Re: c++/9298: [3.4 regression] [new parser] ICE with function-pointer-typetemplate args

[Wolfgang Bangerth <bangerth at ticam dot utexas dot edu>]

The following reply was made to PR c++/9298; it has been noted by GNATS.

From: Wolfgang Bangerth <bangerth@ticam.utexas.edu>
To: Volker Reichelt <reichelt@igpm.rwth-aachen.de>
Cc: gcc-bugs@gcc.gnu.org, <gcc-gnats@gcc.gnu.org>
Subject: Re: c++/9298: [3.4 regression] [new parser] ICE with function-pointer-type
 template args
Date: Wed, 15 Jan 2003 15:35:39 -0600 (CST)

 > sorry for the confusion, I was *quite* wrong.
 
 No problem :-) You're hereby sentenced to three days in the C++ testcase 
 diction jail:
   struct bars {
     bool behind ();
   };
   ...
   if (bars().behind())
     printf("foo :-)");
 
 
 > Let's have another look at the code:
 > 
 > The declaration of foo as a static template function is of course legal.
 > But I disagree with you when you say
 > 
 > > It's illegal, though, to use its address as a template parameter!
 > 
 > Why should it be illegal? You can use the address of a static variable as
 > a template parameter, too:
 > 
 > ------------snip here-------------
 > int i;
 > template <int*> void foo() {}
 > template void foo<&i> ();
 > ----------------------------------
 
 Values for pointer- or reference-type template arguments need to have 
 _external_ linkage. "int i" does have that, "static void foo()" doesn't.
 
 
 > Given that, the following code should compile IMHO (the EDG front-end
 > thinks so, too):
 > 
 > ----------------------snip here-------------------
 > struct A
 > {
 >     typedef void (*pfun)();
 > 
 >     template <pfun> static void bar() {}
 > };
 > 
 > template <int> static void foo() {}
 > 
 > template void A::bar< &foo<0> >();
 > ----------------------snip here-------------------
 > 
 > Alas, each gcc version since 2.95.x rejects the code with a message similar to
 > 
 > bug.cc:10: `&foo<0>' is not a valid template argument
 > bug.cc:10: template-id `bar<(&foo<0>)>' for `void A::bar()' does not match any 
 >    template declaration
 
 EDG is wrong (surprisingly), and gcc is right: foo has internal linkage, 
 and its address is therefore not a valid one for a template parameter.
 Once you remove the static from the declaration of foo, it compiles 
 cleanly.
 
 It's just too bad that here the error message is not really good:
 
 GCC2.96:
 tmp/g> gcc -c x.cc
 x.cc:10: `&foo<0>' is not a valid template argument
 x.cc:10: template-id `bar<(&foo<0>)>' for `A::bar ()' does not match
 any template declaration
 
 PRESENT CVS:
 tmp/g> ../build-gcc/gcc-install/bin/gcc -c x.cc
 x.cc:10: error: template-id `bar<(&foo<0>)>' for `void A::bar()' does not match
    any template declaration
 
 
 It's particularly bad since gcc-CVS knows about this. If I take your 
 example from above, and make it invalid by adding "static" to the 
 declaration of "int i", then I get:
 
 x.cc:3: error: address of non-extern `i' cannot be used as template argument
 x.cc:3: error: template-id `foo<(&i)>' for `void foo()' does not match any
    template declaration
 
 
 Regards
   Wolfgang
 
 -------------------------------------------------------------------------
 Wolfgang Bangerth             email:            bangerth@ticam.utexas.edu
                               www: http://www.ticam.utexas.edu/~bangerth/