Re: [PATCH] c++: ahead of time variable template-id coercion [PR89442]

[Patrick Palka <ppalka@redhat.com>]

On Wed, 3 May 2023, Patrick Palka wrote:

> This patch makes us coerce the arguments of a variable template-id ahead
> of time, as we do for other template-ids, which allows us to immediately
> diagnose template parameter/argument kind mismatches and arity mismatches.
> 
> Unfortunately this causes a regression in cpp1z/constexpr-if20.C: coercing
> the variable template-id m<ar, as> ahead of time means we strip it of
> typedefs, yielding m<typename C<i>::q, typename C<j>::q>, but in this
> stripped form we're directly using 'i' and so we expect to have captured
> it.  This is PR107437 but with a variable template instead of a class
> template.  I'm not sure how to fix this :(

... though it doesn't seem like a serious regression since it has easy
workarounds: use decltype(i){} instead of i, or just capture i.

> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?
> 
> 	PR c++/89442
> 	PR c++/107437
> 
> gcc/cp/ChangeLog:
> 
> 	* cp-tree.h (lookup_template_variable): Add complain parameter.
> 	* parser.cc (cp_parser_template_id): Pass tf_warning_or_error
> 	to lookup_template_variable.
> 	* pt.cc (lookup_template_variable): Add complain parameter.
> 	Coerce template arguments here ...
> 	(finish_template_variable): ... instead of here.
> 	(lookup_and_finish_template_variable): Check for error_mark_node
> 	result from lookup_template_variable.
> 	(tsubst_copy) <case TEMPLATE_ID_EXPR>: Pass complain to
> 	lookup_template_variable.
> 	(instantiate_template): Use build2 instead of
> 	lookup_template_variable to build a TEMPLATE_ID_EXPR
> 	for most_specialized_partial_spec.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp/pr64127.C: Expect "expected unqualified-id at end
> 	of input" error.
> 	* g++.dg/cpp0x/alias-decl-ttp1.C: Fix template parameter/argument
> 	kind mismatch for variable template has_P_match_V.
> 	* g++.dg/cpp1y/pr72759.C: Expect "template argument 1 is invalid"
> 	error.
> 	* g++.dg/cpp1z/constexpr-if20.C: XFAIL test due to bogus "'i' is
> 	not captured" error.
> 	* g++.dg/cpp1z/noexcept-type21.C: Fix arity of variable template d.
> 	* g++.dg/diagnostic/not-a-function-template-1.C: Add default
> 	template argument to variable template A so that A<> is valid.
> 	* g++.dg/parse/error56.C: Don't expect "ISO C++ forbids
> 	declaration with no type" error.
> 	* g++.dg/parse/template30.C: Don't expect "parse error in
> 	template argument list" error.
> 	* g++.dg/cpp1y/var-templ80.C: New test.
> ---
>  gcc/cp/cp-tree.h                              |  2 +-
>  gcc/cp/parser.cc                              |  2 +-
>  gcc/cp/pt.cc                                  | 20 ++++++++++---------
>  gcc/testsuite/g++.dg/cpp/pr64127.C            |  2 +-
>  gcc/testsuite/g++.dg/cpp0x/alias-decl-ttp1.C  |  2 +-
>  gcc/testsuite/g++.dg/cpp1y/pr72759.C          |  2 +-
>  gcc/testsuite/g++.dg/cpp1y/var-templ80.C      | 12 +++++++++++
>  gcc/testsuite/g++.dg/cpp1z/constexpr-if20.C   |  1 +
>  gcc/testsuite/g++.dg/cpp1z/noexcept-type21.C  |  2 +-
>  .../diagnostic/not-a-function-template-1.C    |  2 +-
>  gcc/testsuite/g++.dg/parse/error56.C          |  1 -
>  gcc/testsuite/g++.dg/parse/template30.C       |  3 +--
>  12 files changed, 32 insertions(+), 19 deletions(-)
>  create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ80.C
> 
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index c9c4cd6f32f..96807282ec5 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -7349,7 +7349,7 @@ extern bool redeclare_class_template		(tree, tree, tree);
>  extern tree lookup_template_class		(tree, tree, tree, tree,
>  						 int, tsubst_flags_t);
>  extern tree lookup_template_function		(tree, tree);
> -extern tree lookup_template_variable		(tree, tree);
> +extern tree lookup_template_variable		(tree, tree, tsubst_flags_t);
>  extern bool uses_template_parms			(tree);
>  extern bool uses_template_parms_level		(tree, int);
>  extern bool uses_outer_template_parms_in_constraints (tree);
> diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
> index d89553e7da8..4982583809b 100644
> --- a/gcc/cp/parser.cc
> +++ b/gcc/cp/parser.cc
