Merge #13769: Mark single-argument constructors "explicit"

1ac3c983bf Mark single-argument constructors "explicit" (practicalswift)

Pull request description:

  Mark single-argument constructors `explicit`.

  Rationale:
  * Avoid unexpected implicit promotions.

  From the developer notes:

  > **By default, declare single-argument constructors explicit.**
  > Rationale: This is a precaution to avoid unintended conversions that might arise when single-argument constructors are used as implicit conversion functions.

Tree-SHA512: 7901ed5be808c9d0ecb5ca501e1bc0395987fe1b7941b8548cebac2ff08a14f7dab61fab374a69b9ba29a9295a04245c814325c7f95b97ae558af0780f111dfa

src/test/miner_tests.cpp

@@ -29,7 +29,7 @@ BOOST_FIXTURE_TEST_SUITE(miner_tests, TestingSetup)
 // BOOST_CHECK_EXCEPTION predicates to check the specific validation error
 class HasReason {
 public:
-    HasReason(const std::string& reason) : m_reason(reason) {}
+    explicit HasReason(const std::string& reason) : m_reason(reason) {}
     bool operator() (const std::runtime_error& e) const {
         return std::string(e.what()).find(m_reason) != std::string::npos;
     };