Re: [PATCH] c++: Fix explicit instantiation of const variable templates after earlier implicit instantation [PR113976]

[Patrick Palka <ppalka@redhat.com>]

On Tue, 20 Feb 2024, Jakub Jelinek wrote:

> Hi!
> 
> Already previously instantiated const variable templates had
> cp_apply_type_quals_to_decl called when they were instantiated,
> but if they need runtime initialization, their TREE_READONLY flag
> has been subsequently cleared.
> Explicit variable template instantiation calls grokdeclarator which
> calls cp_apply_type_quals_to_decl on them again, setting TREE_READONLY
> flag again, but nothing clears it afterwards, so we emit such
> instantiations into rodata sections and segfault when the dynamic
> initialization attempts to initialize them.
> 
> The following patch fixes that by not calling cp_apply_type_quals_to_decl
> on already instantiated variable declarations.

LGTM, this seems like the safest approach for backporting.  Note
we can't check DECL_EXPLICIT_INSTANTIATION at this point because
that doesn't get set until later from do_decl_instantiation.

> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> 
> 2024-02-19  Jakub Jelinek  <jakub@redhat.com>
> 	    Patrick Palka  <ppalka@redhat.com>
> 
> 	PR c++/113976
> 	* decl.cc (grokdeclarator): Don't call cp_apply_type_quals_to_decl
> 	on DECL_TEMPLATE_INSTANTIATED VAR_DECLs.
> 
> 	* g++.dg/cpp1y/var-templ87.C: New test.
> 
> --- gcc/cp/decl.cc.jj	2024-02-15 09:51:34.460065992 +0100
> +++ gcc/cp/decl.cc	2024-02-19 19:18:09.839188137 +0100
> @@ -15263,7 +15263,12 @@ grokdeclarator (const cp_declarator *dec
>      /* Record constancy and volatility on the DECL itself .  There's
>         no need to do this when processing a template; we'll do this
>         for the instantiated declaration based on the type of DECL.  */
> -    if (!processing_template_decl)
> +    if (!processing_template_decl
> +	/* Don't do it for instantiated variable templates either,
> +	   cp_apply_type_quals_to_decl should have been called on it
> +	   already and might have been overridden in cp_finish_decl
> +	   if initializer needs runtime initialization.  */
> +	&& (!VAR_P (decl) || !DECL_TEMPLATE_INSTANTIATED (decl)))
>        cp_apply_type_quals_to_decl (type_quals, decl);
>  
>      return decl;
> --- gcc/testsuite/g++.dg/cpp1y/var-templ87.C.jj	2024-02-19 19:21:49.668129195 +0100
> +++ gcc/testsuite/g++.dg/cpp1y/var-templ87.C	2024-02-19 19:21:42.218232862 +0100
> @@ -0,0 +1,43 @@
> +// PR c++/113976
> +// { dg-do run { target c++14 } }
> +
> +int
> +foo ()
> +{
> +  return 42;
> +}
> +
> +template <int N>
> +const int a = foo ();
> +const int *b = &a <0>;
> +template <int N>
> +const int c = foo ();
> +template const int c <0>;
> +template <int N>
> +const int d = foo ();
> +const int *e = &d <0>;
> +template const int d <0>;
> +template <int N>
> +const int f = foo ();
> +template const int f <0>;
> +const int *g = &f <0>;
> +struct S { int a, b; };
> +template <int N>
> +const S h = { 42, foo () };
> +const S *i = &h <0>;
> +template <int N>
> +const S j =  { 42, foo () };
> +template const S j <0>;
> +template <int N>
> +const S k =  { 42, foo () };
> +const S *l = &k <0>;
> +template const S k <0>;
> +template <int N>
> +const S m =  { 42, foo () };
> +template const S m <0>;
> +const S *n = &m <0>;
> +
> +int
> +main ()
> +{
> +}
> 
> 	Jakub
> 
>