Backport changes from 7bea1e2ac36ac54b648ae5c9d381a59bc69db912 to fix #2273 for 2.x

projects/SelfTest/UsageTests/Approx.tests.cpp

@@ -212,4 +212,11 @@ TEST_CASE( "Comparison with explicitly convertible types", "[Approx]" )
 
 }
 
+TEST_CASE("Approx::operator() is const correct", "[Approx][.approvals]") {
+  const Approx ap = Approx(0.0).margin(0.01);
+
+  // As long as this compiles, the test should be considered passing
+  REQUIRE(1.0 == ap(1.0));
+}
+
 }} // namespace ApproxTests