[Sema] Do not emit cast to unrelated warning from existential to Anyhashable

Author: LucianoPAlmeida

lib/Sema/TypeCheckProtocol.cpp

@@ -5725,7 +5725,13 @@ TypeChecker::couldDynamicallyConformToProtocol(Type type, ProtocolDecl *Proto,
   // we cannot know statically.
   if (type->isExistentialType())
     return true;
-  
+
+  // The underlying concrete type may have a `Hashable` conformance that is
+  // not possible to know statically.
+  if (type->isAnyHashable()) {
+    return true;
+  }
+
   // A generic archetype may have protocol conformances we cannot know
   // statically.
   if (type->is<ArchetypeType>())

test/Constraints/casts.swift

@@ -693,3 +693,16 @@ func SR_16058_tests() {
   // More than one optionality wrapping
   let _: String? = foo.flatMap { dict.SR16058(_: $0) } as? String // OK
 }
+
+// https://github.com/apple/swift/issues/59405
+func isHashable(_ error: Error) -> Bool {
+  (error as? AnyHashable) != nil // OK
+}
+
+func isHashable_is(_ error: Error) -> Bool {
+  error is AnyHashable // OK
+}
+
+func isHashable_composition(_ error: Error & AnyObject) -> Bool {
+  error is AnyHashable // OK
+}