refactor: compund assignment and pre-/post-increment operators are now lvalue-qualified member functions

test/static/chrono_test.cpp

@@ -137,8 +137,8 @@ static_assert(std::chrono::nanoseconds(quantity{1 * ns}) == 1ns);
 static_assert(std::chrono::nanoseconds(quantity{1 * s}) == 1s);
 
 // operators
-static_assert((1 * s += quantity{1s}) == 2 * s);
-static_assert((2 * s -= quantity{1s}) == 1 * s);
+static_assert([q = 1 * s]() mutable { return q += quantity{1s}; }() == 2 * s);
+static_assert([q = 2 * s]() mutable { return q -= quantity{1s}; }() == 1 * s);
 static_assert(quantity{1s} + 1 * s == 2 * s);
 static_assert(quantity{1s} + 1 * min == 61 * s);
 static_assert(1 * s + quantity{1s} == 2 * s);