Detect double toplevel definitions in separate files

Flag as error if two separate files contain toplevel definitions with
the same name.

Author: odersky

Diff for: compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -302,21 +302,31 @@ class Namer { typer: Typer =>
 
     typr.println(i"creating symbol for $tree in ${ctx.mode}")
 
-    def checkNoConflict(name: Name): Name = {
+    def checkNoConflict(name: Name): Name =
       def errorName(msg: => String) = {
         ctx.error(msg, tree.sourcePos)
         name.freshened
       }
+      def conflict(conflicting: Symbol) =
+        val f = conflicting.associatedFile
+        val where = if f == null then "" else i" in $f"
+        errorName(i"$name is already defined as $conflicting$where")
+
       def preExisting = ctx.effectiveScope.lookup(name)
-      if (ctx.owner.is(PackageClass))
-        if (preExisting.isDefinedInCurrentRun)
-          errorName(s"${preExisting.showLocated} has already been compiled once during this run")
-        else name
-      else
-        if ((!ctx.owner.isClass || name.isTypeName) && preExisting.exists)
-          errorName(i"$name is already defined as $preExisting")
+      if ctx.owner.is(PackageClass) then
+        if preExisting.isDefinedInCurrentRun then conflict(preExisting)
         else name
-    }
+      else if ctx.owner.isPackageObject && name != nme.CONSTRUCTOR then
+        def conflicts(sym: Symbol) =
+          sym.is(Package)
+          || sym.isDefinedInCurrentRun && sym.associatedFile != ctx.compilationUnit.source.file
+        ctx.owner.skipWeakOwner.info.member(name).altsWith(conflicts) match
+          case conflicting :: _ => conflict(conflicting.symbol)
+          case _ => name
+      else if (!ctx.owner.isClass || name.isTypeName) && preExisting.exists then
+        errorName(i"$name is already defined as $preExisting")
+      else name
+    end checkNoConflict
 
     /** Create new symbol or redefine existing symbol under lateCompile. */
     def createOrRefine[S <: Symbol](

Diff for: tests/neg/toplevel-doubledef/defs_1.scala

@@ -0,0 +1,2 @@
+def hello(x: String): Unit = // error: has already been compiled
+  println(s"hello: $x")

Diff for: tests/neg/toplevel-doubledef/moredefs_1.scala

@@ -0,0 +1,2 @@
+def hello(x: String): Unit =
+  println(s"hi, $x")

Diff for: tests/neg/toplevel-overload/defs_1.scala

@@ -0,0 +1,3 @@
+trait A
+
+def f(x: A) = s"A"   // error: has already been compiled

Diff for: tests/neg/toplevel-overload/moredefs_1.scala

@@ -0,0 +1,3 @@
+trait B
+
+def f(x: B) = s"B"

Diff for: tests/neg/toplevel-package/defs_1.scala

@@ -0,0 +1,6 @@
+package foo
+package bar
+  class C
+  def hello(x: String): Unit =
+    println(s"hello: $x")
+

Diff for: tests/neg/toplevel-package/moredefs_2.scala

@@ -0,0 +1,2 @@
+package foo
+def bar = 2  // error: bar is a package

Diff for: tests/run/toplevel-mixed/B_1.scala

@@ -9,5 +9,5 @@ object X2 {
   def bar = "hi"
 }
 
-class X3
+class X4