[CSBindings] Revert changes in BindingSet::isViable

This change although correct cases performance issues
in some scenarios.

Author: xedin

lib/Sema/CSBindings.cpp

@@ -1208,19 +1208,7 @@ bool BindingSet::isViable(PotentialBinding &binding, bool isTransitive) {
     if (!existingNTD || NTD != existingNTD)
       continue;
 
-    // What is going on here needs to be thoroughly re-evaluated,
-    // but at least for now, let's not filter bindings of different
-    // kinds so if we have a situation like: `Array<$T0> conv $T1`
-    // and `$T1 conv Array<(String, Int)>` we can't lose `Array<$T0>`
-    // as a binding because `$T0` could be inferred to
-    // `(key: String, value: Int)` and binding `$T1` to `Array<(String, Int)>`
-    // eagerly would be incorrect.
-    if (existing->Kind != binding.Kind) {
-      // Array, Set and Dictionary allow conversions, everything else
-      // requires their generic arguments to match exactly.
-      if (existingType->isKnownStdlibCollectionType())
-        continue;
-    }
+    // FIXME: What is going on here needs to be thoroughly re-evaluated.
 
     // If new type has a type variable it shouldn't
     // be considered  viable.

test/Constraints/rdar139812024.swift

@@ -0,0 +1,37 @@
+// RUN: %target-typecheck-verify-swift %clang-importer-sdk
+
+// REQUIRES: objc_interop
+
+// rdar://139812024 - error: cannot convert return expression of type 'NSObject' to return type 'NSImage'
+
+import Foundation
+
+@objc
+final class Image: NSObject {
+}
+
+@available(*, unavailable)
+extension Image: Sendable {} // expected-note {{explicitly marked unavailable here}}
+
+class Lock<State> {
+  func withLock<R: Sendable>(_: @Sendable (inout State) -> R) -> R {
+    fatalError()
+  }
+}
+
+extension Lock where State == Void {
+  func withLock<R: Sendable>(_: @Sendable () -> R) -> R {
+    fatalError()
+  }
+}
+
+class Test {
+  var images: [Int: Image] = [:]
+
+  func fetchImage(lock: Lock<Void>, id: Int) -> Image? {
+    if let existingImage = lock.withLock({ return images[id] }) { // expected-warning {{unavailable}}
+      return existingImage
+    }
+    return nil
+  }
+}