Improve message for nested package missing braces

Move the caret closer to the real problem; clarify the error.

Author: som-snytt

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -4692,6 +4692,10 @@ object Parsers {
     def packaging(start: Int): Tree =
       val pkg = qualId()
       possibleTemplateStart()
+      if in.token != INDENT && in.token != LBRACE then
+        val prefix = "':' or "
+        val suffix = "\nNested package statements that are not at the beginning of the file require braces or `:` with an indented body."
+        syntaxErrorOrIncomplete(ExpectedTokenButFound(LBRACE, in.token, prefix = prefix, suffix = suffix), in.lastOffset)
       val stats = inDefScopeBraces(topStatSeq(), rewriteWithColon = true)
       makePackaging(start, pkg, stats)
 

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

@@ -1231,7 +1231,7 @@ extends ReferenceMsg(ForwardReferenceExtendsOverDefinitionID) {
         |"""
 }
 
-class ExpectedTokenButFound(expected: Token, found: Token, prefix: String = "")(using Context)
+class ExpectedTokenButFound(expected: Token, found: Token, prefix: String = "", suffix: String = "")(using Context)
 extends SyntaxMsg(ExpectedTokenButFoundID) {
 
   private def foundText = Tokens.showToken(found)
@@ -1240,7 +1240,7 @@ extends SyntaxMsg(ExpectedTokenButFoundID) {
     val expectedText =
       if (Tokens.isIdentifier(expected)) "an identifier"
       else Tokens.showToken(expected)
-    i"""$prefix$expectedText expected, but $foundText found"""
+    i"""$prefix$expectedText expected, but $foundText found$suffix"""
 
   def explain(using Context) =
     if (Tokens.isIdentifier(expected) && Tokens.isKeyword(found))

tests/neg/i23815.check

@@ -0,0 +1,9 @@
+-- [E040] Syntax Error: tests/neg/d17874.scala:7:10 --------------------------------------------------------------------
+7 |package t2 // error
+  |          ^
+  |  ':' or '{' expected, but 'end of statement' found
+  |  Nested package statements that are not at the beginning of the file require braces or `:` with an indented body.
+-- [E040] Syntax Error: tests/neg/d17874.scala:10:0 --------------------------------------------------------------------
+10 |
+   |^
+   |'}' expected, but eof found

tests/neg/i23815.scala

@@ -0,0 +1,9 @@
+
+package test6
+
+package t1
+trait T1
+
+package t2 // error
+trait T2
+// anypos-error