Honor strict unused warnings

Author: som-snytt

compiler/src/dotty/tools/dotc/transform/CheckUnused.scala

@@ -171,7 +171,8 @@ class CheckUnused private (phaseMode: PhaseMode, suffix: String) extends MiniPha
     tree
 
   override def prepareForDefDef(tree: DefDef)(using Context): Context =
-    def trivial = tree.symbol.is(Deferred) || isUnconsuming(tree.rhs)
+    def fewerUnused = ctx.settings.Yunused.value != "strict"
+    def trivial = tree.symbol.is(Deferred) || fewerUnused && isUnconsuming(tree.rhs)
     def nontrivial = tree.symbol.isConstructor || tree.symbol.isAnonymousFunction
     if !nontrivial && trivial then
       refInfos.skip.addOne(tree.symbol)

tests/warn/i22742b.scala

@@ -0,0 +1,4 @@
+
+//> using options -Wunused:all -Yunused:strict
+
+def f(x: Int) = () // warn strict means warn in obviously trivial definitions