Re: c++/10394: type inference gets confused during template instantiation

Re: c++/10394: type inference gets confused during template instantiation

[Giovanni Bajo]

The following reply was made to PR c++/10394; it has been noted by GNATS.

From: "Giovanni Bajo" <giovannibajo@libero.it>
To: <colby@alum.mit.edu>,
	<gcc-gnats@gcc.gnu.org>,
	<gcc-bugs@gcc.gnu.org>,
	<nobody@gcc.gnu.org>,
	<gcc-prs@gcc.gnu.org>
Cc: "Wolfgang Bangerth" <bangerth@ices.utexas.edu>
Subject: Re: c++/10394: type inference gets confused during template instantiation
Date: Mon, 14 Apr 2003 03:24:12 +0200

 http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&p
 r=10394
 
 Your code is ill-formed, and correctly rejected by any GCC version I have
 (even if the error message could use a lifting).
 
 To invoke correctly the function, you have to use this syntax:
 
     a.f(typename A<typename V::X>::template B<int>());
 
 Because:
 
 - B<> is a nested template within A<>. A<> is dependent on C's template
 parameter, so the keyword 'template' is needed in front of it to
 disambiguate the parser.
 - The whole expression A<typename V::X>::template B<int> refers to a type
 name, which is dependent on C's template paremter, so the keyword 'typename'
 is needed in front of it to disambiguate the parser.
 
 Your line marked as [1] compiles because it is a function declaration, not
 an object instantiation. Specifically, you're declaring a function called
 "c" which returns an object of type C<E>, and accepts as parameter a pointer
 to a function which gets no parameters and returns a A<int>.
 
 Nonetheless, I found a regression on the mainline while playing with this:
 
 -------------------------------------------------------
 template <class T>
 struct A
 {
   template <class U>
   class B {};
 };
 
 template <class V>
 struct C
 {
   C()
   {
       A<typename V::X>::template B<int> k;
   }
 };
 
 struct E
 {
   typedef int X;
 };
 
 template struct C<E>;
 
 int main()
 {}
 -------------------------------------------------------
 pr10394.cpp: In constructor `C<V>::C()':
 pr10394.cpp:13: error: expected `;'
 pr10394.cpp: In constructor `C<V>::C() [with V = E]':
 pr10394.cpp:22:   instantiated from here
 pr10394.cpp:13: internal compiler error: in resolve_offset_ref, at
 cp/init.c:
    1841
 Please submit a full bug report,
 
 on 3.4 20030413. Since the code is ill-formed (missing 'typename' keyword at
 the start of the definition of k), this is a 3.4 regression,
 ice-on-illegal-code. Previous versions (2.95 -> 3.3) reports some kind of
 (unreadable) error message at least.
 
 Giovanni Bajo
 

Re: c++/10394: type inference gets confused during template instantiation

[ehrhardt]

Synopsis: type inference gets confused during template instantiation

State-Changed-From-To: open->closed
State-Changed-By: cae
State-Changed-When: Mon Apr 14 12:00:29 2003
State-Changed-Why:
    Not a bug. The remaining ICE found by Giovanni is now PR 10398.

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=10394

c++/10394: type inference gets confused during template instantiation

[colby]

>Number:         10394
>Category:       c++
>Synopsis:       type inference gets confused during template instantiation
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          rejects-legal
>Submitter-Id:   net
>Arrival-Date:   Sun Apr 13 21:36:00 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator:     Christopher Colby
>Release:        3.2.2
>Organization:
>Environment:
Windows XP
>Description:
Enclosed is a small test program with class A templated by <class T> and class C templated by <class V>.  C expects V to define a type V::X and instantiates A with that type.  In C's constructor, there are two occurrences of A<typename V::X>.  The first occurrence correctly instantiates A with V::X, but the second occurrence erroneously instantiates A with V, thus causing the compiler to reject the program.

Tweaking the enclosed demonstration program in almost any way makes this error disappear.  For instance, line #1 compiles, but line #2 generates the type error:

    #1   C<E> c(A<int>());
    #2   A<int> a;  C<E> c(a);

Also, removing the unused template argument from A's templated subclass B also eliminates the type error.
>How-To-Repeat:
Read the comments in main() before compiling the program.
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted:
----gnatsweb-attachment----
Content-Type: text/plain; name="test.cpp"
Content-Disposition: inline; filename="test.cpp"

template <class T> class A {
public:
  template <class U> class B {};
  void f(B<int>) {}
};

template <class V> class C {
public:
  C(A<typename V::X> a) { a.f(A<typename V::X>::B<int>()); }
};

template <class V> class D {
public:
  D(A<typename V::X>* a) { a->f(A<typename V::X>::B<int>()); }
};

class E {
public:
  typedef int X;
};

int main() {
  A<int> a;

  // When you uncomment one of the following four lines...

  // C<E> c(A<int>());       // okay; see [1] below.
  // C<E> c(a);              // compile error; see [2] below.
  // D<E> d(new A<int>());   // compile error; see [3] below.
  // D<E> d(&a);             // compile error; see [4] below.
}

/*

c:\cygwin\home\Christopher Colby\df>g++ -v
g++ -v
Reading specs from /usr/lib/gcc-lib/i686-pc-cygwin/3.2.2/specs
Configured with: ../src/configure --enable-languages=c,c++,f77,java --enable-libgcj --enable-threads=posix --with-system-zlib --enable-nls --without-included-gettext --enable-interpreter --disable-sjlj-exceptions --disable-version-specific-runtime-libs --enable-shared --enable-haifa --prefix=/usr --exec-prefix=/usr --sysconfdir=/etc --libdir=/usr/lib --includedir=/nonexistent/include --libexecdir=/usr/sbin
Thread model: posix
gcc version 3.2.2

[1]
c:\cygwin\home\Christopher Colby\df>g++ test.cpp
g++ test.cpp

[2]
c:\cygwin\home\Christopher Colby\df>g++ test.cpp
g++ test.cpp
test.cpp: In constructor `C<V>::C(A<V::X>) [with V = E]':
test.cpp:28:   instantiated from here
test.cpp:9: no matching function for call to `A<int>::f(A<E>::B<int>)'
test.cpp:4: candidates are: void A<T>::f(A<T>::B<int>) [with T = int]

[3]
c:\cygwin\home\Christopher Colby\df>g++ test.cpp
g++ test.cpp
test.cpp: In constructor `D<V>::D(A<V::X>*) [with V = E]':
test.cpp:29:   instantiated from here
test.cpp:14: no matching function for call to `A<int>::f(A<E>::B<int>)'
test.cpp:4: candidates are: void A<T>::f(A<T>::B<int>) [with T = int]

[4]
c:\cygwin\home\Christopher Colby\df>g++ test.cpp
g++ test.cpp
test.cpp: In constructor `D<V>::D(A<V::X>*) [with V = E]':
test.cpp:30:   instantiated from here
test.cpp:14: no matching function for call to `A<int>::f(A<E>::B<int>)'
test.cpp:4: candidates are: void A<T>::f(A<T>::B<int>) [with T = int]

*/