Elide companion defs to a object extending AnyVal (#18451)

Fixes #18274

Author: nicolasstucki

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -820,7 +820,7 @@ object desugar {
       }
       else if (companionMembers.nonEmpty || companionDerived.nonEmpty || isEnum)
         companionDefs(anyRef, companionMembers)
-      else if (isValueClass)
+      else if isValueClass && !isObject then
         companionDefs(anyRef, Nil)
       else Nil
 

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

@@ -1686,10 +1686,15 @@ class CannotExtendAnyVal(sym: Symbol)(using Context)
   extends SyntaxMsg(CannotExtendAnyValID) {
   def msg(using Context) = i"""$sym cannot extend ${hl("AnyVal")}"""
   def explain(using Context) =
-    i"""Only classes (not traits) are allowed to extend ${hl("AnyVal")}, but traits may extend
-        |${hl("Any")} to become ${Green("\"universal traits\"")} which may only have ${hl("def")} members.
-        |Universal traits can be mixed into classes that extend ${hl("AnyVal")}.
-        |"""
+    if sym.is(Trait) then
+      i"""Only classes (not traits) are allowed to extend ${hl("AnyVal")}, but traits may extend
+          |${hl("Any")} to become ${Green("\"universal traits\"")} which may only have ${hl("def")} members.
+          |Universal traits can be mixed into classes that extend ${hl("AnyVal")}.
+          |"""
+    else if sym.is(Module) then
+      i"""Only classes (not objects) are allowed to extend ${hl("AnyVal")}.
+          |"""
+    else ""
 }
 
 class CannotExtendJavaEnum(sym: Symbol)(using Context)

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

@@ -711,6 +711,8 @@ object Checking {
     if (isDerivedValueClass(clazz)) {
       if (clazz.is(Trait))
         report.error(CannotExtendAnyVal(clazz), clazz.srcPos)
+      if clazz.is(Module) then
+        report.error(CannotExtendAnyVal(clazz), clazz.srcPos)
       if (clazz.is(Abstract))
         report.error(ValueClassesMayNotBeAbstract(clazz), clazz.srcPos)
       if (!clazz.isStatic)

tests/neg/i18274.check

@@ -0,0 +1,9 @@
+-- [E068] Syntax Error: tests/neg/i18274.scala:3:7 ---------------------------------------------------------------------
+3 |object Foo extends AnyVal // error
+  |       ^
+  |       object Foo cannot extend AnyVal
+  |---------------------------------------------------------------------------------------------------------------------
+  | Explanation (enabled by `-explain`)
+  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+  | Only classes (not objects) are allowed to extend AnyVal.
+   ---------------------------------------------------------------------------------------------------------------------

tests/neg/i18274.scala

@@ -0,0 +1,3 @@
+//> using options -explain
+
+object Foo extends AnyVal // error