Merge #21317: util: Make Assume() usable as unary expression

fa4ceba util: Make Assume() usable as unary expression (MarcoFalke)

Pull request description:

  Assume shouldn't behave different at the call site depending on build flags. Currently compilation fails if it is used as expression. Fix that by using the lambda approach from `Assert()` without the `assert()`.

ACKs for top commit:
  jnewbery:
    ACK fa4ceba
  practicalswift:
    cr ACK fa4ceba: patch looks correct and commit hash starts with `fa`

Tree-SHA512: 9ec9ac8d410cdaf5e4e28df571a89e3d23d38e05a7027bb726cae3da6e9314734277e5a218e9e090cc17e10db763da71052c229ad642077ca5824ee42022f3ed

Author: MarcoFalke

src/test/util_tests.cpp

@@ -77,6 +77,9 @@ BOOST_AUTO_TEST_CASE(util_check)
     const int two = *Assert(p_two);
     Assert(two == 2);
     Assert(true);
+    // Check that Assume can be used as unary expression
+    const bool result{Assume(two == 2)};
+    Assert(result);
 }
 
 BOOST_AUTO_TEST_CASE(util_criticalsection)

src/util/check.h

@@ -69,7 +69,7 @@ T get_pure_r_value(T&& val)
 #ifdef ABORT_ON_FAILED_ASSUME
 #define Assume(val) Assert(val)
 #else
-#define Assume(val) ((void)(val))
+#define Assume(val) ([&]() -> decltype(get_pure_r_value(val)) { auto&& check = (val); return std::forward<decltype(get_pure_r_value(val))>(check); }())
 #endif
 
 #endif // BITCOIN_UTIL_CHECK_H