Use P0608's trick to detect convertibility without narrowing; and now that we can depend on C++17, use its features. First, this moves the burden of detecting a narrowing conversion on the compiler, rather than us maintaining a complicated series of checks. Of course, this exposes * bugs in compilers (e.g. GCC < 9 thinks that float->bool is not narrowing; * behavior still not (widely) implemented (pointer to bool conversions are narrowing, P1957); * interesting compiler choices, e.g. GCC 9 thinks that unscoped enumerations are non-narrowing convertible to a datatype big enum to contain all the _enumerators_, even if the underlying type of the enum (and/or its sizeof()) is wider than the target datatype. Second, it allows to detect conversions that have a narrowing conversion as an intermediate step. Given a type like struct Bad { operator double() const; }; then an object of type Bad is implictly convertible to a type like int via a narrowing conversion. Therefore, a connection is possible between a signal carrying a Bad and a slot accepting an int. We can now detect and block this. Tests regarding scoped enumerations have been dropped, for the simple reason that a scoped enumeration is not implictly convertible to an integral type, so we don't have that detection (it would constantly fail). Scoped enumerations do not take part in narrowing conversions anyhow, cf. [dcl.init.list]. [ChangeLog][QtCore][QObject] The detection of narrowing conversions when calling QObject::connect() when QT_NO_NARROWING_CONVERSIONS_IN_CONNECT now takes also into account user-defined implicit conversions that undergo through a narrowing conversion. Change-Id: Ie09d59203fe6283378b36dfbc54de1d58098ef51 Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io> Reviewed-by: Lars Knoll <lars.knoll@qt.io>