util: Make Assert work with any value #19491

Goal is to avoid compile failures

Can be tested by reverting the changes to the header and observing compile failures in the test

Concept ACK

Interesting idea.

ACK fa5363538125d996ae5cede55f7f05e88701ace2

<details><summary>gcc 9.3 compile errors with header file reverted.</summary><p>

  CXX      wallet/test/test_test_bitcoin-coinselector_tests.o
In file included from ./test/util/setup_common.h:15,
                 from test/util_tests.cpp:12:
test/util_tests.cpp: In lambda function:
test/util_tests.cpp:47:62: error: cannot bind non-const lvalue reference of type ‘std::unique_ptr<int>&’ to an rvalue of type ‘std::unique_ptr<int>’
   47 |     const std::unique_ptr<int> p_two = Assert(MakeUnique<int>(2));
./util/check.h:57:80: note: in definition of macro ‘Assert’
   57 | #define Assert(val) [&]() -> decltype(get_pure_r_value(val))& { auto& check = (val); assert(#val && check); return check; }()
      |                                                                                ^~~
test/util_tests.cpp: In member function ‘void util_tests::util_check::test_method()’:
./util/check.h:57:125: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = int; _Dp = std::default_delete<int>]’
   57 | #define Assert(val) [&]() -> decltype(get_pure_r_value(val))& { auto& check = (val); assert(#val && check); return check; }()
      |                                                                                                                             ^
test/util_tests.cpp:47:40: note: in expansion of macro ‘Assert’
   47 |     const std::unique_ptr<int> p_two = Assert(MakeUnique<int>(2));
      |                                        ^~~~~~
In file included from /usr/include/c++/9/memory:80,
                 from /usr/include/boost/system/detail/std_interoperability.hpp:12,
                 from /usr/include/boost/system/error_code.hpp:959,
                 from /usr/include/boost/filesystem/path_traits.hpp:22,
                 from /usr/include/boost/filesystem/path.hpp:26,
                 from /usr/include/boost/filesystem.hpp:16,
                 from ./fs.h:14,
                 from ./util/system.h:20,
                 from test/util_tests.cpp:5:
/usr/include/c++/9/bits/unique_ptr.h:414:7: note: declared here
  414 |       unique_ptr(const unique_ptr&) = delete;
      |       ^~~~~~~~~~
In file included from ./test/util/setup_common.h:15,
                 from test/util_tests.cpp:12:
test/util_tests.cpp: In lambda function:
test/util_tests.cpp:50:16: error: cannot bind non-const lvalue reference of type ‘bool&’ to an rvalue of type ‘bool’
   50 |     Assert(two == 2);
./util/check.h:57:80: note: in definition of macro ‘Assert’
   57 | #define Assert(val) [&]() -> decltype(get_pure_r_value(val))& { auto& check = (val); assert(#val && check); return check; }()
      |                                                                                ^~~
test/util_tests.cpp: In lambda function:
./util/check.h:57:83: error: cannot bind non-const lvalue reference of type ‘bool&’ to an rvalue of type ‘bool’
   57 | #define Assert(val) [&]() -> decltype(get_pure_r_value(val))& { auto& check = (val); assert(#val && check); return check; }()
      |                                                                                   ^
test/util_tests.cpp:51:5: note: in expansion of macro ‘Assert’
   51 |     Assert(true);
      |     ^~~~~~
make[2]: *** [Makefile:17997: test/test_bitcoin-util_tests.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[2]: Leaving directory '/home/jon/projects/bitcoin/bitcoin/src'
make[1]: *** [Makefile:18328: all-recursive] Error 1
make[1]: Leaving directory '/home/jon/projects/bitcoin/bitcoin/src'
make: *** [Makefile:788: all-recursive] Error 1

</p></details>

<details><summary>clang 11 nightly compile errors with header file reverted.</summary><p>

  CXX      test/test_bitcoin-validation_flush_tests.o
test/util_tests.cpp:47:40: error: non-const lvalue reference to type 'unique_ptr<...>' cannot bind to a temporary of type 'unique_ptr<...>'
    const std::unique_ptr<int> p_two = Assert(MakeUnique<int>(2));
                                       ^~~~~~~~~~~~~~~~~~~~~~~~~~
./util/check.h:57:71: note: expanded from macro 'Assert'
#define Assert(val) [&]() -> decltype(get_pure_r_value(val))& { auto& check = (val); assert(#val && check); return check; }()
                                                                      ^       ~~~~~
test/util_tests.cpp:47:32: error: call to deleted constructor of 'const std::unique_ptr<int>'
    const std::unique_ptr<int> p_two = Assert(MakeUnique<int>(2));
                               ^       ~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/unique_ptr.h:414:7: note: 'unique_ptr' has been explicitly marked deleted here
      unique_ptr(const unique_ptr&) = delete;
      ^
test/util_tests.cpp:50:5: error: non-const lvalue reference to type 'bool' cannot bind to a temporary of type 'bool'
    Assert(two == 2);
    ^~~~~~~~~~~~~~~~
./util/check.h:57:71: note: expanded from macro 'Assert'
#define Assert(val) [&]() -> decltype(get_pure_r_value(val))& { auto& check = (val); assert(#val && check); return check; }()
                                                                      ^       ~~~~~
test/util_tests.cpp:51:5: error: non-const lvalue reference to type 'bool' cannot bind to a temporary of type 'bool'
    Assert(true);
    ^~~~~~~~~~~~
./util/check.h:57:71: note: expanded from macro 'Assert'
#define Assert(val) [&]() -> decltype(get_pure_r_value(val))& { auto& check = (val); assert(#val && check); return check; }()
                                                                      ^       ~~~~~
4 errors generated.
make[2]: *** [Makefile:17997: test/test_bitcoin-util_tests.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[2]: Leaving directory '/home/jon/projects/bitcoin/bitcoin/src'
make[1]: *** [Makefile:18328: all-recursive] Error 1
make[1]: Leaving directory '/home/jon/projects/bitcoin/bitcoin/src'
make: *** [Makefile:788: all-recursive] Error 1

</p></details>

Code review ACK fa5363538125d996ae5cede55f7f05e88701ace2. Looks like if argument is an lvalue this effectively does:

T& Assert(T& check) { assert(check); return check; }

and if argument is an rvalue it does:

T Assert(T&& check) { assert(check); return std::move(check); }