From ea03640c3581f3ad39868dabadf918e8c336f183 Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Sat, 2 Jul 2022 10:58:53 +0200
Subject: [PATCH] Check type arguments for bad bounds

Fixes #15569
---
 .../dotty/tools/dotc/inlines/InlineReducer.scala  |  2 +-
 .../dotty/tools/dotc/transform/PostTyper.scala    | 12 +++++++++++-
 tests/neg/i15569.scala                            | 15 +++++++++++++++
 3 files changed, 27 insertions(+), 2 deletions(-)
 create mode 100644 tests/neg/i15569.scala

diff --git a/compiler/src/dotty/tools/dotc/inlines/InlineReducer.scala b/compiler/src/dotty/tools/dotc/inlines/InlineReducer.scala
index 7be63fc8ba3a..b5e2e7d475ad 100644
--- a/compiler/src/dotty/tools/dotc/inlines/InlineReducer.scala
+++ b/compiler/src/dotty/tools/dotc/inlines/InlineReducer.scala
@@ -14,7 +14,7 @@ import util.SimpleIdentityMap
 
 import collection.mutable
 
-/** A utility object offering methods for rewriting inlined code */
+/** A utility class offering methods for rewriting inlined code */
 class InlineReducer(inliner: Inliner)(using Context):
   import tpd.*
   import Inliner.{isElideableExpr, DefBuffer}
diff --git a/compiler/src/dotty/tools/dotc/transform/PostTyper.scala b/compiler/src/dotty/tools/dotc/transform/PostTyper.scala
index dda4b89f6d1c..a5ac0d7ba50f 100644
--- a/compiler/src/dotty/tools/dotc/transform/PostTyper.scala
+++ b/compiler/src/dotty/tools/dotc/transform/PostTyper.scala
@@ -14,6 +14,7 @@ import Decorators._
 import Symbols._, SymUtils._, NameOps._
 import ContextFunctionResults.annotateContextResults
 import config.Printers.typr
+import util.SrcPos
 import reporting._
 
 object PostTyper {
@@ -180,6 +181,13 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
         => Checking.checkAppliedTypesIn(tree)
       case _ =>
 
+    private def checkGoodBounds(tpe: Type, pos: SrcPos)(using Context): Unit = tpe.dealias match
+      case tpe: TypeRef =>
+        checkGoodBounds(tpe.info, pos)
+      case TypeBounds(lo, hi) if !(lo <:< hi) =>
+        report.error(i"type argument has unrealizable bounds $tpe", pos)
+      case _ =>
+
     private def removeUnwantedAnnotations(sym: Symbol, metaAnnotSym: Symbol,
         metaAnnotSymBackup: Symbol, keepIfNoRelevantAnnot: Boolean)(using Context): Unit =
       def shouldKeep(annot: Annotation): Boolean =
@@ -342,7 +350,9 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
           if tree.symbol.is(Inline) then
             ctx.compilationUnit.needsInlining = true
           val tree1 @ TypeApply(fn, args) = normalizeTypeArgs(tree)
-          args.foreach(checkInferredWellFormed)
+          for arg <- args do
+            checkInferredWellFormed(arg)
+            checkGoodBounds(arg.tpe, arg.srcPos)
           if (fn.symbol != defn.ChildAnnot.primaryConstructor)
             // Make an exception for ChildAnnot, which should really have AnyKind bounds
             Checking.checkBounds(args, fn.tpe.widen.asInstanceOf[PolyType])
diff --git a/tests/neg/i15569.scala b/tests/neg/i15569.scala
new file mode 100644
index 000000000000..f98345a61691
--- /dev/null
+++ b/tests/neg/i15569.scala
@@ -0,0 +1,15 @@
+trait Foo[X >: Any <: Nothing]
+
+def andThenSub[A, B, C](f: A <:< B, g: B <:< C): A <:< C =
+  f.andThen(g)
+
+@main def Test = (None: Option[Foo[?]]) match {
+  case _: Option[Foo[t]] =>
+    val unsound: Any <:< Nothing = andThenSub[Any, t, Nothing](summon, summon) // error
+    unsound("hi :)")
+}
+@main def Test2 =
+  type t >: Any <: Nothing
+  val unsound: Any <:< Nothing = andThenSub[Any, t, Nothing](summon, summon) // error
+  unsound("hi :)")
+