Backport "Skip checkSafeAnnots for synthetic symbols" to 3.8.4

compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala

@@ -1286,6 +1286,9 @@ class CheckCaptures extends Recheck, SymTransformer:
         else if runInConstructor then
           pushConstructorEnv()
 
+        if sym.is(Synthetic) then
+          tree.tpt.putAttachment(SafeRefs.SkipAnnotsInType, ())
+
         checkInferredResult(super.recheckValDef(tree, sym), tree)
       finally
         if !sym.is(Param) then
@@ -1348,12 +1351,16 @@ class CheckCaptures extends Recheck, SymTransformer:
         SafeRefs.checkSafeAnnots(sym)
         for params <- tree.paramss; param <- params do
           SafeRefs.checkSafeAnnots(param.symbol)
-          param match
-            case param: ValDef => SafeRefs.checkSafeAnnotsInType(param.tpt)
-            case param: TypeDef => SafeRefs.checkSafeAnnotsInType(param.rhs)
+          if !param.symbol.is(Synthetic) then
+            param match
+              case param: ValDef => SafeRefs.checkSafeAnnotsInType(param.tpt)
+              case param: TypeDef => SafeRefs.checkSafeAnnotsInType(param.rhs)
 
         checkNoUnboxedReaches(tree)
 
+        if sym.is(Synthetic) then
+          tree.tpt.putAttachment(SafeRefs.SkipAnnotsInType, ())
+
         try checkInferredResult(super.recheckDefDef(tree, sym)(using bodyCtx), tree)
         finally
           if !sym.isAnonymousFunction then

compiler/src/dotty/tools/dotc/cc/SafeRefs.scala

@@ -6,7 +6,7 @@ import core.*
 import Symbols.*
 import Annotations.*
 import util.Spans.NoSpan
-import util.SrcPos
+import util.{Property, SrcPos}
 import Contexts.Context
 import Constants.Constant
 import Decorators.*
@@ -20,6 +20,8 @@ import typer.ProtoTypes.SelectionProto
 
 /** Check whether references from safe mode should be allowed */
 object SafeRefs {
+
+  val SkipAnnotsInType: Property.Key[Unit] = Property.Key()
 
   val assumedSafePackages = List(
     "scala", "scala.runtime", "scala.collection.immutable", "scala.compiletime.ops",
@@ -195,14 +197,14 @@ object SafeRefs {
     checkNotRejected(ann.symbol, errpos)
 
   def checkSafeAnnots(sym: Symbol)(using Context): Unit =
-    if Feature.safeEnabled then
+    if Feature.safeEnabled && !sym.is(Synthetic) then
       for ann <- sym.annotations do
         checkSafeAnnot(ann, sym.srcPos)
 
   def checkSafeAnnotsInType(tree: Tree)(using Context): Unit =
     def checkAnnotatedType(tp: Type) = tp match
       case AnnotatedType(tp, ann) => checkSafeAnnot(ann, tree.srcPos)
       case _ =>
-    if Feature.safeEnabled then
+    if Feature.safeEnabled && !tree.hasAttachment(SkipAnnotsInType) then
       tree.tpe.foreachPart(checkAnnotatedType(_))
 }

tests/pos-custom-args/captures/safemode-synthetic/A_1.scala

@@ -0,0 +1,11 @@
+import language.experimental.captureChecking
+import caps.assumeSafe
+
+@assumeSafe
+object A:
+  // get @uncheckedVariance on the types
+  def f(
+      a: List[String] = List.empty,
+      b: Option[String] = None,
+      c: Long = 30000
+    ) = ()

tests/pos-custom-args/captures/safemode-synthetic/B_2.scala

@@ -0,0 +1,9 @@
+import language.experimental.safe
+
+def test(i: Int, j: (String, Int)) =
+  // desugared to:
+  // val a$1: List[String] = List.apply[String](["bar" : String]*)
+  // val b$1: Option[String] @uncheckedVariance = A.f$default$2
+  // A.f(a = a$1, b$1, c = 1000L)
+  // We need to skip annotation check for the synthetic b$1 in safe mode
+  A.f(a = List("bar"), c = 1000)