[Bug c++/67152] New: [concepts] bogus "partial specialization of ‘foo<T>’ after instantiation" error

[Bug c++/67152] New: [concepts] bogus "partial specialization of ‘foo<T>’ after instantiation" error

[eric.niebler at gmail dot com]

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67152

            Bug ID: 67152
           Summary: [concepts] bogus "partial specialization of ‘foo<T>’
                    after instantiation" error
           Product: gcc
           Version: c++-concepts
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eric.niebler at gmail dot com
  Target Milestone: ---

The error happens on constrained partial specializations of a template that has
already been instantiated. But the template that got instantiated would fail
the constraint anyway, so the error is complaining about nothing AFAICT.
Consider:

template <class T>
concept bool HasType = requires { typename T::type; };

template<class T>
struct trait {
  using type = void;
};

struct has_type { using type = void; };

// Instantiation here
trait<has_type>::type foo() {}

// constrained version here. Type "has_type" would fail this
// constraint so this partial specialization would not have been
// selected.
template<class T>
  requires !HasType<T>
struct trait<T> {
  using type = void;
};


...yields the following:

test.cpp:17:8: error: partial specialization of ‘struct trait<T>’ after
instantiation of ‘struct trait<has_type>’ [-fpermissive]
 struct trait<T> {
        ^

If the partial specialization is the following instead:

template<class T>
struct trait<T*> {
  using type = void;
};

then there is no error. It seems to me that for partial specializations, a
constraints failure should be treated similarly to a pattern match failure WRT
such errors.
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From: "ncm at cantrip dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/67153] New: integer optimizations 53% slower than std::bitset<>
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idg153

            Bug ID: 67153
           Summary: integer optimizations 53% slower than std::bitset<>
           Product: gcc
           Version: 5.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ncm at cantrip dot org
  Target Milestone: ---

Created attachment 36146
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id6146&actioníit
The std::bitset<> version

I have attached two small, semantically equivalent C++14 programs.

One uses std::bitset<26> for its operations; the other uses raw unsigned int.
The one that uses unsigned int runs 53% slower than the bitset<> version, as
compiled with g++-5.1 and running on a 2013-era Haswell i7-4770.  While this
represents, perhaps, a stunning triumph in the optimization of inline member
and lambda functions operating on structs, it may represent an equally
intensely embarrassing, even mystifying, failure for optimization of the
underlying raw integer operations.

For both, build and test was with

  $ g++-5 -O3 -march=native -mtune=native -g3 -Wall $PROG.cc
  $ time ./a.out | wc -l
  2818

Times on a 3.2GHz Haswell are consistently 0.25s for the unsigned int
version, 0.16s for the std::bitset<26> version.

These programs are archived at <https://github.com/ncm/nytm-spelling-bee/>.

The runtimes of the two versions are identical as built and run on my
2009 Westmere 2.4GHz i5-M520, and about the same as the integer version
on Haswell.

[Bug c++/67152] [concepts] bogus "partial specialization of ‘foo<T>’ after instantiation" error

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67152

--- Comment #1 from Jason Merrill <jason at gcc dot gnu.org> ---
Author: jason
Date: Sat Aug  8 22:01:39 2015
New Revision: 226739

URL: https://gcc.gnu.org/viewcvs?rev=226739&root=gcc&view=rev
Log:
        PR c++/67152
        * pt.c (process_partial_specialization): Call
        associate_classtype_constraints.

Added:
    trunk/gcc/testsuite/g++.dg/concepts/partial-spec6.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/pt.c

[Bug c++/67152] [concepts] bogus "partial specialization of ‘foo<T>’ after instantiation" error

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67152

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |FIXED
           Assignee|unassigned at gcc dot gnu.org      |jason at gcc dot gnu.org
   Target Milestone|---                         |6.0

--- Comment #2 from Jason Merrill <jason at gcc dot gnu.org> ---
Fixed.