Warning if code after an empty template body is indented

Under -noindent, warn id code that follows an empty template is
indented more deeply than the template header. Inserting braces
automatically would change meaning in this case.

Example:

```scala
  trait A
    case class B() extends A  // error: Line is indented too far to the right
    case object C extends A   // error: Line is indented too far to the right
```

Author: odersky

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

@@ -619,6 +619,16 @@ object Parsers {
         else body()
       case _ => body()
 
+    /** If indentation is not significant, check that this is not the start of a
+     *  statement that's indented relative to the current region.
+     */
+    def checkNextNotIndented(): Unit = in.currentRegion match
+      case r: InBraces if in.token == NEWLINE || in.token == NEWLINES =>
+        val nextIndentWidth = in.indentWidth(in.next.offset)
+        if r.indentWidth < nextIndentWidth then
+          warning(i"Line is indented too far to the right, or a `{' is missing", in.next.offset)
+      case _ =>
+
 /* -------- REWRITES ----------------------------------------------------------- */
 
     /** The last offset where a colon at the end of line would be required if a subsequent { ... }
@@ -3460,22 +3470,27 @@ object Parsers {
 
     /** TemplateOpt = [Template]
      */
-    def templateOpt(constr: DefDef): Template = {
+    def templateOpt(constr: DefDef): Template =
       possibleTemplateStart()
-      if (in.token == EXTENDS || isIdent(nme.derives))
+      if in.token == EXTENDS || isIdent(nme.derives) then
         template(constr)
       else
-        if (in.isNestedStart) template(constr)
-        else Template(constr, Nil, Nil, EmptyValDef, Nil)
-    }
+        if in.isNestedStart then
+          template(constr)
+        else
+          checkNextNotIndented()
+          Template(constr, Nil, Nil, EmptyValDef, Nil)
 
     /** TemplateBody ::= [nl] `{' TemplateStatSeq `}'
      */
-    def templateBodyOpt(constr: DefDef, parents: List[Tree], derived: List[Tree]): Template = {
+    def templateBodyOpt(constr: DefDef, parents: List[Tree], derived: List[Tree]): Template =
       val (self, stats) =
-        if (in.isNestedStart) templateBody() else (EmptyValDef, Nil)
+        if in.isNestedStart then
+          templateBody()
+        else
+          checkNextNotIndented()
+          (EmptyValDef, Nil)
       Template(constr, parents, derived, self, stats)
-    }
 
     def templateBody(): (ValDef, List[Tree]) = {
       val r = inDefScopeBraces { templateStatSeq() }

compiler/test/debug/Gen.scala

@@ -126,38 +126,38 @@ object Gen {
             |""".stripMargin
     }).mkString("\n\n")
 
-  s"""|#!/usr/bin/expect
-      |
-      |# log_user 1
-      |# exp_internal 1
-      |# set timeout 5
-      |
-      |send_user "spawning job...\\n"
-      |
-      |spawn jdb -attach 5005 -sourcepath $source
-      |
-      |send_user "interacting...\\n"
-      |
-      |expect {
-      |  "*VM Started*" { send_user "success - connected to server \\n" }
-      |  timeout {
-      |      send_user "timeout while waiting for: *VM Started*\\n"
-      |      exit 1
-      |  }
-      |}
-      |
-      |send_user "setting breakpoints...\\n"
-      |
-      |# breakpoints
-      |$breakpoints
-      |
-      |# run
-      |send_user "run program...\\n"
-      |send "run\\r"
-      |expect "Breakpoint hit"
-      |
-      |# interactions
-      |$commands""".stripMargin
+    s"""|#!/usr/bin/expect
+        |
+        |# log_user 1
+        |# exp_internal 1
+        |# set timeout 5
+        |
+        |send_user "spawning job...\\n"
+        |
+        |spawn jdb -attach 5005 -sourcepath $source
+        |
+        |send_user "interacting...\\n"
+        |
+        |expect {
+        |  "*VM Started*" { send_user "success - connected to server \\n" }
+        |  timeout {
+        |      send_user "timeout while waiting for: *VM Started*\\n"
+        |      exit 1
+        |  }
+        |}
+        |
+        |send_user "setting breakpoints...\\n"
+        |
+        |# breakpoints
+        |$breakpoints
+        |
+        |# run
+        |send_user "run program...\\n"
+        |send "run\\r"
+        |expect "Breakpoint hit"
+        |
+        |# interactions
+        |$commands""".stripMargin
   }
 
   def main(args: Array[String]): Unit = {

tests/neg-custom-args/indentRight.scala

@@ -17,6 +17,10 @@ object Test {
 
   }
 
+  trait A
+    case class B() extends A  // error: Line is indented too far to the right
+    case object C extends A   // error: Line is indented too far to the right
+
   if (true)   // OK
     println("hi")
   }