[Bug c++/64446] New: Misleading error message when inheriting from a template class w/o the template params

[Bug c++/64446] New: Misleading error message when inheriting from a template class w/o the template params

[petschy at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64446

            Bug ID: 64446
           Summary: Misleading error message when inheriting from a
                    template class w/o the template params
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: petschy at gmail dot com

When compiling the under code, g++ gives a misleading error message:

$ g++-5.0.0 -Wall 20141230-templ_base.cpp 
20141230-templ_base.cpp:7:1: error: expected class-name before ‘{’ token

'Base' is definitely a valid class name.

The problem is that the name given is a class, but it's a template and the
template argument is missing.

However, in the second case, when inheriting from Base2 and only one template
argument is given of the two, the error message is OK: wrong number of template
arguments (1, should be 2).

Something similar would be desirable in the first case, not to waste time
staring at the screen, searching for a typo in the class name and finding none.
Like '<classname> is a template but no template arguments are given'.

$ g++-5.0.0 -v
Using built-in specs.
COLLECT_GCC=g++-5.0.0
COLLECT_LTO_WRAPPER=/usr/local/libexec/gcc/x86_64-unknown-linux-gnu/5.0.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../configure --enable-languages=c,c++ --disable-multilib
--program-suffix=-5.0.0
Thread model: posix
gcc version 5.0.0 20141222 (experimental) (GCC) 

4.9 and 4.8 gives the same misleading error message.

----8<----8<----8<----8<----
template<typename T>
struct Base
{
};

struct Foo : Base
{       // error: expected class-name before ‘{’ token
};

template<typename T, typename U>
struct Base2
{
};

struct Foo2 : Base2<int> // OK: wrong number of template arguments (1, should
be 2)
{       
};
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From: "mikpelinux at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/64442] -O1 modify output of a simple computation with rounding
Date: Tue, 30 Dec 2014 15:08:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idd442

--- Comment #3 from Mikael Pettersson <mikpelinux at gmail dot com> ---
See PR323 and https://gcc.gnu.org/bugs/#known

You could experiment with -ffloat-store, -mpc64, or -msse2 (if your CPU
supports SSE2, which it should if it isn't ancient).

[Bug c++/64446] Misleading error message when inheriting from a template class w/o the template params

[petschy at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64446

--- Comment #1 from petschy at gmail dot com ---
One subtlety:

template<typename T=void>
struct Base3
{
};
struct Foo3 : Base3 
{
};

In this case complaining about missing template params is probably
inappropriate, since Base3<> is perfectly valid.

So on second thought, the error should be about the missing <> after the class
name.

[Bug c++/64446] Misleading error message when inheriting from a template class w/o the template params

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64446

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-03-09
     Ever confirmed|0                           |1

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to petschy from comment #0)
> 'Base' is definitely a valid class name.

No it isn't, Base is a class template, and that is not a class. The error
message is completely accurate, although it could be improved to say how to fix
the code.


Clang also says:

p.cc:6:14: error: expected class name
struct Foo : Base
             ^


EDG gives a better error:

"p.cc", line 6: error: argument list for class template "Base" is missing
  struct Foo : Base
               ^

"p.cc", line 6: error: not a class or struct name
  struct Foo : Base
               ^


Ideally the compiler would recognise that Base is a template and point out that
the template argument list is missing.