[CSGen] If assignment destination is r-value record fix early

If type of the assignment destination expression has been determined
to be a r-value type or a type variable which can't be bound to an
l-value type, let's fix the type and record a fix for that early.

Author: xedin

lib/Sema/CSGen.cpp

@@ -2757,10 +2757,25 @@ namespace {
         }
         return TupleType::get(destTupleTypes, CS.getASTContext());
       } else {
-        Type destTy = CS.createTypeVariable(CS.getConstraintLocator(expr),
-                                            TVO_CanBindToNoEscape);
-        CS.addConstraint(ConstraintKind::Bind, LValueType::get(destTy), CS.getType(expr),
-                         CS.getConstraintLocator(expr));
+        auto *locator = CS.getConstraintLocator(expr);
+
+        auto isOrCanBeLValueType = [](Type type) {
+          if (auto *typeVar = type->getAs<TypeVariableType>()) {
+            return typeVar->getImpl().canBindToLValue();
+          }
+          return type->is<LValueType>();
+        };
+
+        auto exprType = CS.getType(expr);
+        if (!isOrCanBeLValueType(exprType)) {
+          // Pretend that destination is an l-value type.
+          exprType = LValueType::get(exprType);
+          (void)CS.recordFix(TreatRValueAsLValue::create(CS, locator));
+        }
+
+        auto *destTy = CS.createTypeVariable(locator, TVO_CanBindToNoEscape);
+        CS.addConstraint(ConstraintKind::Bind, LValueType::get(destTy),
+                         exprType, locator);
         return destTy;
       }
     }