Prevent unconditional joins, and naked bool in where() or boolean expressions

- `.from(t1, t2)` produces an unconditional join if you forget to add a condition in the .where()
  sqlpp11 therefore now deprecates unconditional joins.
- more often than not, writing  something like `where(name == "doe")`, you
  meant to write `where(t.name == "doe")`. It is hard to find bugs when
  the former expression compiles because you happen to have a variable
  `name` in the current scope as well.
  sqlpp11 therefore now deprecates `.where(bool)` and disallows
  raw bool values boolean expression like `something and bool`
  wrap bools in sqlpp::value(), if you REALLY want a bool value here

test_static_asserts/unwrapped_bool.cpp

@@ -0,0 +1,115 @@
+/*
+ * Copyright (c) 2015-2016, Roland Bock
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without modification,
+ * are permitted provided that the following conditions are met:
+ *
+ *  * Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+ * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
+ * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+ * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
+ * OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <iostream>
+#include "MockDb.h"
+#include "Sample.h"
+#include <sqlpp11/sqlpp11.h>
+
+namespace
+{
+  constexpr auto t = test::TabBar{};
+
+  template <typename T>
+  struct wrap
+  {
+  };
+
+  template <typename T>
+  void print_type_on_error(std::true_type)
+  {
+  }
+
+  template <typename T>
+  void print_type_on_error(std::false_type)
+  {
+    wrap<T>::_print_me_;
+  }
+
+  template <typename Assert, typename Lhs, typename Rhs>
+  void and_check(const Lhs& lhs, const Rhs& rhs)
+  {
+    using ReturnType = decltype(lhs and rhs);
+    using ExpectedReturnType =
+        sqlpp::logic::all_t<Assert::value xor std::is_same<ReturnType, sqlpp::bad_expression<sqlpp::boolean>>::value>;
+    print_type_on_error<ReturnType>(ExpectedReturnType{});
+    static_assert(ExpectedReturnType::value, "Unexpected return type");
+  }
+
+  template <typename Assert, typename Lhs, typename Rhs>
+  void or_check(const Lhs& lhs, const Rhs& rhs)
+  {
+    using ReturnType = decltype(lhs or rhs);
+    using ExpectedReturnType =
+        sqlpp::logic::all_t<Assert::value xor std::is_same<ReturnType, sqlpp::bad_expression<sqlpp::boolean>>::value>;
+    print_type_on_error<ReturnType>(ExpectedReturnType{});
+    static_assert(ExpectedReturnType::value, "Unexpected return type");
+  }
+
+  template <typename Assert, typename Lhs>
+  void not_check(const Lhs& lhs)
+  {
+    using ReturnType = decltype(not lhs);
+    using ExpectedReturnType =
+        sqlpp::logic::all_t<Assert::value xor std::is_same<ReturnType, sqlpp::bad_expression<sqlpp::boolean>>::value>;
+    print_type_on_error<ReturnType>(ExpectedReturnType{});
+    static_assert(ExpectedReturnType::value, "Unexpected return type");
+  }
+
+  template <typename Assert, typename Condition>
+  void where_check(const Condition& condition)
+  {
+    using ReturnType = decltype(sqlpp::where(condition));
+    using ExpectedReturnType =
+        sqlpp::logic::all_t<Assert::value xor std::is_same<ReturnType, sqlpp::bad_statement>::value>;
+    print_type_on_error<ReturnType>(ExpectedReturnType{});
+    static_assert(ExpectedReturnType::value, "Unexpected return type");
+  }
+
+  void boolean()
+  {
+    and_check<sqlpp::consistent_t>(t.gamma, t.gamma);
+    and_check<sqlpp::assert_valid_operands>(t.gamma, true);
+    // and_check<sqlpp::assert_valid_operands>(true, t.gamma); // Cannot currently do that
+
+    or_check<sqlpp::consistent_t>(t.gamma, t.gamma);
+    or_check<sqlpp::assert_valid_operands>(t.gamma, true);
+    // or_check<sqlpp::assert_valid_operands>(true, t.gamma); // Cannot currently do that
+
+    not_check<sqlpp::consistent_t>(t.gamma);
+  }
+
+  void where()
+  {
+    where_check<sqlpp::consistent_t>(t.gamma);
+    where_check<sqlpp::bad_statement>(true);
+  }
+}
+
+int main(int, char* [])
+{
+  boolean();
+  where();
+}