Detect clashes involving renamed exports

Fixes scala#14818

Author: odersky

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

@@ -1072,11 +1072,23 @@ class Namer { typer: Typer =>
     /** The forwarders defined by export `exp` */
     private def exportForwarders(exp: Export)(using Context): List[tpd.MemberDef] =
       val buf = new mutable.ListBuffer[tpd.MemberDef]
-      val Export(expr, selectors) = exp
+      val Export(expr, selectors0) = exp
       if expr.isEmpty then
         report.error(em"Export selector must have prefix and `.`", exp.srcPos)
         return Nil
 
+      val renamed = mutable.Set[Name]()
+      val selectors = selectors0 map {
+        case sel @ ImportSelector(imported, id @ Ident(alias), bound) if alias != nme.WILDCARD =>
+          if renamed.contains(alias) then
+            report.error(i"duplicate rename target", id.srcPos)
+            cpy.ImportSelector(sel)(imported, EmptyTree, bound).asInstanceOf[ImportSelector]
+          else
+            renamed += alias
+            sel
+        case sel => sel
+      }
+
       val path = typedAheadExpr(expr, AnySelectionProto)
       checkLegalExportPath(path, selectors)
       lazy val wildcardBound = importBound(selectors, isGiven = false)
@@ -1091,6 +1103,8 @@ class Namer { typer: Typer =>
           Skip
         else if cls.derivesFrom(sym.owner) && (sym.owner == cls || !sym.is(Deferred)) then
           No(i"is already a member of $cls")
+        else if renamed.contains(sym.name.toTermName) then
+          No(i"clashes with a renamed export")
         else if sym.is(Override) then
           sym.allOverriddenSymbols.find(
             other => cls.derivesFrom(other.owner) && !other.is(Deferred)

tests/neg/i14818.check

@@ -0,0 +1,15 @@
+-- Error: tests/neg/i14818.scala:9:12 ----------------------------------------------------------------------------------
+9 |  export M.{A, B as A}  // error
+  |            ^
+  |            no eligible member A at M
+  |            M.A cannot be exported because it clashes with a renamed export
+-- [E050] Type Error: tests/neg/i14818.scala:16:10 ---------------------------------------------------------------------
+16 |  val x = b(1)  // error
+   |          ^
+   |          method b in object T3 does not take parameters
+   |
+   | longer explanation available when compiling with `-explain`
+-- Error: tests/neg/i14818.scala:19:25 ---------------------------------------------------------------------------------
+19 |  export M.{A as C, B as C} // error
+   |                         ^
+   |                         duplicate rename target

tests/neg/i14818.scala

@@ -0,0 +1,20 @@
+object M {
+  type A
+  type B
+  def a = 1
+  def b(x: Int) = x
+}
+
+object T1:
+  export M.{A, B as A}  // error
+
+object T2:
+  export M.{A as B, *}
+
+object T3:
+  export M.{a as b, *}
+  val x = b(1)  // error
+
+object T4:
+  export M.{A as C, B as C} // error
+