[CSBindings] Detect situations when type variable bindings could be incomplete

When `bind param` constraint is associated with a given
type variable a set of bindings collected for it is
potentially incomplete, because binding gathering doesn't
look through related type variables. Which means that
such type variable has to be de-prioritized until
`bind param` constraint is resolved or there is just
nothing else to try, otherwise there is a risk that
solver would skip some of the valid solutions.

This only affects type variable that appears one the
right-hand side of the `bind param` constraint and
represents result type of the closure body, because
left-hand side gets types from overload choices.

Resolves: rdar://problem/45659733

Author: xedin

lib/Sema/CSBindings.cpp

@@ -424,6 +424,20 @@ ConstraintSystem::getPotentialBindings(TypeVariableType *typeVar) {
     case ConstraintKind::ArgumentConversion:
     case ConstraintKind::OperatorArgumentConversion:
     case ConstraintKind::OptionalObject: {
+      // If there is a `bind param` constraint associated with
+      // current type variable, result should be aware of that
+      // fact. Binding set might be incomplete until
+      // this constraint is resolved, because we currently don't
+      // look-through constraints expect to `subtype` to try and
+      // find related bindings.
+      // This only affects type variable that appears one the
+      // right-hand side of the `bind param` constraint and
+      // represents result type of the closure body, because
+      // left-hand side gets types from overload choices.
+      if (constraint->getKind() == ConstraintKind::BindParam &&
+          constraint->getSecondType()->isEqual(typeVar))
+        result.PotentiallyIncomplete = true;
+
       auto binding = getPotentialBindingForRelationalConstraint(
           result, constraint, hasDependentMemberRelationalConstraints,
           hasNonDependentMemberRelationalConstraints,

lib/Sema/ConstraintSystem.h

@@ -2831,6 +2831,12 @@ class ConstraintSystem {
     /// Whether the bindings of this type involve other type variables.
     bool InvolvesTypeVariables = false;
 
+    /// Whether the bindings represent (potentially) incomplete set,
+    /// there is no way to say with absolute certainty if that's the
+    /// case, but that could happen when certain constraints like
+    /// `bind param` are present in the system.
+    bool PotentiallyIncomplete = false;
+
     /// Whether this type variable has literal bindings.
     LiteralBindingKind LiteralBinding = LiteralBindingKind::None;
 
@@ -2875,9 +2881,14 @@ class ConstraintSystem {
       if (formBindingScore(y) < formBindingScore(x))
         return false;
 
-      // If the only difference is default types,
+      // If there is a difference in number of default types,
       // prioritize bindings with fewer of them.
-      return x.NumDefaultableBindings < y.NumDefaultableBindings;
+      if (x.NumDefaultableBindings != y.NumDefaultableBindings)
+        return x.NumDefaultableBindings < y.NumDefaultableBindings;
+
+      // As a last resort, let's check if the bindings are
+      // potentially incomplete, and if so, let's de-prioritize them.
+      return x.PotentiallyIncomplete < y.PotentiallyIncomplete;
     }
 
     void foundLiteralBinding(ProtocolDecl *proto) {
@@ -2910,6 +2921,8 @@ class ConstraintSystem {
     void dump(llvm::raw_ostream &out,
               unsigned indent = 0) const LLVM_ATTRIBUTE_USED {
       out.indent(indent);
+      if (PotentiallyIncomplete)
+        out << "potentially_incomplete ";
       if (FullyBound)
         out << "fully_bound ";
       if (SubtypeOfExistentialType)

test/Constraints/closures.swift

@@ -823,3 +823,29 @@ func rdar_40537960() {
   _ = A(arr, fn: { L($0.v) }) // expected-error {{cannot convert value of type 'L' to closure result type 'R<T>'}}
 }
 
+// rdar://problem/45659733
+func rdar_45659733() {
+  func foo<T : BinaryInteger>(_: AnyHashable, _: T) {}
+  func bar(_ a: Int, _ b: Int) {
+    _ = (a ..< b).map { i in foo(i, i) } // Ok
+  }
+
+  struct S<V> {
+    func map<T>(
+      get: @escaping (V) -> T,
+      set: @escaping (inout V, T) -> Void
+    ) -> S<T> {
+      fatalError()
+    }
+
+    subscript<T>(
+      keyPath: WritableKeyPath<V, T?>,
+      default defaultValue: T
+    ) -> S<T> {
+      return map(
+        get: { $0[keyPath: keyPath] ?? defaultValue },
+        set: { $0[keyPath: keyPath] = $1 }
+      ) // Ok, make sure that we deduce result to be S<T>
+    }
+  }
+}