Re: [PATCH] c++: Fix deduction from the type of an NTTP

[Jason Merrill <jason@redhat.com>]

On 1/4/21 5:50 PM, Patrick Palka wrote:
> In the testcase nontype-auto17.C below, the calls to f and g are invalid
> because neither deduction nor defaulting of the template parameter T
> yields a valid specialization.  Deducing T doesn't work because T is
> only used in a non-deduced context, and defaulting T doesn't work
> because its default argument makes the type of M invalid.
> 
> But with -std=c++17 or later, we incorrectly accept both calls.  With
> C++17 (specifically P0127R2), we're allowed to try to deduce T from
> the argument 42 that's been tentatively deduced for M.  The problem is
> that when unify walks into the type of M, it immediately gives up on
> the TYPENAME_TYPE and doesn't register any new unifications (so the type
> of M is still unknown) -- and then we go on to unify M with 42 anyway.
> Later in type_unification_real, we blindly use the default argument for
> T to complete the template argument vector, and we end up with the bogus
> specializations f<void, 42> and g<S, 42>.
> 
> This patch fixes this issue by checking whether the type of a NTTP is
> still dependent after walking into the type.  If it is, it means we
> couldn't deduce all the template parameters used in its type, and so we
> shouldn't yet unify the NTTP.
> 
> (The new testcase ttp33.C demonstrates the need for the TEMPLATE_PARM_LEVEL
> check; without it, we would ICE on this testcase from the call to tsubst.)
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?
> 
> gcc/cp/ChangeLog:
> 
> 	* pt.c (unify) <case TEMPLATE_PARM_INDEX>: After walking into
> 	the type of the TEMPLATE_PARM_INDEX, substitute into the type a
> 	second time.  If the type is still dependent, don't unify it.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/template/partial5.C: Adjust directives to expect the
> 	same errors across all dialects.
> 	* g++.dg/cpp1z/nontype-auto17.C: New test.
> 	* g++.dg/cpp1z/nontype-auto18.C: New test.
> 	* g++.dg/template/ttp33.C: New test.
> ---
>   gcc/cp/pt.c                                 | 10 +++++++++-
>   gcc/testsuite/g++.dg/cpp1z/nontype-auto17.C | 10 ++++++++++
>   gcc/testsuite/g++.dg/cpp1z/nontype-auto18.C |  6 ++++++
>   gcc/testsuite/g++.dg/template/partial5.C    |  2 +-
>   gcc/testsuite/g++.dg/template/ttp33.C       | 10 ++++++++++
>   5 files changed, 36 insertions(+), 2 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp1z/nontype-auto17.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp1z/nontype-auto18.C
>   create mode 100644 gcc/testsuite/g++.dg/template/ttp33.C
> 
> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> index 19fd4c1d8a4..f1e8b01bc01 100644
> --- a/gcc/cp/pt.c
> +++ b/gcc/cp/pt.c
> @@ -23581,13 +23581,21 @@ unify (tree tparms, tree targs, tree parm, tree arg, int strict,
>   	  /* We haven't deduced the type of this parameter yet.  */
>   	  if (cxx_dialect >= cxx17
>   	      /* We deduce from array bounds in try_array_deduction.  */
> -	      && !(strict & UNIFY_ALLOW_INTEGER))
> +	      && !(strict & UNIFY_ALLOW_INTEGER)
> +	      && TEMPLATE_PARM_LEVEL (parm) <= TMPL_ARGS_DEPTH (targs))
>   	    {
>   	      /* Deduce it from the non-type argument.  */
>   	      tree atype = TREE_TYPE (arg);
>   	      RECUR_AND_CHECK_FAILURE (tparms, targs,
>   				       tparm, atype,
>   				       UNIFY_ALLOW_NONE, explain_p);
> +	      /* Now check whether the type of this parameter is still
> +		 dependent, and give up if so.  */
> +	      ++processing_template_decl;
> +	      tparm = tsubst (tparm, targs, tf_none, NULL_TREE);
> +	      --processing_template_decl;
> +	      if (uses_template_parms (tparm))
> +		return unify_success (explain_p);

Hmm, I was wondering about returning success without checking whether 
the type is still dependent, and relying on the retrying in 
type_unification_real, but that only works for function templates.

The patch is OK.

>   	    }
>   	  else
>   	    /* Try again later.  */
> diff --git a/gcc/testsuite/g++.dg/cpp1z/nontype-auto17.C b/gcc/testsuite/g++.dg/cpp1z/nontype-auto17.C
> new file mode 100644
> index 00000000000..509eb0e98e3
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1z/nontype-auto17.C
> @@ -0,0 +1,10 @@
> +// { dg-do compile { target c++11 } }
> +
> +template <int> struct K { };
> +
> +template <class T = void, typename T::type M> int f(K<M>); // { dg-error "void" }
> +int a = f(K<42>{}); // { dg-error "no match" }
> +
> +struct S { using type = void; };
> +template <class T = S, typename T::type M> int g(K<M>); // { dg-message "deduction" }
> +int b = g(K<42>{}); // { dg-error "no match" }
> diff --git a/gcc/testsuite/g++.dg/cpp1z/nontype-auto18.C b/gcc/testsuite/g++.dg/cpp1z/nontype-auto18.C
> new file mode 100644
> index 00000000000..46873672714
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1z/nontype-auto18.C
> @@ -0,0 +1,6 @@
> +// { dg-do compile { target c++11 } }
> +
> +template <int> struct K { };
> +struct S { using type = int; };
> +template <class T = S, typename T::type M> int f(K<M>);
> +int c = f(K<42>{});
> diff --git a/gcc/testsuite/g++.dg/template/partial5.C b/gcc/testsuite/g++.dg/template/partial5.C
> index a56229770f4..40d8c45b087 100644
> --- a/gcc/testsuite/g++.dg/template/partial5.C
> +++ b/gcc/testsuite/g++.dg/template/partial5.C
> @@ -14,7 +14,7 @@ template<typename T, typename T::foo V>
>   struct Y { };
>   
>   template<typename T, typename U, U v>
> -struct Y<T, v> { }; // { dg-error "" "" { target { ! c++17 } } }
> +struct Y<T, v> { }; // { dg-error "" }
>   
>   
>   template<typename T, T V>
> diff --git a/gcc/testsuite/g++.dg/template/ttp33.C b/gcc/testsuite/g++.dg/template/ttp33.C
> new file mode 100644
> index 00000000000..cd0de8ca641
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/ttp33.C
> @@ -0,0 +1,10 @@
> +// A slight variation of ttp31.C.
> +// { dg-do compile { target c++11 } }
> +
> +template<class TA,
> +	 template<typename TA::type...> class TTA, TA... VA>
> +struct A { };
> +
> +template<class UC, class TC,
> +	 template<typename TC::type...> class TTC, TC... VC>
> +struct C : A<TC, TTC, VC...> { };
>