Forward reference error includes line numbers

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

@@ -1190,7 +1190,10 @@ extends DeclarationMsg(OverrideErrorID), NoDisambiguation:
 
 class ForwardReferenceExtendsOverDefinition(value: Symbol, definition: Symbol)(using Context)
 extends ReferenceMsg(ForwardReferenceExtendsOverDefinitionID) {
-  def msg(using Context) = i"${definition.name} is a forward reference extending over the definition of ${value.name}"
+  extension (sym: Symbol) def srcLine = sym.line + 1
+  def msg(using Context) = i"${definition.name}${
+    if value != definition then s" (defined on L${definition.srcLine})" else ""
+  } is a forward reference extending over the definition of ${value.name} (on L${value.srcLine})"
 
   def explain(using Context) =
     i"""|${definition.name} is used before you define it, and the definition of ${value.name}

tests/neg/i14401.check

@@ -0,0 +1,24 @@
+-- [E039] Reference Error: tests/neg/i14401.scala:6:17 -----------------------------------------------------------------
+6 |    def f: Int = x; // error
+  |                 ^
+  |                 x is a forward reference extending over the definition of x (on L7)
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E039] Reference Error: tests/neg/i14401.scala:10:17 ----------------------------------------------------------------
+10 |    def f: Int = g; // error
+   |                 ^
+   |                 g (defined on L12) is a forward reference extending over the definition of x (on L11)
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E039] Reference Error: tests/neg/i14401.scala:15:17 ----------------------------------------------------------------
+15 |    def f: Int = g; // error
+   |                 ^
+   |                 g (defined on L17) is a forward reference extending over the definition of x (on L16)
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E039] Reference Error: tests/neg/i14401.scala:28:28 ----------------------------------------------------------------
+28 |    class NotUsed {val xs = args} // error
+   |                            ^^^^
+   |                    args (defined on L31) is a forward reference extending over the definition of dummy (on L29)
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg/i14401.scala

@@ -0,0 +1,33 @@
+object Test {
+  def f: Int = x;
+  val x: Int = f;
+
+  {
+    def f: Int = x; // error
+    val x: Int = f;
+  }
+  {
+    def f: Int = g; // error
+    val x: Int = f;
+    def g: Int = x;
+  }
+  {
+    def f: Int = g; // error
+    var x: Int = f;
+    def g: Int = x;
+  }
+  {
+    def f: Int = g;
+    Console.println("foo");
+    def g: Int = f;
+  }
+}
+
+object MyApp {
+  def main(args: Array[String]) = {
+    class NotUsed {val xs = args} // error
+    val dummy = false
+    // oops, shadows the parameter
+    def args = Seq("a","b","c")
+  }
+}