[Constraint system] Separate out constraint generation for closures.

Author: DougGregor

lib/Sema/CSGen.cpp

@@ -4418,10 +4418,20 @@ bool ConstraintSystem::generateConstraints(
   llvm_unreachable("BOOM");
 }
 
-Expr *ConstraintSystem::generateConstraints(ClosureExpr *closure) {
+bool ConstraintSystem::generateConstraints(
+    ClosureExpr *closure, Type resultType) {
   assert(closure->hasSingleExpressionBody());
-  return generateConstraintsFor(*this, closure->getSingleExpressionBody(),
-                                closure);
+  auto closureBody = generateConstraintsFor(
+      *this, closure->getSingleExpressionBody(), closure);
+  if (!closureBody)
+    return true;
+
+  bool hasReturn = hasExplicitResult(closure);
+  addConstraint(
+      ConstraintKind::Conversion, getType(closureBody),
+      resultType,
+      getConstraintLocator(closure, LocatorPathElt::ClosureBody(hasReturn)));
+  return false;
 }
 
 Expr *ConstraintSystem::generateConstraints(Expr *expr, DeclContext *dc) {

lib/Sema/CSSimplify.cpp

@@ -7162,14 +7162,8 @@ bool ConstraintSystem::resolveClosure(TypeVariableType *typeVar,
   // If this is a multi-statement closure its body doesn't participate
   // in type-checking.
   if (closure->hasSingleExpressionBody()) {
-    auto *closureBody = generateConstraints(closure);
-    if (!closureBody)
+    if (generateConstraints(closure, closureType->getResult()))
       return false;
-
-    addConstraint(
-        ConstraintKind::Conversion, getType(closureBody),
-        closureType->getResult(),
-        getConstraintLocator(closure, LocatorPathElt::ClosureBody(hasReturn)));
   } else if (!hasReturn) {
     // If this closure has an empty body and no explicit result type
     // let's bind result type to `Void` since that's the only type empty body

lib/Sema/ConstraintSystem.h

@@ -3576,10 +3576,13 @@ class ConstraintSystem {
   bool generateConstraints(SolutionApplicationTarget &target,
                            FreeTypeVariableBinding allowFreeTypeVariables);
 
-  /// Generate constraints for the body of the given single-statement closure.
+  /// Generate constraints for the body of the given closure.
   ///
-  /// \returns a possibly-sanitized expression, or null if an error occurred.
-  Expr *generateConstraints(ClosureExpr *closure);
+  /// \param closure the closure expression
+  /// \param resultType the closure's result type
+  ///
+  /// \returns \c true if constraint generation failed, \c false otherwise
+  bool generateConstraints(ClosureExpr *closure, Type resultType);
 
   /// Generate constraints for the given (unchecked) expression.
   ///