Fix imported twice error messages

compiler/src/dotty/tools/dotc/ast/untpd.scala

@@ -137,6 +137,8 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
     val rename: TermName = renamed match
       case Ident(rename: TermName) => rename
       case _ => name
+
+    def isUnimport = rename == nme.WILDCARD
   }
 
   case class Number(digits: String, kind: NumberKind)(implicit @constructorOnly src: SourceFile) extends TermTree

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

@@ -135,7 +135,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case JavaSymbolIsNotAValueID // errorNumber: 119
   case DoubleDefinitionID // errorNumber: 120
   case MatchCaseOnlyNullWarningID // errorNumber: 121
-  case ImportRenamedTwiceID // errorNumber: 122
+  case ImportedTwiceID // errorNumber: 122
   case TypeTestAlwaysDivergesID // errorNumber: 123
   case TermMemberNeedsNeedsResultTypeForImplicitSearchID // errorNumber: 124
   case ClassCannotExtendEnumID // errorNumber: 125
@@ -198,6 +198,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case NotConstantID // errorNumber: 182
   case ClosureCannotHaveInternalParameterDependenciesID // errorNumber: 183
   case MatchTypeNoCasesID // errorNumber: 184
+  case UnimportedAndImportedID // errorNumber: 185
 
   def errorNumber = ordinal - 1
 

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

@@ -2234,8 +2234,15 @@ extends NamingMsg(DoubleDefinitionID) {
   def explain(using Context) = ""
 }
 
-class ImportRenamedTwice(ident: untpd.Ident)(using Context) extends SyntaxMsg(ImportRenamedTwiceID) {
-  def msg(using Context) = s"${ident.show} is renamed twice on the same import line."
+class ImportedTwice(sel: Name)(using Context) extends SyntaxMsg(ImportedTwiceID) {
+  def msg(using Context) = s"${sel.show} is imported twice on the same import line."
+  def explain(using Context) = ""
+}
+
+class UnimportedAndImported(sel: Name, isImport: Boolean)(using Context) extends SyntaxMsg(UnimportedAndImportedID) {
+  def msg(using Context) =
+    val otherStr = if isImport then "and imported" else "twice"
+    s"${sel.show} is unimported $otherStr on the same import line."
   def explain(using Context) = ""
 }
 

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

@@ -1511,17 +1511,23 @@ trait Checking {
    *  (2) Check that no import selector is renamed more than once.
    */
   def checkImportSelectors(qualType: Type, selectors: List[untpd.ImportSelector])(using Context): Unit =
-    val seen = mutable.Set.empty[Name]
+    val originals = mutable.Set.empty[Name]
+    val targets = mutable.Set.empty[Name]
 
     def checkIdent(sel: untpd.ImportSelector): Unit =
       if sel.name != nme.ERROR
           && !qualType.member(sel.name).exists
           && !qualType.member(sel.name.toTypeName).exists
       then
         report.error(NotAMember(qualType, sel.name, "value"), sel.imported.srcPos)
-      if seen.contains(sel.name) then
-        report.error(ImportRenamedTwice(sel.imported), sel.imported.srcPos)
-      seen += sel.name
+      if sel.isUnimport then
+        if originals.contains(sel.name) then
+          report.error(UnimportedAndImported(sel.name, targets.contains(sel.name)), sel.imported.srcPos)
+      else
+        if targets.contains(sel.rename) then
+          report.error(ImportedTwice(sel.rename), sel.renamed.orElse(sel.imported).srcPos)
+        targets += sel.rename
+      originals += sel.name
 
     if !ctx.compilationUnit.isJava then
       for sel <- selectors do

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

@@ -109,7 +109,7 @@ class ImportInfo(symf: Context ?=> Symbol,
       else
         if sel.rename != sel.name then
           myExcluded = myExcluded.nn + sel.name
-        if sel.rename != nme.WILDCARD then
+        if !sel.isUnimport then
           myForwardMapping = myForwardMapping.uncheckedNN.updated(sel.name, sel.rename)
           myReverseMapping = myReverseMapping.uncheckedNN.updated(sel.rename, sel.name)
 

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

@@ -1309,7 +1309,7 @@ class Namer { typer: Typer =>
           if sel.isWildcard then
             addWildcardForwarders(seen, sel.span)
           else
-            if sel.rename != nme.WILDCARD then
+            if !sel.isUnimport then
               addForwardersNamed(sel.name, sel.rename, sel.span)
             addForwarders(sels1, sel.name :: seen)
         case _ =>

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

@@ -325,7 +325,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                 fail(em"reference to `$name` is ambiguous; it is imported twice")
               found
 
-            if selector.rename == termName && selector.rename != nme.WILDCARD then
+            if selector.rename == termName && !selector.isUnimport then
               val memberName =
                 if selector.name == termName then name
                 else if name.isTypeName then selector.name.toTypeName

tests/neg/i18082.check

@@ -0,0 +1,20 @@
+-- [E122] Syntax Error: tests/neg/i18082.scala:5:23 --------------------------------------------------------------------
+5 |import O.{a as b, b as b} // error
+  |                       ^
+  |                       b is imported twice on the same import line.
+-- [E122] Syntax Error: tests/neg/i18082.scala:6:18 --------------------------------------------------------------------
+6 |import O.{a as b, b} // error
+  |                  ^
+  |                  b is imported twice on the same import line.
+-- [E122] Syntax Error: tests/neg/i18082.scala:7:13 --------------------------------------------------------------------
+7 |import O.{a, a} // error
+  |             ^
+  |             a is imported twice on the same import line.
+-- [E185] Syntax Error: tests/neg/i18082.scala:10:18 -------------------------------------------------------------------
+10 |import O.{a as _, a as _} // error
+   |                  ^
+   |                  a is unimported twice on the same import line.
+-- [E185] Syntax Error: tests/neg/i18082.scala:11:13 -------------------------------------------------------------------
+11 |import O.{a, a as _} // error
+   |             ^
+   |             a is unimported and imported on the same import line.

tests/neg/i18082.scala

@@ -0,0 +1,12 @@
+object O { val a = 1; val b = 2 }
+
+import O.{a as b, a} // OK
+import O.{a as b, b as a} // OK
+import O.{a as b, b as b} // error
+import O.{a as b, b} // error
+import O.{a, a} // error
+
+import O.{a as _, b as _} // ok
+import O.{a as _, a as _} // error
+import O.{a, a as _} // error
+