> @@ -18525,7 +18525,7 @@ cp_parser_template_id (cp_parser *parser,
>      }
>    else if (variable_template_p (templ))
>      {
> -      template_id = lookup_template_variable (templ, arguments);
> +      template_id = lookup_template_variable (templ, arguments, tf_warning_or_error);
>        if (TREE_CODE (template_id) == TEMPLATE_ID_EXPR)
>  	SET_EXPR_LOCATION (template_id, combined_loc);
>      }
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 930291917f2..abf99feab20 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -10329,11 +10329,16 @@ lookup_template_class (tree d1, tree arglist, tree in_decl, tree context,
>  /* Return a TEMPLATE_ID_EXPR for the given variable template and ARGLIST.  */
>  
>  tree
> -lookup_template_variable (tree templ, tree arglist)
> +lookup_template_variable (tree templ, tree arglist, tsubst_flags_t complain)
>  {
>    if (flag_concepts && variable_concept_p (templ))
>      return build_concept_check (templ, arglist, tf_none);
>  
> +  tree parms = DECL_INNERMOST_TEMPLATE_PARMS (templ);
> +  arglist = coerce_template_parms (parms, arglist, templ, complain);
> +  if (arglist == error_mark_node)
> +    return error_mark_node;
> +
>    /* The type of the expression is NULL_TREE since the template-id could refer
>       to an explicit or partial specialization. */
>    return build2 (TEMPLATE_ID_EXPR, NULL_TREE, templ, arglist);
> @@ -10354,11 +10359,6 @@ finish_template_variable (tree var, tsubst_flags_t complain)
>        || any_dependent_template_arguments_p (arglist))
>      return var;
>  
> -  tree parms = DECL_TEMPLATE_PARMS (templ);
> -  arglist = coerce_template_parms (parms, arglist, templ, complain);
> -  if (arglist == error_mark_node)
> -    return error_mark_node;
> -
>    if (flag_concepts && !constraints_satisfied_p (templ, arglist))
>      {
>        if (complain & tf_error)
> @@ -10380,7 +10380,9 @@ tree
>  lookup_and_finish_template_variable (tree templ, tree targs,
>  				     tsubst_flags_t complain)
>  {
> -  tree var = lookup_template_variable (templ, targs);
> +  tree var = lookup_template_variable (templ, targs, complain);
> +  if (var == error_mark_node)
> +    return error_mark_node;
>    /* We may be called while doing a partial substitution, but the
>       type of the variable template may be auto, in which case we
>       will call do_auto_deduction in mark_used (which clears tf_partial)
> @@ -17793,7 +17795,7 @@ tsubst_copy (tree t, tree args, tsubst_flags_t complain, tree in_decl)
>  	  targs = tsubst_template_args (targs, args, complain, in_decl);
>  
>  	if (variable_template_p (tmpl))
> -	  return lookup_template_variable (tmpl, targs);
> +	  return lookup_template_variable (tmpl, targs, complain);
>  	else
>  	  return lookup_template_function (tmpl, targs);
>        }
> @@ -22097,7 +22099,7 @@ instantiate_template (tree tmpl, tree orig_args, tsubst_flags_t complain)
>        /* We need to determine if we're using a partial or explicit
>  	 specialization now, because the type of the variable could be
>  	 different.  */
> -      tree tid = lookup_template_variable (tmpl, targ_ptr);
> +      tree tid = build2 (TEMPLATE_ID_EXPR, NULL_TREE, tmpl, targ_ptr);
>        tree elt = most_specialized_partial_spec (tid, complain);
>        if (elt == error_mark_node)
>  	pattern = error_mark_node;
> diff --git a/gcc/testsuite/g++.dg/cpp/pr64127.C b/gcc/testsuite/g++.dg/cpp/pr64127.C
> index 29c3bf2662a..8dc3336a1f9 100644
> --- a/gcc/testsuite/g++.dg/cpp/pr64127.C
> +++ b/gcc/testsuite/g++.dg/cpp/pr64127.C
> @@ -1,4 +1,4 @@
>  /* { dg-do compile { target c++98_only } } */
>  
>  template <0> int __copy_streambufs_eof; // { dg-error "expected identifier|numeric constant|variable templates" }
> -__copy_streambufs_eof < // { dg-error "template argument|parse error|not name a type" }
> +__copy_streambufs_eof < // { dg-error "template argument|expected unqualified-id" }
> diff --git a/gcc/testsuite/g++.dg/cpp0x/alias-decl-ttp1.C b/gcc/testsuite/g++.dg/cpp0x/alias-decl-ttp1.C
> index d1af8d1bbb2..52c0362ec01 100644
> --- a/gcc/testsuite/g++.dg/cpp0x/alias-decl-ttp1.C
> +++ b/gcc/testsuite/g++.dg/cpp0x/alias-decl-ttp1.C
> @@ -2,5 +2,5 @@
>  // { dg-do compile { target c++14 } }
>  
