Re: [PATCH] c++: ICE on loopy var tmpl auto deduction [PR109300]

[Jason Merrill <jason@redhat.com>]

On 3/28/23 13:37, Patrick Palka wrote:
> Now that we resolve non-dependent variable template-ids ahead of time,
> cp_finish_decl needs to handle a new invalid situation: we can end up
> trying to instantiate a variable template with deduced return type
> before we fully parsed (and attached) its initializer.
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this OK for
> trunK?
> 
> 	PR c++/109300
> 
> gcc/cp/ChangeLog:
> 
> 	* decl.cc (cp_finish_decl): Diagnose ordinary auto deduction
> 	with no initializer instead of asserting.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp1y/var-templ79.C: New test.
> ---
>   gcc/cp/decl.cc                           | 15 ++++++++++++++-
>   gcc/testsuite/g++.dg/cpp1y/var-templ79.C |  5 +++++
>   2 files changed, 19 insertions(+), 1 deletion(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ79.C
> 
> diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
> index 20b980f68c8..2c91693b99d 100644
> --- a/gcc/cp/decl.cc
> +++ b/gcc/cp/decl.cc
> @@ -8276,7 +8276,20 @@ cp_finish_decl (tree decl, tree init, bool init_const_expr_p,
>   	      return;
>   	    }
>   
> -	  gcc_assert (CLASS_PLACEHOLDER_TEMPLATE (auto_node));
> +	  if (CLASS_PLACEHOLDER_TEMPLATE (auto_node))
> +	    /* Class deduction with no initializer is OK.  */;
> +	  else
> +	    {
> +	      /* Ordinary auto deduction without an initializer, a situation
> +		 which grokdeclarator already catches and rejects for the most
> +		 part.  But we can still get here if we're instantiating a
> +		 variable template before we've fully parsed (and attached) its
> +		 initializer, e.g. template<class> auto x = x<int>;  */

In the case of recursively dependent instantiation I'd hope to have an 
error_mark_node initializer, rather than none?

> +	      error_at (DECL_SOURCE_LOCATION (decl),
> +			"declaration of %q#D has no initializer", decl);
> +	      TREE_TYPE (decl) = error_mark_node;
> +	      return;
> +	    }
>   	}
>         d_init = init;
>         if (d_init)
> diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ79.C b/gcc/testsuite/g++.dg/cpp1y/var-templ79.C
> new file mode 100644
> index 00000000000..3c0d276153a
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/var-templ79.C
> @@ -0,0 +1,5 @@
> +// PR c++/109300
> +// { dg-do compile { target c++14 } }
> +
> +template<class>
> +auto x = x<int>; // { dg-error "" }