Print double definitions signatures at Typer

Fixes #14966

Author: odersky

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

@@ -2174,10 +2174,12 @@ import transform.SymUtils._
         else
           "Name clash between inherited members"
 
-      em"""$clashDescription:
-          |${previousDecl.showDcl} ${symLocation(previousDecl)} and
-          |${decl.showDcl} ${symLocation(decl)}
-          |""" + details
+      atPhase(typerPhase) {
+        em"""$clashDescription:
+            |${previousDecl.showDcl} ${symLocation(previousDecl)} and
+            |${decl.showDcl} ${symLocation(decl)}
+            |"""
+      } + details
     }
     def explain = ""
   }

tests/neg/i14966.check

@@ -0,0 +1,10 @@
+-- [E120] Naming Error: tests/neg/i14966.scala:2:11 --------------------------------------------------------------------
+2 |  export s.* // error
+  |           ^
+  |           Double definition:
+  |           final def concat[B >: T](suffix: IterableOnce[B]): Set[B] in class B at line 2 and
+  |           final def concat(that: IterableOnce[T]): Set[T] in class B at line 2
+  |           have the same type after erasure.
+  |
+  |           Consider adding a @targetName annotation to one of the conflicting definitions
+  |           for disambiguation.

tests/neg/i14966.scala

@@ -0,0 +1,2 @@
+class B[T](val s: Set[T]):
+  export s.* // error