Add hint to error message

In case of a double definition error that's due to an erasure clash,
add a hint that one can resolve it with a @TargetNAME annotation

Author: odersky

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

@@ -2079,7 +2079,10 @@ import transform.SymUtils._
             case MethodNotAMethodMatch =>
               "neither has parameters."
             case FullMatch =>
-              i"have the same$nameAnd type after erasure."
+              i"""have the same$nameAnd type after erasure.
+                 |
+                 |Consider adding a @targetName annotation to one of the conflicting definitions
+                 |for disambiguation."""
           }
         }
         else ""

tests/neg/doubleDefinition.check

@@ -5,6 +5,9 @@
    |	    def foo(x: List[A]): A => A in class Test2 at line 13 and
    |	    def foo(x: List[B]): B => B in class Test2 at line 14
    |	    have the same type after erasure.
+   |
+   |	    Consider adding a @targetName annotation to one of the conflicting definitions
+   |	    for disambiguation.
 -- [E120] Naming Error: tests/neg/doubleDefinition.scala:21:4 ----------------------------------------------------------
 21 |	def foo(x: List[A]): Function2[B, B, B] = ??? // error
    |	    ^
@@ -43,6 +46,9 @@
    |	    def foo(x: List[A]): A => A in trait Test6 at line 54 and
    |	    def foo(x: List[B]): B => B in trait Test6 at line 55
    |	    have the same type after erasure.
+   |
+   |	    Consider adding a @targetName annotation to one of the conflicting definitions
+   |	    for disambiguation.
 -- [E120] Naming Error: tests/neg/doubleDefinition.scala:62:4 ----------------------------------------------------------
 62 |	def foo(x: List[A]): Function2[B, B, B] = ??? // error
    |	    ^

tests/neg/exports.check

@@ -19,20 +19,29 @@
    |                                 def status: => List[String] in class Copier at line 28 and
    |                                 final def status: => List[String] in class Copier at line 23
    |                                 have the same type after erasure.
+   |
+   |                                 Consider adding a @targetName annotation to one of the conflicting definitions
+   |                                 for disambiguation.
 -- [E120] Naming Error: tests/neg/exports.scala:24:20 ------------------------------------------------------------------
 24 |    export scanUnit._               // error: double definition
    |                    ^
    |                    Double definition:
    |                    final def status: => List[String] in class Copier at line 23 and
    |                    final def status: => List[String] in class Copier at line 24
    |                    have the same type after erasure.
+   |
+   |                    Consider adding a @targetName annotation to one of the conflicting definitions
+   |                    for disambiguation.
 -- [E120] Naming Error: tests/neg/exports.scala:26:21 ------------------------------------------------------------------
 26 |    export printUnit.status         // error: double definition
    |                     ^
    |                     Double definition:
    |                     final def status: => List[String] in class Copier at line 24 and
    |                     final def status: => List[String] in class Copier at line 26
    |                     have the same type after erasure.
+   |
+   |                     Consider adding a @targetName annotation to one of the conflicting definitions
+   |                     for disambiguation.
 -- Error: tests/neg/exports.scala:35:24 --------------------------------------------------------------------------------
 35 |    export this.{concat => ++}   // error: no eligible member
    |                 ^^^^^^^^^^^^

tests/neg/i7359-f.check

@@ -5,3 +5,6 @@
   |      def equals: [T >: Boolean <: Boolean](obj: Any): T in trait SAMTrait at line 3 and
   |      def equals(x$0: Any): Boolean in class Any
   |      have the same type after erasure.
+  |
+  |      Consider adding a @targetName annotation to one of the conflicting definitions
+  |      for disambiguation.

tests/neg/mixin-forwarder-clash1.check

@@ -5,3 +5,6 @@
    |      def concat(suffix: Int): X in trait One at line 4 and
    |      def concat: [Dummy](suffix: Int): Y in trait Two at line 8
    |      have the same type after erasure.
+   |
+   |      Consider adding a @targetName annotation to one of the conflicting definitions
+   |      for disambiguation.

tests/neg/mixin-forwarder-clash2.check

@@ -6,3 +6,6 @@
   |      def concat(suffix: Int): X in trait One at line 4 and
   |      def concat: [Dummy](suffix: Int): Y in trait Two at line 8
   |      have the same type after erasure.
+  |
+  |      Consider adding a @targetName annotation to one of the conflicting definitions
+  |      for disambiguation.

tests/neg/override-erasure-clash.check

@@ -5,3 +5,6 @@
   |      def f(): Int in class A at line 3 and
   |      def g(): Int in class B at line 5
   |      have the same name and type after erasure.
+  |
+  |      Consider adding a @targetName annotation to one of the conflicting definitions
+  |      for disambiguation.