Help renaming conflicting givens

Author: som-snytt

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -2125,8 +2125,27 @@ extends NamingMsg(AlreadyDefinedID):
         i" in ${conflicting.associatedFile}"
       else if conflicting.owner == owner then ""
       else i" in ${conflicting.owner}"
+    def print(tpe: Type): String =
+      def addParams(tpe: Type): List[String] = tpe match
+        case tpe: MethodType =>
+          val s = if tpe.isContextualMethod then i"(${tpe.paramInfos}%, %) =>" else ""
+          s :: addParams(tpe.resType)
+        case tpe: PolyType =>
+          i"[${tpe.paramNames}%, %] =>" :: addParams(tpe.resType)
+        case tpe =>
+          i"$tpe" :: Nil
+      addParams(tpe).mkString(" ")
     def note =
-      if owner.is(Method) || conflicting.is(Method) then
+      if conflicting.is(Given) && name.startsWith("given_") then
+        i"""|
+            |
+            |Provide an explicit, unique name to given definitions,
+            |since the names assigned to anonymous givens may clash. For example:
+            |
+            |      given myGiven: ${print(atPhase(typerPhase)(conflicting.info))}  // define an instance
+            |      given myGiven @ ${print(atPhase(typerPhase)(conflicting.info))} // as a pattern variable
+            |"""
+      else if owner.is(Method) || conflicting.is(Method) then
         "\n\nNote that overloaded methods must all be defined in the same group of toplevel definitions"
       else ""
     if conflicting.isTerm != name.isTermName then

tests/neg/i23119.check

@@ -1,6 +1,20 @@
--- [E161] Naming Error: tests/neg/i23119.scala:7:4 ---------------------------------------------------------------------
-7 |    given Option[List[Int]] = Some(List(x)) // error
+-- [E161] Naming Error: tests/neg/i23119.scala:8:4 ---------------------------------------------------------------------
+8 |    given Option[List[Int]] = Some(List(x)) // error
   |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |    given_Option_List is already defined as given instance given_Option_List
   |
-  |    Note that overloaded methods must all be defined in the same group of toplevel definitions
+  |    Provide an explicit, unique name to given definitions,
+  |    since the names assigned to anonymous givens may clash. For example:
+  |
+  |          given myGiven: Option[List[String]]  // define an instance
+  |          given myGiven @ Option[List[String]] // as a pattern variable
+-- [E161] Naming Error: tests/neg/i23119.scala:18:8 --------------------------------------------------------------------
+18 |  given [A] => List[A] = ??? // error
+   |  ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |  given_List_A is already defined as given instance given_List_A
+   |
+   |  Provide an explicit, unique name to given definitions,
+   |  since the names assigned to anonymous givens may clash. For example:
+   |
+   |        given myGiven: [A] => List[A]  // define an instance
+   |        given myGiven @ [A] => List[A] // as a pattern variable

tests/neg/i23119.scala

@@ -1,3 +1,4 @@
+//> using options -explain
 
 @main def test = println:
   for x <- 1 to 2
@@ -11,3 +12,8 @@
 //  given_Option_List is already defined as given instance given_Option_List
 // Previously the naming clash was noticed when extracting values in the map or do function:
 //  duplicate pattern variable: given_Option_List
+
+def also =
+  given [A] => List[A] = ???
+  given [A] => List[A] = ??? // error
+  ()