Merge pull request swiftlang#12545 from slavapestov/fewer-type-vars-for-closures

Sema: Don't create redundant type variables for multiple references to the same closure parameter type

Author: slavapestov

lib/Sema/ConstraintSystem.cpp

@@ -922,12 +922,17 @@ ConstraintSystem::getTypeOfReference(ValueDecl *value,
   // constraint.
   if (auto *param = dyn_cast<ParamDecl>(varDecl)) {
     if (param->isLet() && valueType->is<TypeVariableType>()) {
-      Type paramType = valueType;
-      valueType = createTypeVariable(getConstraintLocator(locator),
+      auto found = OpenedParameterTypes.find(param);
+      if (found != OpenedParameterTypes.end())
+        return { found->second, found->second };
+
+      auto typeVar = createTypeVariable(getConstraintLocator(locator),
                                      TVO_CanBindToLValue |
                                      TVO_CanBindToInOut);
-      addConstraint(ConstraintKind::BindParam, paramType, valueType,
+      addConstraint(ConstraintKind::BindParam, valueType, typeVar,
                     getConstraintLocator(locator));
+      OpenedParameterTypes.insert(std::make_pair(param, typeVar));
+      return { typeVar, typeVar };
     }
   }
 

lib/Sema/ConstraintSystem.h

@@ -959,6 +959,10 @@ class ConstraintSystem {
   /// the types on the expression nodes.
   llvm::DenseMap<const Expr *, TypeBase *> ExprTypes;
 
+  /// Maps closure parameters to type variables.
+  llvm::DenseMap<const ParamDecl *, TypeVariableType *>
+    OpenedParameterTypes;
+
   /// There can only be a single contextual type on the root of the expression
   /// being checked.  If specified, this holds its type along with the base
   /// expression, and the purpose of it.