Make fatal warnings not fail compilation early & aggregate warns (#19245)

Final PR. Adds functionality that changes the behaviour of
fatal-warnings - fixes #18634
PR 5/5 (merge consecutively, per [Nicolas'
suggestion](#18829 (review))

Split up version of #18829

Author: szymon-rd

compiler/src/dotty/tools/dotc/Run.scala

@@ -348,6 +348,7 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
     runCtx.withProgressCallback: cb =>
       _progress = Progress(cb, this, fusedPhases.map(_.traversals).sum)
     runPhases(allPhases = fusedPhases)(using runCtx)
+    ctx.reporter.finalizeReporting()
     if (!ctx.reporter.hasErrors)
       Rewrites.writeBack()
     suppressions.runFinished(hasErrors = ctx.reporter.hasErrors)

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

@@ -172,19 +172,14 @@ abstract class Reporter extends interfaces.ReporterResult {
   end issueUnconfigured
 
   def issueIfNotSuppressed(dia: Diagnostic)(using Context): Unit =
-    def toErrorIfFatal(dia: Diagnostic) = dia match
-      case w: Warning if ctx.settings.silentWarnings.value => dia
-      case w: ConditionalWarning if w.isSummarizedConditional => dia
-      case w: Warning if ctx.settings.XfatalWarnings.value => w.toError
-      case _ => dia
 
     def go() =
       import Action.*
       dia match
         case w: Warning => WConf.parsed.action(dia) match
           case Error   => issueUnconfigured(w.toError)
-          case Warning => issueUnconfigured(toErrorIfFatal(w))
-          case Verbose => issueUnconfigured(toErrorIfFatal(w.setVerbose()))
+          case Warning => issueUnconfigured(w)
+          case Verbose => issueUnconfigured(w.setVerbose())
           case Info    => issueUnconfigured(w.toInfo)
           case Silent  =>
         case _ => issueUnconfigured(dia)
@@ -214,6 +209,10 @@ abstract class Reporter extends interfaces.ReporterResult {
   def incomplete(dia: Diagnostic)(using Context): Unit =
     incompleteHandler(dia, ctx)
 
+  def finalizeReporting()(using Context) =
+    if (hasWarnings && ctx.settings.XfatalWarnings.value)
+      report(new Error("No warnings can be incurred under -Werror (or -Xfatal-warnings)", NoSourcePosition))
+
   /** Summary of warnings and errors */
   def summary: String = {
     val b = new mutable.ListBuffer[String]

compiler/test-resources/repl/rewrite-messages

@@ -1,17 +1,20 @@
 //> using options -source:future-migration -deprecation -Werror
 scala> import scala.util._
--- Error: ----------------------------------------------------------------------
+-- Migration Warning: ----------------------------------------------------------
 1 | import scala.util._
   |                   ^
   |         `_` is no longer supported for a wildcard import; use `*` instead
-
+No warnings can be incurred under -Werror (or -Xfatal-warnings)
+1 warning found
+1 error found
 scala> extension (x: Int) def foo(y: Int) = x + y
 def foo(x: Int)(y: Int): Int
-
 scala> 2 foo 4
--- Error: ----------------------------------------------------------------------
+-- Migration Warning: ----------------------------------------------------------
 1 | 2 foo 4
   |   ^^^
   |Alphanumeric method foo is not declared infix; it should not be used as infix operator.
   |Instead, use method syntax .foo(...) or backticked identifier `foo`.
-1 error found
+No warnings can be incurred under -Werror (or -Xfatal-warnings)
+1 warning found
+1 error found

