[ConstraintGraph] Disallow subtype constraint contractions

lib/Sema/ConstraintGraph.cpp

@@ -641,13 +641,6 @@ static bool shouldContractEdge(ConstraintKind kind) {
   case ConstraintKind::BindParam:
   case ConstraintKind::BindToPointerType:
   case ConstraintKind::Equal:
-
-  // We currently only allow subtype contractions for the purpose of 
-  // parameter binding constraints.
-  // TODO: We do this because of how inout parameter bindings are handled
-  // for implicit closure parameters. We should consider adjusting our
-  // current approach to unlock more opportunities for subtype contractions.
-  case ConstraintKind::Subtype:
     return true;
 
   default:
@@ -671,12 +664,7 @@ static bool isStrictInoutSubtypeConstraint(Constraint *constraint) {
     t1 = tt->getElementType(0).getPointer();
   }
 
-  auto iot = t1->getAs<InOutType>();
-
-  if (!iot)
-    return false;
-
-  return !iot->getObjectType()->isTypeVariableOrMember();
+  return t1->is<InOutType>();
 }
 
 bool ConstraintGraph::contractEdges() {
@@ -698,14 +686,6 @@ bool ConstraintGraph::contractEdges() {
         auto t1 = constraint->getFirstType()->getDesugaredType();
         auto t2 = constraint->getSecondType()->getDesugaredType();
 
-        if (kind == ConstraintKind::Subtype) {
-          if (auto iot1 = t1->getAs<InOutType>()) {
-            t1 = iot1->getObjectType().getPointer();
-          } else {
-            continue;
-          }
-        }
-
         auto tyvar1 = t1->getAs<TypeVariableType>();
         auto tyvar2 = t2->getAs<TypeVariableType>();
 

validation-test/Sema/rdar34333874.swift

@@ -0,0 +1,42 @@
+// RUN: %target-run-simple-swift
+// REQUIRES: executable_test
+
+class C {
+  var a: Int = 0
+  var b: Int = 0
+}
+
+@inline(never)
+func foo<T>(_ item: T, update: (inout T) throws -> Void) rethrows -> T {
+  var this = item
+  try update(&this)
+  return this
+}
+
+// Test single statement closure because it's type-checked
+// together with the call to `foo`
+
+let rdar34333874_1 = foo(C()) {
+  $0.a = 42
+}
+
+// The multi-statement closure which is type-checked
+// separately from call to `foo`
+
+let rdar34333874_2 = foo(C()) {
+  $0.a = 42
+  $0.b = 0
+}
+
+print(rdar34333874_1)
+print(rdar34333874_2)
+
+// Example which avoids mutating fields of the class
+
+@inline(never)
+func bar(_ o : C) {
+  let _ = foo(o) { (item) in
+  }
+}
+
+bar(C())