[Bug c++/57941] New: [c++11] confusing error message with invalid constexpr constructor in template struct

[Bug c++/57941] New: [c++11] confusing error message with invalid constexpr constructor in template struct

[f.heckenbach@fh-soft.de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57941

            Bug ID: 57941
           Summary: [c++11] confusing error message with invalid constexpr
                    constructor in template struct
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: f.heckenbach@fh-soft.de

% cat test.cpp
template <typename T> struct a
{
  int i;
  constexpr a (): i (b ()) { }
  constexpr int b () { return c (); }
  int c () const { throw 42; }
  constexpr operator bool () { return true; }
};

static_assert (a <int> (), "foo");
%g++-4.7 -std=c++11 test.cpp 
test.cpp:10:1: error: non-constant condition for static assertion
test.cpp:10:1: error: 'constexpr a<T>::a() [with T = int]' called in a constant
expression
test.cpp:4:13: note: 'constexpr a<T>::a() [with T = int]' is not usable as a
constexpr function because:
%

Because what?

This message is particularly confusing if other unrelated messages appear
afterwards, as it wrongly claims those to be the reason for this one.

(I understand that the program is wrong, it's just about the message.)

[Bug c++/57941] [c++11] confusing error message with invalid constexpr constructor in template struct

[manu at gcc dot gnu.org]

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57941

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-07-20
                 CC|                            |manu at gcc dot gnu.org
     Ever confirmed|0                           |1
      Known to fail|                            |4.8.0, 4.9.0

--- Comment #1 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
Confirmed. It also fails on trunk.

Clang++:

test.cc:10:16: error: static_assert expression is not an integral constant
expression
static_assert (a <int> (), "foo");
               ^~~~~~~~~~
test.cc:5:31: note: non-constexpr function 'c' cannot be used in a constant
expression
  constexpr int b () { return c (); }
                              ^
test.cc:4:22: note: in call to ')'
  constexpr a (): i (b ()) { }
                     ^
test.cc:10:16: note: in call to 'a()'
static_assert (a <int> (), "foo");
               ^
test.cc:6:7: note: declared here
  int c () const { throw 42; }
      ^
1 error generated.

which is not perfect but it is clearly better.

The code that handles this is at explain_invalid_constexpr_fn at
/home/manuel/test1/src/gcc/cp/semantics.c:6461

  save_loc = input_location;
  input_location = DECL_SOURCE_LOCATION (fun);
  inform (0, "%q+D is not usable as a constexpr function because:", fun);
  /* First check the declaration.  */
  if (is_valid_constexpr_fn (fun, true))
    {
      /* Then if it's OK, the body.  */
      if (DECL_DEFAULTED_FN (fun))
        explain_implicit_non_constexpr (fun);
      else
        {
          body = massage_constexpr_body (fun, DECL_SAVED_TREE (fun));
          require_potential_rvalue_constant_expression (body);
          if (DECL_CONSTRUCTOR_P (fun))
            cx_check_missing_mem_inits (fun, body, true);
        }
    }
  input_location = save_loc;

I don't understand the point of the first "if", since if it is ever not taken
it would be a bug.

However, in this case, interestingly, the "if" is taken, but then nothing else
prints any explanation. I didn't investigate in detail why.

Also, playing tricks with input_location is not so nice.
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From: "david-gcc at klaftenegger dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/57942] New: g++-4.8.1 tries to instantiate wrong constructor (C++11)
Date: Sat, 20 Jul 2013 22:09:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idW942

            Bug ID: 57942
           Summary: g++-4.8.1 tries to instantiate wrong constructor
                    (C++11)
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: david-gcc at klaftenegger dot de

Created attachment 30532
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id0532&actioníit
code exampe to trigger the bug

In the attached small C++11 code example g++ tries to instantiate the second
constructor of MyClass<int, false>, which should not happen.
As this code never requires MyClass<int, true>, it should compile, but due to
this useless instantiation g++ rejects the code.

To trigger the problem just run
$ g++-4.8 -std=c++11 g++-bug.cpp

This produces the following error output:
g++-bug.cpp: In instantiation of "class MyClass<int, true>":
g++-bug.cpp:26:27:   required from here
g++-bug.cpp:6:7: error: base type "int" fails to be a struct or class type

Here is the exact compiler version used:
$ g++-4.8 -v
Using built-in specs.
COLLECT_GCC=g++-4.8
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.8/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
4.8.1-2ubuntu1~13.04' --with-bugurl=file:///usr/share/doc/gcc-4.8/README.Bugs
--enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.8 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.8 --libdir=/usr/lib --enable-nls
--with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --enable-gnu-unique-object --enable-plugin
--with-system-zlib --disable-browser-plugin --enable-java-awt=gtk
--enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64/jre
--enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64,mx32 --with-tune=generic
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 4.8.1 (Ubuntu 4.8.1-2ubuntu1~13.04)

[Bug c++/57941] [c++11] confusing error message with invalid constexpr constructor in template struct

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57941

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |paolo.carlini at oracle dot com

--- Comment #2 from Paolo Carlini <paolo.carlini at oracle dot com> ---
In mainline we made progress on this issue (I should track down when and why):
after the "because" note we print:

57941.C:5:33: error: call to non-constexpr function ‘int a<T>::c() const [with
T = int]’
   constexpr int b () { return c (); }
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Subject: [Bug rtl-optimization/63712] [5.0 regression] ICE in expand_expr_addr_expr_1, at expr.c:7662
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63712

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #2 from Martin Liška <marxin at gcc dot gnu.org> ---
Dup.

*** This bug has been marked as a duplicate of bug 63573 ***
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Subject: [Bug bootstrap/63573] [5 Regression] libgo: ICE building libgo on powerpc-linux-gnu
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63573

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |schwab@linux-m68k.org

--- Comment #9 from Martin Liška <marxin at gcc dot gnu.org> ---
*** Bug 63712 has been marked as a duplicate of this bug. ***
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Subject: [Bug tree-optimization/63666] [5.0 regression] FAIL: gcc.dg/vect/pr45752.c (internal compiler error)
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idc666

--- Comment #13 from Marc Glisse <glisse at gcc dot gnu.org> ---
Author: glisse
Date: Mon Nov  3 10:36:35 2014
New Revision: 217033

URL: https://gcc.gnu.org/viewcvs?rev!7033&root=gcc&view=rev
Log:
2014-11-03  Marc Glisse  <marc.glisse@inria.fr>

    PR tree-optimization/63666
    * fold-const.c: Include "optabs.h".
    (fold_ternary_loc) <VEC_PERM_EXPR>: Avoid canonicalizing a
    can_vec_perm_p permutation to one that is not.


Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/fold-const.c

[Bug c++/57941] [c++11] confusing error message with invalid constexpr constructor in template struct

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57941

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |jason at gcc dot gnu.org
         Resolution|---                         |FIXED
   Target Milestone|---                         |5.0

--- Comment #3 from Jason Merrill <jason at gcc dot gnu.org> ---
Fixed for GCC 5, then.