WUnused: Fix for symbols with synthetic names and unused transparent …

…inlines (scala#17061)

Resolves scala#16863

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

@@ -368,7 +368,7 @@ object CheckUnused:
 
     /** Register an import */
     def registerImport(imp: tpd.Import)(using Context): Unit =
-      if !tpd.languageImport(imp.expr).nonEmpty && !imp.isGeneratedByEnum then
+      if !tpd.languageImport(imp.expr).nonEmpty && !imp.isGeneratedByEnum && !isTransparentAndInline(imp) then
         impInScope.top += imp
         unusedImport ++= imp.selectors.filter { s =>
           !shouldSelectorBeReported(imp, s) && !isImportExclusion(s)
@@ -432,6 +432,7 @@ object CheckUnused:
         else
           exists
       }
+
       // if there's an outer scope
       if usedInScope.nonEmpty then
         // we keep the symbols not referencing an import in this scope
@@ -450,6 +451,7 @@ object CheckUnused:
      */
     def getUnused(using Context): UnusedResult =
       popScope()
+
       val sortedImp =
         if ctx.settings.WunusedHas.imports || ctx.settings.WunusedHas.strictNoImplicitWarn then
           unusedImport.map(d => d.srcPos -> WarnTypes.Imports).toList
@@ -460,34 +462,39 @@ object CheckUnused:
           localDefInScope
             .filterNot(d => d.symbol.usedDefContains)
             .filterNot(d => usedInPosition.exists { case (pos, name) => d.span.contains(pos.span) && name == d.symbol.name})
+            .filterNot(d => containsSyntheticSuffix(d.symbol))
             .map(d => d.namePos -> WarnTypes.LocalDefs).toList
         else
           Nil
       val sortedExplicitParams =
         if ctx.settings.WunusedHas.explicits then
           explicitParamInScope
             .filterNot(d => d.symbol.usedDefContains)
+            .filterNot(d => containsSyntheticSuffix(d.symbol))
             .map(d => d.namePos -> WarnTypes.ExplicitParams).toList
         else
           Nil
       val sortedImplicitParams =
         if ctx.settings.WunusedHas.implicits then
           implicitParamInScope
             .filterNot(d => d.symbol.usedDefContains)
+            .filterNot(d => containsSyntheticSuffix(d.symbol))
             .map(d => d.namePos -> WarnTypes.ImplicitParams).toList
         else
           Nil
       val sortedPrivateDefs =
         if ctx.settings.WunusedHas.privates then
           privateDefInScope
             .filterNot(d => d.symbol.usedDefContains)
+            .filterNot(d => containsSyntheticSuffix(d.symbol))
             .map(d => d.namePos -> WarnTypes.PrivateMembers).toList
         else
           Nil
       val sortedPatVars =
         if ctx.settings.WunusedHas.patvars then
           patVarsInScope
             .filterNot(d => d.symbol.usedDefContains)
+            .filterNot(d => containsSyntheticSuffix(d.symbol))
             .filterNot(d => usedInPosition.exists { case (pos, name) => d.span.contains(pos.span) && name == d.symbol.name})
             .map(d => d.namePos -> WarnTypes.PatVars).toList
         else
@@ -500,6 +507,23 @@ object CheckUnused:
     end getUnused
     //============================ HELPERS ====================================
 
+
+    /**
+     * Checks if import selects a def that is transparent and inline
+     */
+    private def isTransparentAndInline(imp: tpd.Import)(using Context): Boolean =
+      imp.selectors.exists { sel =>
+        val qual = imp.expr
+        val importedMembers = qual.tpe.member(sel.name).alternatives.map(_.symbol)
+        importedMembers.exists(s => s.is(Transparent) && s.is(Inline))
+      }
+
+    /**
+     * Heuristic to detect synthetic suffixes in names of symbols
+     */
+    private def containsSyntheticSuffix(symbol: Symbol)(using Context): Boolean =
+      symbol.name.mangledString.contains("$")
+
     /**
      * Is the the constructor of synthetic package object
      * Should be ignored as it is always imported/used in package

tests/neg-custom-args/fatal-warnings/i15503-scala2/scala2-t11681.scala

@@ -100,9 +100,9 @@ trait Anonymous {
 trait Context[A]
 trait Implicits {
   def f[A](implicit ctx: Context[A]) = answer // error
-  def g[A: Context] = answer // error
+  def g[A: Context] = answer // OK
 }
-class Bound[A: Context] // error
+class Bound[A: Context] // OK
 object Answers {
   def answer: Int = 42
 }

tests/neg-custom-args/fatal-warnings/i15503b.scala

@@ -75,7 +75,7 @@ package foo.scala2.tests:
 
   object Types {
     def l1() = {
-      object HiObject { def f = this } // error
+      object HiObject { def f = this } // OK
       class Hi { // error
         def f1: Hi = new Hi
         def f2(x: Hi) = x

tests/neg-custom-args/fatal-warnings/i15503f.scala

@@ -5,8 +5,9 @@ val default_int = 1
 
 def f1(a: Int) = a // OK
 def f2(a: Int) = 1 // OK
-def f3(a: Int)(using Int) = a // error
-def f4(a: Int)(using Int) = default_int // error
+def f3(a: Int)(using Int) = a // OK
+def f4(a: Int)(using Int) = default_int // OK
 def f6(a: Int)(using Int) = summon[Int] // OK
 def f7(a: Int)(using Int) = summon[Int] + a // OK
+def f8(a: Int)(using foo: Int) = a // error
 

tests/neg-custom-args/fatal-warnings/i15503g.scala

@@ -5,8 +5,8 @@ val default_int = 1
 
 def f1(a: Int) = a // OK
 def f2(a: Int) = default_int // error
-def f3(a: Int)(using Int) = a // error
-def f4(a: Int)(using Int) = default_int // error // error
+def f3(a: Int)(using Int) = a // OK
+def f4(a: Int)(using Int) = default_int // error
 def f6(a: Int)(using Int) = summon[Int] // error
 def f7(a: Int)(using Int) = summon[Int] + a // OK
 

tests/neg-custom-args/fatal-warnings/i15503i.scala

@@ -209,6 +209,34 @@ package foo.test.i16925:
       _ = println(i) // OK
     } yield ()
 
+package foo.test.i16863a:
+  import scala.quoted.*
+  def fn(using Quotes) =
+    val x = Expr(1)
+    '{ $x + 2 } // OK
+
+package foo.test.i16863b:
+  import scala.quoted.*
+  def fn[A](using Quotes, Type[A]) = // OK
+    val numeric = Expr.summon[Numeric[A]].getOrElse(???)
+    '{ $numeric.fromInt(3) } // OK
+
+package foo.test.i16863c:
+  import scala.quoted.*
+  def fn[A](expr: Expr[Any])(using Quotes) =
+    val imp = expr match
+      case '{ ${ _ }: a } => Expr.summon[Numeric[a]] // OK
+    println(imp)
+
+package foo.test.i16863d:
+  import scala.quoted.*
+  import scala.compiletime.asMatchable // OK
+  def fn[A](using Quotes, Type[A]) =
+    import quotes.reflect.*
+    val imp = TypeRepr.of[A].widen.asMatchable match
+      case Refinement(_,_,_) => ()
+    println(imp)
+
 package foo.test.i16679a:
   object myPackage:
     trait CaseClassName[A]: