Make safe init checker skip global objects (#18906)

Decouple the checking of classes and global objects. The
`Ysafe-init-global` flag can be used for checking global objects
instead.

Author: olhotak

compiler/src/dotty/tools/dotc/transform/init/Semantic.scala

@@ -1140,7 +1140,7 @@ object Semantic:
    */
   def checkClasses(classes: List[ClassSymbol])(using Context): Unit =
     given Cache.Data()
-    for classSym <- classes if isConcreteClass(classSym) do
+    for classSym <- classes if isConcreteClass(classSym) && !classSym.isStaticObject do
       checkClass(classSym)
 
 // ----- Semantic definition --------------------------------

tests/init-global/neg/i15883.scala

@@ -0,0 +1,2 @@
+val a = b // error
+val b = 1

tests/init/neg/apply2.scala

@@ -1,4 +1,4 @@
-object O:
+class O:
   case class A(b: B):
     println(n)
 

tests/init/neg/final-fields.scala

@@ -12,14 +12,14 @@ trait U {
   val f2: Int
 }
 
-object Test0 extends U {
+class Test0 extends U {
   final val f1 = 1
   final val f2 = 2
   final val f3 = f1 + f2
   val f4: 3 = f3
 }
 
-object Test1 extends U {
+class Test1 extends U {
   final val f1 = 1
   final val f3 = f1 + f2
   final val f2 = 2
@@ -28,7 +28,7 @@ object Test1 extends U {
 
 }
 
-object Test extends T {
+class Test extends T {
  override final val f1 = /*super.f1*/ 1 + f2  // error
  override final val f2 = 2                    // error
  override final val f3 = {println(3); 3}      // error
@@ -37,7 +37,7 @@ object Test extends T {
  def g: 3 = { println("g"); 3 }
  final val x = g + 1
  def main(args: Array[String]): Unit = {
-   Test0
-   Test1
+   new Test0
+   new Test1
  }
 }

tests/init/neg/i12544.scala

@@ -2,18 +2,23 @@ enum Enum:
   case Case
   case Case2(x: Int)
 
-def g(b: Enum.B): Int = b.foo()
+class Outer:
+  val e = new Enum2
 
-object Enum:
-  object nested:
-    val a: Enum = Case
+  class Enum2:
+    class nested:
+      val a: Enum = Enum.Case
 
-  val b: Enum = f(nested.a)
+    val b: Enum = f((new nested).a)
 
-  def f(e: Enum): Enum = e
+    def f(e: Enum): Enum = e
 
-  class B() { def foo() = n + 1 }
-  g(new B())                       // error
-  val n: Int = 10
+    class B() { def foo() = n + 1 }
+    def g(b: B): Int = b.foo()
+    g(new B()) // error
+    val n: Int = 10
 
-@main def main(): Unit = println(Enum.b)
+@main def main(): Unit = {
+  val o = new Outer
+  print(o.e.b)
+}

tests/init/neg/i15883.scala

@@ -1,4 +1,4 @@
-object App {
+class App {
   trait A { type L >: Any}
   def upcast(a: A, x: Any): a.L = x
   val p: A { type L <: Nothing } = p        // error

tests/init/neg/i4031.scala

@@ -1,4 +1,4 @@
-object Scanners {
+class Scanners {
   enum IndentWidth {
     case Run(ch: Char, n: Int)
     case Conc(l: IndentWidth, r: Run)

tests/init/neg/inner30.scala

@@ -1,20 +1,23 @@
-object Flags {
-  class Inner {
-    println(b)
-  }
+class Outer:
+  val flags = new Flags // error
 
-  new Flags.Inner
+  class Flags {
+    class Inner {
+      println(b)
+    }
 
-  val a = this.b + 3
-  val b = 5              // error
-}
+    new flags.Inner
 
-object Flags2 {
-  class Inner {
-    println(b)
+    val a = this.b + 3
+    val b = 5 // error
   }
 
+  class Flags2 {
+    class Inner {
+      println(b)
+    }
+
 
-  lazy val a = 3
-  val b = 5
-}
+    lazy val a = 3
+    val b = 5
+  }

tests/init/neg/inner9.scala

@@ -2,7 +2,7 @@
 19 |    val l2 = l.map(_.m()) // error
    |             ^^^^^^^^^^^^
    |             Call method method map on an uninitialized (Cold) object. Calling trace:
-   |             ├── object leakWarm {	[ leak-warm.scala:1 ]
+   |             ├── class leakWarm {	[ leak-warm.scala:1 ]
    |             │   ^
    |             └── val l2 = l.map(_.m()) // error	[ leak-warm.scala:19 ]
    |                          ^^^^^^^^^^^^