>  template <bool> using enable_if_t = int;
> -template <class> bool has_P_match_v;
> +template <template <class> class> bool has_P_match_v;
>  template <template <class> class... List> enable_if_t<has_P_match_v<List...>> a;
> diff --git a/gcc/testsuite/g++.dg/cpp1y/pr72759.C b/gcc/testsuite/g++.dg/cpp1y/pr72759.C
> index 4af6ea47120..5c2fb9fba0b 100644
> --- a/gcc/testsuite/g++.dg/cpp1y/pr72759.C
> +++ b/gcc/testsuite/g++.dg/cpp1y/pr72759.C
> @@ -14,5 +14,5 @@ template <> struct SpecPerType<Specializer> {
>  };
>  template <unsigned X> void Specializer::A<X>::InnerMemberFn() {
>    using Spec = SpecPerType<Specializer>;
> -  Spec ErrorSite = Spec::SpecMbrFnPtr<SpecMbrFnPtr>;  // { dg-error "not declared" }
> +  Spec ErrorSite = Spec::SpecMbrFnPtr<SpecMbrFnPtr>;  // { dg-error "not declared|invalid" }
>  }
> 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..5670f232a84
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/var-templ80.C
> @@ -0,0 +1,12 @@
> +// PR c++/89442
> +// { dg-do compile { target c++14 } }
> +
> +template<class T, class U> bool vt;
> +
> +template<class T>
> +void f() {
> +  bool a = vt<T>;       // { dg-error "wrong number" }
> +  bool b = vt<T, T>;
> +  bool c = vt<T, T, T>; // { dg-error "number" }
> +  bool d = vt<T, 42>;   // { dg-error "mismatch" }
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-if20.C b/gcc/testsuite/g++.dg/cpp1z/constexpr-if20.C
> index 2fd678bb383..cd4f80253bb 100644
> --- a/gcc/testsuite/g++.dg/cpp1z/constexpr-if20.C
> +++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-if20.C
> @@ -12,6 +12,7 @@ void ao() {
>        [](auto j) {
>  	using as = typename C<j>::q;
>  	if constexpr (m<ar, as>) {}
> +	// { dg-bogus "'i' is not captured" "PR107437" { xfail { *-*-* } } .-1 }
>        }(g());
>    }(g());
>  }
> diff --git a/gcc/testsuite/g++.dg/cpp1z/noexcept-type21.C b/gcc/testsuite/g++.dg/cpp1z/noexcept-type21.C
> index d0a61d95e87..4e0a9eaa91a 100644
> --- a/gcc/testsuite/g++.dg/cpp1z/noexcept-type21.C
> +++ b/gcc/testsuite/g++.dg/cpp1z/noexcept-type21.C
> @@ -2,7 +2,7 @@
>  // { dg-do compile { target c++17 } }
>  
>  template <typename a> using b = typename a ::c;
> -template <typename> bool d;
> +template <typename, typename> bool d;
>  template <typename, typename> struct e {
>    template <typename f, typename g> e(f, g) {}
>    template <typename h, typename i, typename j>
> diff --git a/gcc/testsuite/g++.dg/diagnostic/not-a-function-template-1.C b/gcc/testsuite/g++.dg/diagnostic/not-a-function-template-1.C
> index caf8afa2697..56f15609405 100644
> --- a/gcc/testsuite/g++.dg/diagnostic/not-a-function-template-1.C
> +++ b/gcc/testsuite/g++.dg/diagnostic/not-a-function-template-1.C
> @@ -1,6 +1,6 @@
>  // { dg-do compile { target c++14 } }
>  
> -template<typename> int A;  // { dg-message "24:variable template" }
> +template<typename=int> int A;  // { dg-message "28:variable template" }
>  
>  template int A<>();  // { dg-error "14:template<class>" }
>  
> diff --git a/gcc/testsuite/g++.dg/parse/error56.C b/gcc/testsuite/g++.dg/parse/error56.C
> index bd3d27e8fcd..7c81ab4488f 100644
> --- a/gcc/testsuite/g++.dg/parse/error56.C
> +++ b/gcc/testsuite/g++.dg/parse/error56.C
> @@ -2,5 +2,4 @@
>  
>  template <0> int __copy_streambufs_eof; // { dg-error "" }
>  class {
> -// { dg-error "forbids" "" { target *-*-* } .+1 }
>      friend __copy_streambufs_eof <> ( // { dg-error "" }
> diff --git a/gcc/testsuite/g++.dg/parse/template30.C b/gcc/testsuite/g++.dg/parse/template30.C
> index fa89889a2bc..3bd7dda1a12 100644
> --- a/gcc/testsuite/g++.dg/parse/template30.C
> +++ b/gcc/testsuite/g++.dg/parse/template30.C
> @@ -29,8 +29,7 @@ qux ()
>  {
>    return z<A<int>>=0;		// { dg-error "'>>=' should be '>> =' to terminate a template argument list" "" { target c++11 } }
>  }				// { dg-error "'>>=' should be '> > =' to terminate a template argument list" "" { target c++98_only } .-1 }
> -				// { dg-error "parse error in template argument list" "" { target *-*-* } .-2 }
> -				// { dg-error "template argument 1 is invalid" "" { target *-*-* } .-3 }
> +				// { dg-error "template argument 1 is invalid" "" { target *-*-* } .-2 }
>  
>  void
>  quux ()
> -- 
> 2.40.1.459.g48d89b51b3
> 
>