Merge pull request swiftlang#69237 from xedin/rdar-115603144

[TypeChecker] Make `couldDynamicallyConformToProtocol` more leanient

Author: xedin

lib/Sema/TypeCheckProtocol.cpp

@@ -5904,7 +5904,8 @@ TypeChecker::couldDynamicallyConformToProtocol(Type type, ProtocolDecl *Proto,
   // as an intermediate collection cast can dynamically change if the conditions
   // are met or not.
   if (type->isKnownStdlibCollectionType())
-    return !M->lookupConformance(type, Proto).isInvalid();
+    return !M->lookupConformance(type, Proto, /*allowMissing=*/true)
+                .isInvalid();
   return !conformsToProtocol(type, Proto, M).isInvalid();
 }
 

test/Constraints/casts.swift

@@ -751,3 +751,10 @@ do {
     _ = a is String // OK
   }
 }
+
+// rdar://115603144 - casting `any Sendable` to a collection warns about cast failure although the cast could succeed at runtime
+func test_existential_sendable_cast(v: any Sendable) {
+  let _ = v as! [Any] // Ok
+  let _ = v as! [String: Any] // Ok
+  let _ = v as! Set<Int> // Ok
+}