Make SIP 54 (Multi-Source Extension Overloads) a standard feature

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

@@ -29,7 +29,6 @@ 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/reporting/messages.scala

@@ -1336,7 +1336,8 @@ class ConstrProxyShadows(proxy: TermRef, shadowed: Type, shadowedIsApply: Boolea
        |or use a full prefix for ${shadowed.termSymbol.name} if you mean the latter."""
 end ConstrProxyShadows
 
-class AmbiguousReference(name: Name, newPrec: BindingPrec, prevPrec: BindingPrec, prevCtx: Context)(using Context)
+class AmbiguousReference(
+    name: Name, newPrec: BindingPrec, prevPrec: BindingPrec, prevCtx: Context, isExtension: => Boolean = false)(using Context)
   extends ReferenceMsg(AmbiguousReferenceID), NoDisambiguation {
 
   /** A string which explains how something was bound; Depending on `prec` this is either
@@ -1358,10 +1359,17 @@ class AmbiguousReference(name: Name, newPrec: BindingPrec, prevPrec: BindingPrec
       i"""$howVisible$qualifier in ${whereFound.owner}"""
   }
 
+  def importHint =
+    if (newPrec == BindingPrec.NamedImport || newPrec == BindingPrec.WildImport)
+        && prevPrec == newPrec
+        && isExtension
+    then i"\n\n Hint: This error may arise if extension method `$name` is called as a normal method."
+    else ""
+
   def msg(using Context) =
     i"""|Reference to $name is ambiguous.
         |It is both ${bindingString(newPrec, ctx)}
-        |and ${bindingString(prevPrec, prevCtx, " subsequently")}"""
+        |and ${bindingString(prevPrec, prevCtx, " subsequently")}$importHint"""
 
   def explain(using Context) =
     val precedent =

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

@@ -233,7 +233,8 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           found
         else
           if !scala2pkg && !previous.isError && !found.isError then
-            fail(AmbiguousReference(name, newPrec, prevPrec, prevCtx))
+            fail(AmbiguousReference(name, newPrec, prevPrec, prevCtx,
+              isExtension = previous.termSymbol.is(ExtensionMethod) && found.termSymbol.is(ExtensionMethod)))
           previous
 
       /** Assemble and check alternatives to an imported reference. This implies:
@@ -260,7 +261,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           then
             altImports.uncheckedNN += altImp
 
-        if Feature.enabled(Feature.relaxedExtensionImports) && altImports != null && ctx.isImportContext then
+        if altImports != null && ctx.isImportContext then
           val curImport = ctx.importInfo.uncheckedNN
           namedImportRef(curImport) match
             case altImp: TermRef =>

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

@@ -254,7 +254,8 @@ The following two rewritings are tried in order:
     not a wildcard import, pick the expansion from that import. Otherwise, report
     an ambiguous reference error.
 
-    **Note**: This relaxation is currently enabled only under the `experimental.relaxedExtensionImports` language import.
+    **Note**: This relaxation of the import rules applies only if the method `m` is used as an extension method. If it is used as a normal method in prefix form, the usual import rules apply, which means that importing `m` from
+    multiple places can lead to an ambiguity error.
 
  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

@@ -69,14 +69,6 @@ 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/i16920.check

@@ -1,5 +1,5 @@
--- [E008] Not Found Error: tests/neg/i16920.scala:20:11 ----------------------------------------------------------------
-20 |    "five".wow // error
+-- [E008] Not Found Error: tests/neg/i16920.scala:19:11 ----------------------------------------------------------------
+19 |    "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:28:6 -----------------------------------------------------------------
-28 |    5.wow // error
+-- [E008] Not Found Error: tests/neg/i16920.scala:27:6 -----------------------------------------------------------------
+27 |    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:29:11 ----------------------------------------------------------------
-29 |    "five".wow // error
+-- [E008] Not Found Error: tests/neg/i16920.scala:28:11 ----------------------------------------------------------------
+28 |    "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:36:6 -----------------------------------------------------------------
-36 |    5.wow // error
+-- [E008] Not Found Error: tests/neg/i16920.scala:35:6 -----------------------------------------------------------------
+35 |    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:44:11 ----------------------------------------------------------------
-44 |    "five".wow // error
+-- [E008] Not Found Error: tests/neg/i16920.scala:43:11 ----------------------------------------------------------------
+43 |    "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:51:11 ----------------------------------------------------------------
-51 |    "five".wow // error
+-- [E008] Not Found Error: tests/neg/i16920.scala:50:11 ----------------------------------------------------------------
+50 |    "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:58:6 -----------------------------------------------------------------
-58 |    5.wow // error
+-- [E008] Not Found Error: tests/neg/i16920.scala:57:6 -----------------------------------------------------------------
+57 |    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

@@ -1,4 +1,3 @@
-import language.experimental.relaxedExtensionImports
 
 object One:
   extension (s: String)

tests/neg/sip54.check

@@ -0,0 +1,17 @@
+-- [E049] Reference Error: tests/neg/sip54.scala:12:8 ------------------------------------------------------------------
+12 |val _ = meth(foo)() // error // error
+   |        ^^^^
+   |        Reference to meth is ambiguous.
+   |        It is both imported by import A._
+   |        and imported subsequently by import B._
+   |
+   |         Hint: This error may arise if extension method `meth` is called as a normal method.
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg/sip54.scala:12:13 -------------------------------------------------------------
+12 |val _ = meth(foo)() // error // error
+   |             ^^^
+   |             Found:    (foo : Foo)
+   |             Required: Bar
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg/sip54.scala

@@ -0,0 +1,12 @@
+class Foo
+class Bar
+object A:
+  extension (foo: Foo) def meth(): Foo = foo
+object B:
+  extension (bar: Bar) def meth(): Bar = bar
+
+import A.*
+import B.*
+
+val foo = new Foo
+val _ = meth(foo)() // error // error

tests/pos/i13558.scala

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

tests/pos/i16920.scala

@@ -1,4 +1,3 @@
-import language.experimental.relaxedExtensionImports
 
 object One:
   extension (s: String)