swiftlang · DougGregor · Jun 6, 2020 · Jun 5, 2020
diff --git a/include/swift/Basic/LangOptions.h b/include/swift/Basic/LangOptions.h
@@ -549,7 +549,10 @@ namespace swift {
     /// Enable constraint solver support for experimental
     ///        operator protocol designator feature.
     bool SolverEnableOperatorDesignatedTypes = false;
-
+
+    /// Enable experimental support for one-way constraints for the
+    /// parameters of closures.
+    bool EnableOneWayClosureParameters = false;
   };
 } // end namespace swift
 

diff --git a/include/swift/Option/FrontendOptions.td b/include/swift/Option/FrontendOptions.td
@@ -642,6 +642,10 @@ def experimental_print_full_convention :
  HelpText<"When emitting a module interface, emit additional @convention "
           "arguments, regardless of whether they were written in the source">;
 
+def experimental_one_way_closure_params :
+  Flag<["-"], "experimental-one-way-closure-params">,
+  HelpText<"Enable experimental support for one-way closure parameters">;
+
 def prebuilt_module_cache_path :
   Separate<["-"], "prebuilt-module-cache-path">,
   HelpText<"Directory of prebuilt modules for loading module interfaces">;

diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
@@ -724,6 +724,8 @@ static bool ParseTypeCheckerArgs(TypeCheckerOptions &Opts, ArgList &Args,
 
   Opts.SolverEnableOperatorDesignatedTypes |=
       Args.hasArg(OPT_solver_enable_operator_designated_types);
+  Opts.EnableOneWayClosureParameters |=
+      Args.hasArg(OPT_experimental_one_way_closure_params);
 
   Opts.DebugConstraintSolver |= Args.hasArg(OPT_debug_constraints);
   Opts.DebugGenericSignatures |= Args.hasArg(OPT_debug_generic_signatures);

diff --git a/lib/Sema/CSBindings.cpp b/lib/Sema/CSBindings.cpp
@@ -710,7 +710,8 @@ ConstraintSystem::getPotentialBindings(TypeVariableType *typeVar) const {
       }
       break;
 
-    case ConstraintKind::OneWayEqual: {
+    case ConstraintKind::OneWayEqual:
+    case ConstraintKind::OneWayBindParam: {
       // Don't produce any bindings if this type variable is on the left-hand
       // side of a one-way binding.
       auto firstType = constraint->getFirstType();

diff --git a/lib/Sema/CSSimplify.cpp b/lib/Sema/CSSimplify.cpp
@@ -1322,6 +1322,7 @@ ConstraintSystem::matchTupleTypes(TupleType *tuple1, TupleType *tuple2,
   case ConstraintKind::FunctionInput:
   case ConstraintKind::FunctionResult:
   case ConstraintKind::OneWayEqual:
+  case ConstraintKind::OneWayBindParam:
   case ConstraintKind::DefaultClosureType:
     llvm_unreachable("Not a conversion");
   }
@@ -1387,6 +1388,7 @@ static bool matchFunctionRepresentations(FunctionTypeRepresentation rep1,
   case ConstraintKind::FunctionInput:
   case ConstraintKind::FunctionResult:
   case ConstraintKind::OneWayEqual:
+  case ConstraintKind::OneWayBindParam:
   case ConstraintKind::DefaultClosureType:
     return false;
   }
@@ -1699,6 +1701,7 @@ ConstraintSystem::matchFunctionTypes(FunctionType *func1, FunctionType *func2,
   case ConstraintKind::FunctionInput:
   case ConstraintKind::FunctionResult:
   case ConstraintKind::OneWayEqual:
+  case ConstraintKind::OneWayBindParam:
   case ConstraintKind::DefaultClosureType:
     llvm_unreachable("Not a relational constraint");
   }
@@ -4406,6 +4409,7 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
     case ConstraintKind::FunctionInput:
     case ConstraintKind::FunctionResult:
     case ConstraintKind::OneWayEqual:
+    case ConstraintKind::OneWayBindParam:
     case ConstraintKind::DefaultClosureType:
       llvm_unreachable("Not a relational constraint");
     }
@@ -7121,9 +7125,16 @@ ConstraintSystem::simplifyOneWayConstraint(
     return SolutionKind::Solved;
   }
 
