[TypeChecker] Make couldDynamicallyConformToProtocol more leanient

Current check is too strict for stdlib collection types because
it checks conditional conformances which leads to erroneous warnings.

To determine whether a type dynamically conforms to a protocol,
for stdlib collection types, the method is employing
`Module::lookupConformance` check which has `allowMissing` conformances
set to `false` by default. That directly contradicts the comment
which states that the conditional conformances are to be skipped
and doesn't fit well with `TypeChecker::conformsToProtocol` check
below it that defaults `allowMissing` to `true`.

Resolves: rdar://115603144

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
+}