Support simple renaming imports in Typer

Support imports

    import a as b

where `a` is a simple identifier.

Author: odersky

compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -340,7 +340,7 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
 
       def selectorText(sel: untpd.ImportSelector): Text =
         val id: Text =
-          if sel.isGiven then keywordText("given") else toText(sel.imported)
+          if sel.isGiven then keywordText("given") else toText(sel.imported.name)
         val rename: Text =
           if sel.renamed.isEmpty then "" else Str(" => ") ~ toText(sel.renamed)
         val bound: Text =

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

@@ -693,7 +693,7 @@ class Namer { typer: Typer =>
     // make sure testing contexts are not captured by completers
     assert(!ictx.reporter.isInstanceOf[ExploringReporter])
 
-    protected def typeSig(sym: Symbol): Type = original match {
+    protected def typeSig(sym: Symbol): Type = original match
       case original: ValDef =>
         if (sym.is(Module)) moduleValSig(sym)
         else valOrDefDefSig(original, sym, Nil, identity)(using localContext(sym).setNewScope)
@@ -702,16 +702,12 @@ class Namer { typer: Typer =>
         nestedTyper(sym) = typer1
         typer1.defDefSig(original, sym)(using localContext(sym).setTyper(typer1))
       case imp: Import =>
-        try {
-          val expr1 = typedAheadExpr(imp.expr, AnySelectionProto)
+        try
+          val expr1 = typedImportQualifier(imp, typedAheadExpr)
           ImportType(expr1)
-        }
-        catch {
-          case ex: CyclicReference =>
-            typr.println(s"error while completing ${imp.expr}")
-            throw ex
-        }
-    }
+        catch case ex: CyclicReference =>
+          typr.println(s"error while completing ${imp.expr}")
+          throw ex
 
     final override def complete(denot: SymDenotation)(using Context): Unit = {
       if (Config.showCompletions && ctx.typerState != creationContext.typerState) {

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

@@ -2314,8 +2314,22 @@ class Typer extends Namer
         .asInstanceOf[untpd.ImportSelector]
     }
 
+  def typedImportQualifier(imp: untpd.Import, typd: (untpd.Tree, Type) => Tree)(using Context): Tree =
+    if imp.expr == untpd.EmptyTree then
+      assert(imp.selectors.length == 1, imp)
+      val from = imp.selectors.head.imported
+      val sel = typd(from, WildcardType)
+      sel.tpe.dealias match
+        case TermRef(prefix: SingletonType, _) =>
+          singleton(prefix).withSpan(from.span)
+        case _ =>
+          errorTree(from,
+            em"""Illegal import selector: $from
+                |The selector is not a member of an object or package.""")
+    else typd(imp.expr, AnySelectionProto)
+
   def typedImport(imp: untpd.Import, sym: Symbol)(using Context): Import = {
-    val expr1 = typedExpr(imp.expr, AnySelectionProto)
+    val expr1 = typedImportQualifier(imp, typedExpr)
     checkLegalImportPath(expr1)
     val selectors1 = typedSelectors(imp.selectors)
     assignType(cpy.Import(imp)(expr1, selectors1), sym)

tests/neg/renaming-imports.scala

@@ -0,0 +1,6 @@
+def test =
+  object foo:
+    var x = 0
+  import foo as f  // error
+  f.x
+

tests/pos/renaming-imports.scala

@@ -0,0 +1,9 @@
+import java as j
+import collection as c
+
+val x: j.io.IOException = ???
+
+import c.mutable as mut
+import mut.ArrayBuffer as Buf
+
+val y = Buf(1, 2, 3)