-  // Translate this constraint into a one-way binding constraint.
-  return matchTypes(first, secondSimplified, ConstraintKind::Equal, flags,
-                    locator);
+  // Translate this constraint into an equality or bind-parameter constraint,
+  // as appropriate.
+  if (kind == ConstraintKind::OneWayEqual) {
+    return matchTypes(first, secondSimplified, ConstraintKind::Equal, flags,
+                      locator);
+  }
+
+  assert(kind == ConstraintKind::OneWayBindParam);
+  return matchTypes(
+      secondSimplified, first, ConstraintKind::BindParam, flags, locator);
 }
 
 static Type getFunctionBuilderTypeFor(ConstraintSystem &cs, unsigned paramIdx,
@@ -7175,12 +7186,27 @@ bool ConstraintSystem::resolveClosure(TypeVariableType *typeVar,
 
     Type internalType;
 
+    bool oneWayConstraints =
+      getASTContext().TypeCheckerOpts.EnableOneWayClosureParameters;
     if (paramList->get(i)->getTypeRepr()) {
       // Internal type is the type used in the body of the closure,
       // so "external" type translates to it as follows:
       //  - `Int...` -> `[Int]`,
       //  - `inout Int` -> `@lvalue Int`.
       internalType = param.getParameterType();
+
+      // When there are type variables in the type and we have enabled
+      // one-way constraints, create a fresh type variable to handle the
+      // binding.
+      if (oneWayConstraints && internalType->hasTypeVariable()) {
+        auto *paramLoc =
+            getConstraintLocator(closure, LocatorPathElt::TupleElement(i));
+        auto *typeVar = createTypeVariable(paramLoc, TVO_CanBindToLValue |
+                                                         TVO_CanBindToNoEscape);
+        addConstraint(
+            ConstraintKind::OneWayBindParam, typeVar, internalType, paramLoc);
+        internalType = typeVar;
+      }
     } else {
       auto *paramLoc =
           getConstraintLocator(closure, LocatorPathElt::TupleElement(i));
@@ -7194,7 +7220,13 @@ bool ConstraintSystem::resolveClosure(TypeVariableType *typeVar,
           param.isVariadic() ? ArraySliceType::get(typeVar) : Type(typeVar);
 
       auto externalType = param.getOldType();
-      addConstraint(ConstraintKind::BindParam, externalType, typeVar, paramLoc);
+      if (oneWayConstraints) {
+        addConstraint(
+            ConstraintKind::OneWayBindParam, typeVar, externalType, paramLoc);
+      } else {
+        addConstraint(
+            ConstraintKind::BindParam, externalType, typeVar, paramLoc);
+      }
     }
 
     setType(paramList->get(i), internalType);
@@ -9759,6 +9791,7 @@ ConstraintSystem::addConstraintImpl(ConstraintKind kind, Type first,
                                                subflags, locator);
 
   case ConstraintKind::OneWayEqual:
+  case ConstraintKind::OneWayBindParam:
     return simplifyOneWayConstraint(kind, first, second, subflags, locator);
 
   case ConstraintKind::ValueMember:
@@ -10271,6 +10304,7 @@ ConstraintSystem::simplifyConstraint(const Constraint &constraint) {
     return SolutionKind::Unsolved;
 
   case ConstraintKind::OneWayEqual:
+  case ConstraintKind::OneWayBindParam:
     return simplifyOneWayConstraint(constraint.getKind(),
                                     constraint.getFirstType(),
                                     constraint.getSecondType(),

diff --git a/lib/Sema/CSSolver.cpp b/lib/Sema/CSSolver.cpp
@@ -1688,6 +1688,7 @@ void ConstraintSystem::ArgumentInfoCollector::walk(Type argType) {
       case ConstraintKind::ConformsTo:
       case ConstraintKind::Defaultable:
       case ConstraintKind::OneWayEqual:
+      case ConstraintKind::OneWayBindParam:
       case ConstraintKind::DefaultClosureType:
         break;
       }

