_]) = println("Array(" + x.mkString(", ") + ")")
+
+  def main(args: Array[String]): Unit = {
+    prettyPrintArray(Array(1,2,3) :+ 4)
+    prettyPrintArray(1 +: Array(2,3,4))
+    prettyPrintArray(Array() :+ 1)
+    prettyPrintArray(1 +: Array())
+  }
+}
+
-- 
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