Add document and test for overload resolution

docs/docs/reference/changed-features/overload-resolution.md

@@ -0,0 +1,31 @@
+---
+layout: doc-page
+title: "Changes in Overload Resolution"
+---
+
+Overload resolution in Dotty takes all argument blocks into account instead of
+just the first argument block.
+
+For example, the following code compiles in Dotty, while it results in an
+ambiguous overload error in Scala2:
+
+```Scala
+def f(x: Int)(y: String): Int = 0
+def f(x: Int)(y: Int): Int = 0
+
+f(3)("")     // ok
+```
+
+The following code compiles as well:
+
+```Scala
+def g(x: Int)(y: Int)(z: Int): Int = 0
+def g(x: Int)(y: Int)(z: String): Int = 0
+
+g(2)(3)(4)     // ok
+g(2)(3)("")    // ok
+```
+
+This change is motivated by the new language feature [extension
+methods](../contextual/extension-methods.html), where emerges the need to do
+overload resolution based on additional argument blocks.

docs/sidebar.yml

@@ -99,6 +99,8 @@ sidebar:
               url: docs/reference/changed-features/implicit-resolution.html
             - title: Implicit Conversions
               url: docs/reference/changed-features/implicit-conversions.html
+            - title: Overload Resolution
+              url: docs/reference/changed-features/overload-resolution.html
             - title: Vararg Patterns
               url: docs/reference/changed-features/vararg-patterns.html
             - title: Pattern matching

tests/pos/i6116.scala

@@ -0,0 +1,14 @@
+class Test {
+  def f(x: Int)(y: String): Int = ???
+  def f(x: Int)(y: Int): Int = ???
+
+  f(3)("")       // ok
+  f(3)(4)        // ok
+
+  def g(x: Int)(y: Int)(z: Int): Int = ???
+  def g(x: Int)(y: Int)(z: String): Int = ???
+
+  g(2)(3)(4)     // ok
+  g(2)(3)("")    // ok
+}
+