Put changes under experimental language import

As long as this is not SIP approved, we need to put this under experimental.

Author: odersky

compiler/src/dotty/tools/dotc/config/Feature.scala

@@ -29,6 +29,7 @@ object Feature:
   val fewerBraces = experimental("fewerBraces")
   val saferExceptions = experimental("saferExceptions")
   val clauseInterleaving = experimental("clauseInterleaving")
+  val relaxedExtensionImports = experimental("relaxedExtensionImports")
   val pureFunctions = experimental("pureFunctions")
   val captureChecking = experimental("captureChecking")
   val into = experimental("into")

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -260,7 +260,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           then
             altImports.uncheckedNN += altImp
 
-        if altImports != null && ctx.isImportContext then
+        if Feature.enabled(Feature.relaxedExtensionImports) && altImports != null && ctx.isImportContext then
           val curImport = ctx.importInfo.uncheckedNN
           namedImportRef(curImport) match
             case altImp: TermRef =>

docs/_docs/reference/contextual/extension-methods.md

@@ -252,7 +252,9 @@ The following two rewritings are tried in order:
     If only one import leads to an expansion that typechecks without errors, pick
     that expansion. If there are several such imports, but only one import which is
     not a wildcard import, pick the expansion from that import. Otherwise, report
-    an ambiguous reference error.    
+    an ambiguous reference error.
+
+    **Note**: This relaxation is currently enabled only under the `experimental.relaxedExtensionImports` language import.
 
  2. If the first rewriting does not typecheck with expected type `T`,
     and there is an extension method `m` in some eligible object `o`, the selection is rewritten to `o.m[Ts](e)`. An object `o` is _eligible_ if

library/src/scala/runtime/stdLibPatches/language.scala

@@ -69,6 +69,14 @@ object language:
     @compileTimeOnly("`clauseInterleaving` can only be used at compile time in import statements")
     object clauseInterleaving
 
+    /** Adds support for relaxed imports of extension methods.
+      * Extension methods with the same name can be imported from several places.
+      *
+      * @see [[http://dotty.epfl.ch/docs/reference/contextual/extension-methods]]
+      */
+    @compileTimeOnly("`relaxedExtensionImports` can only be used at compile time in import statements")
+    object relaxedExtensionImports
+
     /** Experimental support for pure function type syntax
      *
      *  @see [[https://dotty.epfl.ch/docs/reference/experimental/purefuns]]

tests/neg/i13558.scala

@@ -0,0 +1,31 @@
+package testcode
+
+class A
+
+class B
+
+object ExtensionA {
+  extension (self: A) {
+    def id = "A"
+  }
+}
+object ExtensionB {
+  extension (self: B) {
+    def id = "B"
+  }
+}
+
+object Main {
+  def main1(args: Array[String]): Unit = {
+    import ExtensionB._
+    import ExtensionA._
+    val a = A()
+    println(a.id) // error
+  }
+  def main2(args: Array[String]): Unit = {
+    import ExtensionA._
+    import ExtensionB._
+    val a = A()
+    println(a.id) // error
+  }
+}

tests/neg/i16920.check

@@ -1,5 +1,5 @@
--- [E008] Not Found Error: tests/neg/i16920.scala:18:11 ----------------------------------------------------------------
-18 |    "five".wow // error
+-- [E008] Not Found Error: tests/neg/i16920.scala:20:11 ----------------------------------------------------------------
+20 |    "five".wow // error
    |    ^^^^^^^^^^
    |    value wow is not a member of String.
    |    An extension method was tried, but could not be fully constructed:
@@ -10,8 +10,8 @@
    |
    |            Found:    ("five" : String)
    |            Required: Int
--- [E008] Not Found Error: tests/neg/i16920.scala:26:6 -----------------------------------------------------------------
-26 |    5.wow // error
+-- [E008] Not Found Error: tests/neg/i16920.scala:28:6 -----------------------------------------------------------------
+28 |    5.wow // error
    |    ^^^^^
    |    value wow is not a member of Int.
    |    An extension method was tried, but could not be fully constructed:
@@ -22,8 +22,8 @@
    |
    |            Found:    (5 : Int)
    |            Required: Boolean
--- [E008] Not Found Error: tests/neg/i16920.scala:27:11 ----------------------------------------------------------------
-27 |    "five".wow // error
+-- [E008] Not Found Error: tests/neg/i16920.scala:29:11 ----------------------------------------------------------------
+29 |    "five".wow // error
    |    ^^^^^^^^^^
    |    value wow is not a member of String.
    |    An extension method was tried, but could not be fully constructed:
@@ -34,8 +34,8 @@
    |
    |            Found:    ("five" : String)
    |            Required: Boolean
--- [E008] Not Found Error: tests/neg/i16920.scala:34:6 -----------------------------------------------------------------
-34 |    5.wow // error
+-- [E008] Not Found Error: tests/neg/i16920.scala:36:6 -----------------------------------------------------------------
+36 |    5.wow // error
    |    ^^^^^
    |    value wow is not a member of Int.
    |    An extension method was tried, but could not be fully constructed:
@@ -48,8 +48,8 @@
    |            both Three.wow(5)
    |            and  Two.wow(5)
    |            are possible expansions of 5.wow
--- [E008] Not Found Error: tests/neg/i16920.scala:42:11 ----------------------------------------------------------------
-42 |    "five".wow // error
+-- [E008] Not Found Error: tests/neg/i16920.scala:44:11 ----------------------------------------------------------------
+44 |    "five".wow // error
    |    ^^^^^^^^^^
    |    value wow is not a member of String.
    |    An extension method was tried, but could not be fully constructed:
@@ -60,8 +60,8 @@
    |
    |            Found:    ("five" : String)
    |            Required: Int
--- [E008] Not Found Error: tests/neg/i16920.scala:49:11 ----------------------------------------------------------------
-49 |    "five".wow // error
+-- [E008] Not Found Error: tests/neg/i16920.scala:51:11 ----------------------------------------------------------------
+51 |    "five".wow // error
    |    ^^^^^^^^^^
    |    value wow is not a member of String.
    |    An extension method was tried, but could not be fully constructed:
@@ -72,8 +72,8 @@
    |
    |            Found:    ("five" : String)
    |            Required: Int
--- [E008] Not Found Error: tests/neg/i16920.scala:56:6 -----------------------------------------------------------------
-56 |    5.wow // error
+-- [E008] Not Found Error: tests/neg/i16920.scala:58:6 -----------------------------------------------------------------
+58 |    5.wow // error
    |    ^^^^^
    |    value wow is not a member of Int.
    |    An extension method was tried, but could not be fully constructed:

tests/neg/i16920.scala

@@ -1,3 +1,5 @@
+import language.experimental.relaxedExtensionImports
+
 object One:
   extension (s: String)
     def wow: Unit = println(s)

tests/pos/i13558.scala

@@ -1,4 +1,5 @@
 package testcode
+import language.experimental.relaxedExtensionImports
 
 class A
 

tests/pos/i16920.scala

@@ -1,3 +1,5 @@
+import language.experimental.relaxedExtensionImports
+
 object One:
   extension (s: String)
     def wow: Unit = println(s)