15503 – [3.4/4.0 regression] nested template problem (original) (raw)

Description Mark Anders 2004-05-17 20:12:27 UTC

For the following code, the compilation fails at the NProperty constructor. I have tried numerous ways of naming the inherited constructor, none of which compile in gcc 3.4. This compiles with gcc 3.3.3 but not with gcc 3.4. Thanks

class I { public: class Type; template class Object; };

class I::Type { public: class Property; };

template class I::Object { public: template class Property; };

class I::Type::Property { };

template template class I::Object::Property : public I::Type::Property { public: Property(D T::*d) : data(d) { } private: D T::*data; };

class C : public I::Object { public: template class NProperty :
public I::Object::template Property { public: NProperty(int T::*d); }; };

template C::NProperty::NProperty(int T::*d) : I::Object::template Property(d) { }

int main() { return 0; }

Comment 1 Drea Pinski 2004-05-17 20:21:45 UTC

I do not know if this is valid C++ (I think it is not, ICC6.0 rejects it in strict mode).

The way I was able to compile it with 3.4.0, is replace the construtor with: template C::NProperty::NProperty(int T::*d) : Property(d) { }

Comment 2 Wolfgang Bangerth 2004-05-17 20:43:34 UTC

I'm sure this is legal, and icc7.1 also accepts it. Here is something smaller:

template struct O {
template struct I {}; };
template struct S : O::template I {
S() : O::template I() { } ; };

g/x> /home/bangerth/bin/gcc-3.3.4-pre/bin/c++ -c x.cc

g/x> /home/bangerth/bin/gcc-3.4-pre/bin/c++ -c x.cc x.cc: In constructor S<T>::S()': x.cc:7: error: expected class-name before "template" x.cc:7: error: expected (' before "template" x.cc:7: error: expected `{' before "template"

g/x> /home/bangerth/bin/gcc-3.5-pre/bin/c++ -c x.cc x.cc: In constructor S<T>::S()': x.cc:7: error: expected class-name before "template" x.cc:7: error: expected (' before "template" x.cc:7: error: expected `{' before "template"

In other words, this is a regression.

W.

Comment 3 Giovanni Bajo 2004-05-18 10:36:50 UTC

I need to know which way to take with this. In my reading of the standard, the keyword 'template' to name nested dependent templates is not allowed in neither the base-specifier nor the mem-initializer clause. The grammar explicitally says:

base-specifier: ::opt nested-name-specifieropt class-name virtual access-specifieropt ::opt nested-name-specifier[opt] class-name access-specifier virtualopt ::opt nested-name-specifier[opt] class-name

mem-initializer-id: ::opt nested-name-specifier[opt] class-name identifier

And there is no "template[opt]". After all, it makes sense as a symbol '<' in that context can only be the start of a template argument list.

This would also match what happens with typename, which is more explicitally disallowed in those contexts ([temp.res]/5): "The keyword typename is not permitted in a base-specifier or in a mem-initializer; in these contexts a qualified-name that depends on a template-parameter (14.6.2) is implicitly assumed to be a type name."

Gaby, do you agree with my reading of the standard? If so, I can make a patch that emits a proper diagnostic when 'template' is found in those contexts, and fix the rejects-valid shown in this bug (if you remove 'template' from the testcase, it still does not accept it).

Comment 4 Mark Mitchell 2004-05-31 19:04:22 UTC

The code in Comment #2 is not allowed by the standard.

This is DR 343, which comes from DR 314, which was opened by me. It looks like the committee will eventually allow both the version with the "template" keyword, and the version without, and that without the keyword the parser should still assume that it is present. Right now, the standard provides no way whatsoever to accomplish the desired functionality.

I'll look into what fixing this would require, but I'm not eager to make major revisions in this area for 3.4.1; they'll likely be somewhat risky.

Comment 5 Giovanni Bajo 2004-06-01 01:40:55 UTC

I have a patch for this. I'll make the 'template' keyword required in this context, much like as we do for the base-specifier.

Comment 6 Giovanni Bajo 2004-06-01 01:54:05 UTC

*** Bug 14566 has been marked as a duplicate of this bug. ***

Comment 8 Mark Mitchell 2004-06-05 19:22:08 UTC

Subject: Re: [3.4/3.5 regression] nested template problem

giovannibajo at libero dot it wrote:

------- Additional Comments From giovannibajo at libero dot it 2004-06-02 11:19 ------- Patch posted, waiting for review: http://gcc.gnu.org/ml/gcc-patches/2004-06/msg00101.html

OK, thanks.

Comment 11 Giovanni Bajo 2004-06-06 03:23:30 UTC

Fixed for GCC 3.4.1 and GCC 3.5.0. Thank your for your report!