[5.5][CSBindings] Account for literal bindings when checking "subtype of existential" property

include/swift/Sema/CSBindings.h

@@ -391,6 +391,13 @@ class BindingSet {
     if (Bindings.empty())
       return false;
 
+    // Literal requirements always result in a subtype/supertype
+    // relationship to a concrete type.
+    if (llvm::any_of(Literals, [](const auto &literal) {
+          return literal.second.viableAsBinding();
+        }))
+      return false;
+
     return llvm::all_of(Bindings, [](const PotentialBinding &binding) {
       return binding.BindingType->isExistentialType() &&
              binding.Kind == AllowedBindingKind::Subtypes;

test/Constraints/protocols.swift

@@ -492,3 +492,30 @@ func test_arg_conformance_with_conditional_reqs(i: Int) {
   let _: Int?? = simple(overloaded_result())
   let _: Int?? = overloaded(overloaded_result())
 }
+
+// rdar://77570994 - regression in type unification for literal collections
+
+protocol Elt {
+}
+
+extension Int : Elt {}
+extension Int64 : Elt {}
+extension Dictionary : Elt where Key == String, Value: Elt {}
+
+struct Object {}
+
+extension Object : ExpressibleByDictionaryLiteral {
+  init(dictionaryLiteral elements: (String, Elt)...) {
+  }
+}
+
+enum E {
+case test(cond: Bool, v: Int64)
+
+  var test_prop: Object {
+    switch self {
+    case let .test(cond, v):
+      return ["obj": ["a": v, "b": cond ? 0 : 42]] // Ok
+    }
+  }
+}