Don't issue errors under -Xfatal-warnings for disabled conditional warnings

Author: odersky

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

@@ -50,7 +50,13 @@ trait Reporting { this: Context =>
 
   def reportWarning(warning: Warning): Unit =
     if (!this.settings.silentWarnings.value) {
-      if (this.settings.XfatalWarnings.value) reporter.report(warning.toError)
+      if (this.settings.XfatalWarnings.value)
+        warning match {
+          case warning: ConditionalWarning if !warning.enablingOption.value =>
+            reporter.report(warning) // conditional warnings that are not enabled are not fatal
+          case _ =>
+            reporter.report(warning.toError)
+        }
       else reporter.report(warning)
     }
 

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

@@ -162,6 +162,7 @@ class CompilationTests extends ParallelTesting {
     compileFile("tests/neg-custom-args/i3882.scala", allowDeepSubtypes) +
     compileFile("tests/neg-custom-args/i4372.scala", allowDeepSubtypes) +
     compileFile("tests/neg-custom-args/i1754.scala", allowDeepSubtypes) +
+    compileFile("tests/neg-custom-args/conditionalWarnings.scala", allowDeepSubtypes.and("-deprecation").and("-Xfatal-warnings")) +
     compileFilesInDir("tests/neg-custom-args/isInstanceOf", allowDeepSubtypes and "-Xfatal-warnings") +
     compileFile("tests/neg-custom-args/i3627.scala", allowDeepSubtypes) +
     compileFile("tests/neg-custom-args/matchtype-loop.scala", allowDeepSubtypes) +