[CSSimplify] Relax isBindable requirements for pack expansion variables

If type variable we are about to bind represents a pack
expansion type, allow the binding to happen regardless of
what the \c type is, because contextual type is just a hint
in this situation and type variable would be bound to its
opened type instead.

Resolves: rdar://112617922

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -4210,7 +4210,16 @@ static void enumerateOptionalConversionRestrictions(
 /// Determine whether we can bind the given type variable to the given
 /// fixed type.
 static bool isBindable(TypeVariableType *typeVar, Type type) {
-  return !ConstraintSystem::typeVarOccursInType(typeVar, type) &&
+  // Disallow recursive bindings.
+  if (ConstraintSystem::typeVarOccursInType(typeVar, type))
+    return false;
+
+  // If type variable we are about to bind represents a pack
+  // expansion type, allow the binding to happen regardless of
+  // what the \c type is, because contextual type is just a hint
+  // in this situation and type variable would be bound to its
+  // opened type instead.
+  return typeVar->getImpl().isPackExpansion() ||
          !type->is<DependentMemberType>();
 }
 

validation-test/Sema/type_checker_crashers_fixed/rdar112617922.swift

@@ -0,0 +1,27 @@
+// RUN: not %target-swift-frontend %s -typecheck
+
+enum MyError: Error {
+  case unknown
+}
+
+extension Result {
+  init<each S>(success: repeat each S, failure: Failure?) where Success == (repeat each S.Wrapped), Failure == any Error, repeat each S: OptionalProtocol {
+  }
+}
+
+protocol OptionalProtocol {
+  associatedtype Wrapped
+  var asOptional: Optional<Wrapped> { get }
+}
+
+extension Optional: OptionalProtocol {
+  var asOptional: Optional<Wrapped> { self }
+}
+
+func test<each EncodedResult: OptionalProtocol>(
+    _ transformed: @escaping (Result<(repeat each EncodedResult.Wrapped), any Error>) -> Void
+) -> ((any Error)?, repeat each EncodedResult) -> Void where repeat each EncodedResult.Wrapped: DataProtocol {
+  return {
+    transformed(Result(success: $1, failure: $0))
+  }
+}