Re: [PATCH] c++: scoped variable template-id of reference type [PR97340]

[Jason Merrill <jason@redhat.com>]

On 5/18/23 13:59, Patrick Palka wrote:
> lookup_and_finish_template_variable calls convert_from_reference, which
> means for a variable template-id of reference type the function returns
> an INDIRECT_REF instead of the bare VAR_DECL.  But the downstream logic
> of two callers, tsubst_qualified_id and finish_class_member_access_expr,
> expect a DECL_P result and so we end up crashing when resolving the
> template-id's in the first testcase.  (Note that these two callers
> eventually call convert_from_reference as appropriate, so this earlier
> call seems at best redundant.)
> 
> This patch fixes this by pulling out the convert_from_reference call
> from lookup_and_finish_template_variable and into the callers that
> actually need it, which turns out to be tsubst_copy_and_build (without
> it we'd mishandle the second testcase).
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?

OK.

> 	PR c++/97340
> 
> gcc/cp/ChangeLog:
> 
> 	* pt.cc (lookup_and_finish_template_variable): Don't call
> 	convert_from_reference.
> 	(tsubst_copy_and_build) <case TEMPLATE_ID_EXPR>: Call
> 	convert_from_reference on the result of
> 	lookup_and_finish_template_variable.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp1y/var-templ80.C: New test.
> 	* g++.dg/cpp1y/var-templ81.C: New test.
> ---
>   gcc/cp/pt.cc                             |  3 ++-
>   gcc/testsuite/g++.dg/cpp1y/var-templ80.C | 22 ++++++++++++++++++++++
>   gcc/testsuite/g++.dg/cpp1y/var-templ81.C | 14 ++++++++++++++
>   3 files changed, 38 insertions(+), 1 deletion(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ80.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ81.C
> 
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 351fc18b600..9e5b29f3099 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -10394,7 +10394,7 @@ lookup_and_finish_template_variable (tree templ, tree targs,
>     complain &= ~tf_partial;
>     var = finish_template_variable (var, complain);
>     mark_used (var);
> -  return convert_from_reference (var);
> +  return var;
>   }
>   
>   /* If the set of template parameters PARMS contains a template parameter
> @@ -20462,6 +20462,7 @@ tsubst_copy_and_build (tree t,
>   	  {
>   	    tree r = lookup_and_finish_template_variable (templ, targs,
>   							  complain);
> +	    r = convert_from_reference (r);
>   	    r = maybe_wrap_with_location (r, EXPR_LOCATION (t));
>   	    RETURN (r);
>   	  }
> diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ80.C b/gcc/testsuite/g++.dg/cpp1y/var-templ80.C
> new file mode 100644
> index 00000000000..4439bee8292
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/var-templ80.C
> @@ -0,0 +1,22 @@
> +// PR c++/97340
> +// { dg-do compile { target c++14 } }
> +
> +template<class>
> +struct A {
> +  template<class>
> +  static constexpr const int& var = 0;
> +};
> +
> +template<class T>
> +struct B {
> +  static constexpr int x1 = A<T>::template var<int>;
> +  static constexpr int y1 = A<T>{}.template var<int>;
> +
> +  static constexpr int x2 = A<int>::template var<T>;
> +  static constexpr int y2 = A<int>{}.template var<T>;
> +
> +  static constexpr int x3 = A<int>::template var<int>;
> +  static constexpr int y3 = A<int>{}.template var<int>;
> +};
> +
> +template struct B<int>;
> diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ81.C b/gcc/testsuite/g++.dg/cpp1y/var-templ81.C
> new file mode 100644
> index 00000000000..f9d2e6b1eed
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/var-templ81.C
> @@ -0,0 +1,14 @@
> +// Verify we don't ICE on an invalid use of unary * for a variable
> +// template-id of reference type.
> +// { dg-do compile { target c++14 } }
> +
> +template<class>
> +static constexpr const int& var = 0;
> +
> +template<class T>
> +struct B {
> +  static constexpr int x = *var<T>; // { dg-error "argument of unary" }
> +  static constexpr const int& y = *var<T>; // { dg-error "argument of unary" }
> +};
> +
> +template struct B<int>;