Names in default template arguments not found during partial specialization matching

[jakub-labenski]

Issue Summary

A class template's default template argument can be looked up in the wrong context. This happens when the default argument is filled in while the front end matches a partial specialization of a different template. Names in the default argument are then looked up from where that partial specialization is declared, instead of where the class template is declared. Anything declared after the partial specialization is not found, and the substitution is wrongly treated as failing.

In practice this breaks the common void_t detection idiom: the "is this well-formed" trait quietly returns false. Nothing is diagnosed at the point of the mistake, so the result is a wrong static_assert result or the wrong overload being chosen. The code compiles with GCC, Clang and MSVC.

Reproducing Source Code

template<class...> using void_t = void;

template<class, template<class...> class TT, class... P>
constexpr bool ok = false;

template<template<class...> class TT, class... P>
constexpr bool ok<void_t<TT<P...>>, TT, P...> = true;

namespace lib { template<class T> T&& declval() noexcept; }

template<class T, class = decltype(&lib::declval<T&>())> struct A {};

struct V {};

static_assert(ok<void, A, V>, "substitution wrongly reported as failing");

Command-line Options

cpfe --c++17 repro.cpp

Type of Issue

[Front End] This code SHOULD compile with these options.

Additional Details

Actual output:

"repro.cpp", line 15: error: static assertion failed with
          "substitution wrongly reported as failing"
  static_assert(ok<void, A, V>, "substitution wrongly reported as failing");

Observations:

• It also reproduces with --c++14, --g++ and --clang.
• It does not reproduce in the default Microsoft mode (--microsoft). That mode turns off dependent name processing, so names in templates are looked up later, when the template is used. It does reproduce with --microsoft --dep_name.
• Moving namespace lib above the partial specialization of ok makes the code compile, so the bug depends on declaration order.
• A<V> itself can be used directly with no problem. The failure only happens when A's default argument is filled in while the ok<void_t<TT<P...>>, TT, P...> partial specialization is being matched.
• Class template partial specializations (struct is_detected<void_t<...>, ...>) behave the same way.

Reproducing (Standard) Configuration(s)

linux-gcc-debug

Reproducing (Non-standard) Configuration

No response

I acknowledge that:

• I have checked open issues and existing discussions before filing this report.
• The reproducing source code that I have provided does not depend on headers that I have not included with my submission.
• I have reduced the reproducing source code to the best of my ability.
• I have selected at least one standard configuration or uploaded a non-standard configuration dump.
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[jakub-labenski]

Pull request: #483

[nv-cmeerw]

created issue GH #484