Boosts - I spent ~hour yesterday fighting an issue with my C++ code, only to later figure...

smurthys, 19 days ago

I spent ~hour yesterday fighting an issue with my C++ code, only to later figure out it's a possible GCC bug, because Clang accepts the same code.

The issue is that GCC does not permit a constrained type parameter in a template template parameter of an aliased template. See the simplified code with the issue.

A cursory search of GCC Bugzilla does not readily show any related bug. I'll look carefully but lemme know if this is a known bug (probably is). 🙏🏽

https://sigcpp.godbolt.org/z/bGTnM3v1q

#cpp #gcc #bug

$Syntax-highlighted C++ code in a dark scheme: template<std::integral T> class P {}; template<template <std::integral T> class V = P> class C {}; template<int I> class Q {}; template<template <int I> class V = Q> class D {}; template<template<typename> class V = P> using A1 = C<V>; //OK template<template<class> class V = P> using A2 = C<V>; //OK template<template<std::integral> class V = P> using A3 = C<V>; //Error: since GCC 11.1; OK in GCC 10.5 and since Clang 11.0 template<template<typename T> requires std::integral<T> class V = P> using A4 = C<V>; //Error: since GCC 11.1; OK in GCC 10.5 and since Clang 11.0 template<template<typename T> requires std::is_integral_v<T> class V = P> using A5 = C<V>; //Error: since GCC 10.5 and up to Clang 15.0; OK since Clang 16.0 //non type parameter in template template parameter template<template<int I> requires (I != 10) class V = Q> using A6 = D<V>; //Error: up to Clang 15.0; OK since Clang 16.0; OK since GCC 10.5$

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+ gracicot

gracicot 17 days ago