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -215,10 +215,8 @@ class CompilationTests {
   // initialization tests
   @Test def checkInitGlobal: Unit = {
     implicit val testGroup: TestGroup = TestGroup("checkInitGlobal")
-    val options = defaultOptions.and("-Ysafe-init-global", "-Xfatal-warnings")
-    compileFilesInDir("tests/init-global/neg", options, FileFilter.exclude(TestSources.negInitGlobalScala2LibraryTastyBlacklisted)).checkExpectedErrors()
     compileFilesInDir("tests/init-global/warn", defaultOptions.and("-Ysafe-init-global"), FileFilter.exclude(TestSources.negInitGlobalScala2LibraryTastyBlacklisted)).checkWarnings()
-    compileFilesInDir("tests/init-global/pos", options, FileFilter.exclude(TestSources.posInitGlobalScala2LibraryTastyBlacklisted)).checkCompile()
+    compileFilesInDir("tests/init-global/pos", defaultOptions.and("-Ysafe-init-global", "-Xfatal-warnings"), FileFilter.exclude(TestSources.posInitGlobalScala2LibraryTastyBlacklisted)).checkCompile()
   }
 
   // initialization tests

tests/init-global/neg/i11262.scala

@@ -0,0 +1,10 @@
+-- Warning: tests/init-global/warn/context-sensitivity.scala:9:21 ------------------------------------------------------
+9 |  def foo(): Int = A.m // warn
+  |                   ^^^
+  |                   Access uninitialized field value m. Calling trace:
+  |                   ├── object A:	[ context-sensitivity.scala:14 ]
+  |                   │   ^
+  |                   ├── val m: Int = box1.value.foo()	[ context-sensitivity.scala:17 ]
+  |                   │                ^^^^^^^^^^^^^^^^
+  |                   └── def foo(): Int = A.m // warn	[ context-sensitivity.scala:9 ]
+  |                                        ^^^

tests/init-global/neg/i15883.scala

@@ -6,7 +6,7 @@ class C(var x: Int) extends Foo {
 }
 
 class D(var y: Int) extends Foo {
-  def foo(): Int = A.m  // error
+  def foo(): Int = A.m // warn
 }
 
 class Box(var value: Foo)
@@ -15,3 +15,4 @@ object A:
   val box1: Box = new Box(new C(5))
   val box2: Box = new Box(new D(10))
   val m: Int = box1.value.foo()
+

tests/init-global/warn/context-sensitivity.check

@@ -1,20 +1,20 @@
--- Error: tests/init-global/neg/global-cycle1.scala:1:7 ----------------------------------------------------------------
-1 |object A {             // error
+-- Warning: tests/init-global/warn/global-cycle1.scala:1:7 -------------------------------------------------------------
+1 |object A { // warn
   |       ^
   |       Cyclic initialization: object A -> object B -> object A. Calling trace:
-  |       ├── object A {             // error	[ global-cycle1.scala:1 ]
+  |       ├── object A { // warn	[ global-cycle1.scala:1 ]
   |       │   ^
   |       ├── val a: Int = B.b	[ global-cycle1.scala:2 ]
   |       │                ^
   |       ├── object B {	[ global-cycle1.scala:5 ]
   |       │   ^
-  |       └── val b: Int = A.a     // error	[ global-cycle1.scala:6 ]
+  |       └── val b: Int = A.a // warn	[ global-cycle1.scala:6 ]
   |                        ^
--- Error: tests/init-global/neg/global-cycle1.scala:6:17 ---------------------------------------------------------------
-6 |  val b: Int = A.a     // error
+-- Warning: tests/init-global/warn/global-cycle1.scala:6:17 ------------------------------------------------------------
+6 |  val b: Int = A.a // warn
   |               ^^^
   |               Access uninitialized field value a. Calling trace:
   |               ├── object B {	[ global-cycle1.scala:5 ]
   |               │   ^
-  |               └── val b: Int = A.a     // error	[ global-cycle1.scala:6 ]
+  |               └── val b: Int = A.a // warn	[ global-cycle1.scala:6 ]
   |                                ^^^

tests/init-global/neg/context-sensitivity.scala renamed to tests/init-global/warn/context-sensitivity.scala

@@ -1,9 +1,9 @@
-object A {             // error
+object A { // warn
   val a: Int = B.b
 }
 
 object B {
-  val b: Int = A.a     // error
+  val b: Int = A.a // warn
 }
 
 @main