diff --git a/lib/Sema/Constraint.cpp b/lib/Sema/Constraint.cpp
@@ -66,6 +66,7 @@ Constraint::Constraint(ConstraintKind Kind, Type First, Type Second,
   case ConstraintKind::FunctionResult:
   case ConstraintKind::OpaqueUnderlyingType:
   case ConstraintKind::OneWayEqual:
+  case ConstraintKind::OneWayBindParam:
     assert(!First.isNull());
     assert(!Second.isNull());
     break;
@@ -138,6 +139,7 @@ Constraint::Constraint(ConstraintKind Kind, Type First, Type Second, Type Third,
   case ConstraintKind::FunctionResult:
   case ConstraintKind::OpaqueUnderlyingType:
   case ConstraintKind::OneWayEqual:
+  case ConstraintKind::OneWayBindParam:
   case ConstraintKind::DefaultClosureType:
     llvm_unreachable("Wrong constructor");
 
@@ -265,6 +267,7 @@ Constraint *Constraint::clone(ConstraintSystem &cs) const {
   case ConstraintKind::FunctionResult:
   case ConstraintKind::OpaqueUnderlyingType:
   case ConstraintKind::OneWayEqual:
+  case ConstraintKind::OneWayBindParam:
   case ConstraintKind::DefaultClosureType:
     return create(cs, getKind(), getFirstType(), getSecondType(), getLocator());
 
@@ -348,6 +351,7 @@ void Constraint::print(llvm::raw_ostream &Out, SourceManager *sm) const {
   case ConstraintKind::EscapableFunctionOf: Out << " @escaping type of "; break;
   case ConstraintKind::OpenedExistentialOf: Out << " opened archetype of "; break;
   case ConstraintKind::OneWayEqual: Out << " one-way bind to "; break;
+  case ConstraintKind::OneWayBindParam: Out << " one-way bind param to "; break;
   case ConstraintKind::DefaultClosureType:
     Out << " closure can default to ";
     break;
@@ -564,6 +568,7 @@ gatherReferencedTypeVars(Constraint *constraint,
   case ConstraintKind::FunctionResult:
   case ConstraintKind::OpaqueUnderlyingType:
   case ConstraintKind::OneWayEqual:
+  case ConstraintKind::OneWayBindParam:
   case ConstraintKind::DefaultClosureType:
     constraint->getFirstType()->getTypeVariables(typeVars);
     constraint->getSecondType()->getTypeVariables(typeVars);

diff --git a/lib/Sema/Constraint.h b/lib/Sema/Constraint.h
@@ -161,6 +161,12 @@ enum class ConstraintKind : char {
   /// type). At that point, this constraint will be treated like an `Equal`
   /// constraint.
   OneWayEqual,
+  /// The second type is the type of a function parameter, and the first type
+  /// is the type of a reference to that function parameter within the body.
+  /// Once the second type has been fully determined (and mapped down to a
+  /// concrete type), this constraint will be treated like a 'BindParam'
+  /// constraint.
+  OneWayBindParam,
   /// If there is no contextual info e.g. `_ = { 42 }` default first type
   /// to a second type (inferred closure type). This is effectively a
   /// `Defaultable` constraint which a couple of differences:
@@ -549,6 +555,7 @@ class Constraint final : public llvm::ilist_node<Constraint>,
     case ConstraintKind::OptionalObject:
     case ConstraintKind::OpaqueUnderlyingType:
     case ConstraintKind::OneWayEqual:
+    case ConstraintKind::OneWayBindParam:
     case ConstraintKind::DefaultClosureType:
       return ConstraintClassification::Relational;
 
@@ -669,7 +676,8 @@ class Constraint final : public llvm::ilist_node<Constraint>,
 
   /// Whether this is a one-way constraint.
   bool isOneWayConstraint() const {
-    return Kind == ConstraintKind::OneWayEqual;
+    return Kind == ConstraintKind::OneWayEqual ||
+        Kind == ConstraintKind::OneWayBindParam;
   }
 
   /// Retrieve the overload choice for an overload-binding constraint.

diff --git a/test/Constraints/one_way_closure_params.swift b/test/Constraints/one_way_closure_params.swift
@@ -0,0 +1,8 @@
+// RUN: %target-typecheck-verify-swift -swift-version 4 -experimental-one-way-closure-params
+
+func testBasic() {
+  let _: (Float) -> Float = { $0 + 1 }
+
+  let _ = { $0 + 1 } // expected-error{{unable to infer type of a closure parameter $0 in the current context}}
+}
+