[CSBindings] Adjust type variable viability condition

To be viable to be attempted next type variable should have
at least one of the following:

- a direct binding (from relational constraint);
- a default (from `Defaultable` constraint);
- a literal protocol conformance with a default type
  described by `LiteralConformsTo` constraint;

Third condition was under-defined in 5.4 since it would consider
_all_ protocol requirements, but that discounts the fact that
only literal conformances could be binding sources, which leads
to problems where type variables with incomplete set of bindings
could be picked to be attempted.

Resolves: rdar://77233864

Author: xedin

lib/Sema/CSBindings.cpp

@@ -577,8 +577,10 @@ ConstraintSystem::determineBestBindings() {
 
     return bindings || !bindings.Defaults.empty() ||
            llvm::any_of(bindings.Protocols, [&](Constraint *constraint) {
-             return bool(
-                 TypeChecker::getDefaultType(constraint->getProtocol(), DC));
+             return constraint->getKind() ==
+                        ConstraintKind::LiteralConformsTo &&
+                    bool(TypeChecker::getDefaultType(constraint->getProtocol(),
+                                                     DC));
            });
   };
 

test/Constraints/generics.swift

@@ -858,3 +858,15 @@ func rdar56212087() {
 
   setValue(foo("", ""), forKey: "") // Ok (T is inferred as a `String` instead of `Any?`)
 }
+
+// rdar://77233864 - Ternary operator fails to deduce non-default literal type in SwiftUI preview
+func test_ternary_operator_with_regular_conformance_to_literal_protocol() {
+  // Note that in this case `ExpressibleByIntegerLiteral` is a non-literal requirement
+  func test<T: ExpressibleByIntegerLiteral>(_: T) -> T {
+    fatalError()
+  }
+
+  func bug(_: Float?) {}
+
+  bug(true ? test(0) : test(42)) // Ok - type is `CGFloat` for 0 and 42
+}