c++: missing SFINAE for consteval calls [PR104620]

  Here we weren't respecting SFINAE when evaluating a substituted call to
a consteval function, which caused us to reject the new testcase below.
This patch fixes this by making build_over_call use the SFINAE-friendly
version of cxx_constant_value.

This change causes us to no longer diagnose ahead of time a couple of
invalid non-dependent consteval calls in consteval-if2.C (with
-fchecking=2).  These errors were apparently coming from the call to
fold_non_dependent_expr in build_non_dependent_expr (for the RHS of the +=)
despite complain=tf_none being passed.  Now that build_over_call
respects the value of complain during constant evaluation of a consteval
call, the errors are gone.

That the errors don't occur without -fchecking=2 is a regression caused
by r12-7264-gc19f317a78c0e4 and is the subject of PR104620.  As described
in comment #5, I think it was only an accident that we were diagnosing
these two calls correctly before r12-7264, so perhaps we can live
without them in GCC 12.  To that end this patch just XFAILs the two tests.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?

	PR c++/104620

gcc/cp/ChangeLog:

	* call.cc (build_over_call): Use cxx_constant_value_sfinae
	instead of cxx_constant_value to evaluate a consteval call.
	* constexpr.cc (cxx_constant_value_sfinae): Add decl parameter
	and pass it to cxx_eval_outermost_constant_expr.
	* cp-tree.h (cxx_constant_value_sfinae): Add decl parameter.
	* pt.cc (fold_targs_r): Pass NULL_TREE as decl to
	cxx_constant_value_sfinae.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/consteval-if2.C: XFAIL two dg-error tests where
	the argument to the non-dependent consteval call is wrapped in
	NON_DEPENDENT_EXPR.
	* g++.dg/cpp2a/consteval30.C: New test.
---
 gcc/cp/call.cc                             |  2 +-
 gcc/cp/constexpr.cc                        |  4 ++--
 gcc/cp/cp-tree.h                           |  2 +-
 gcc/cp/pt.cc                               |  2 +-
 gcc/testsuite/g++.dg/cpp23/consteval-if2.C |  4 ++--
 gcc/testsuite/g++.dg/cpp2a/consteval30.C   | 12 ++++++++++++
 6 files changed, 19 insertions(+), 7 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/consteval30.C
  

On 3/24/22 10:32, Patrick Palka wrote:
> Here we weren't respecting SFINAE when evaluating a substituted call to
> a consteval function, which caused us to reject the new testcase below.
> This patch fixes this by making build_over_call use the SFINAE-friendly
> version of cxx_constant_value.
> 
> This change causes us to no longer diagnose ahead of time a couple of
> invalid non-dependent consteval calls in consteval-if2.C (with
> -fchecking=2).  These errors were apparently coming from the call to
> fold_non_dependent_expr in build_non_dependent_expr (for the RHS of the +=)
> despite complain=tf_none being passed.  Now that build_over_call
> respects the value of complain during constant evaluation of a consteval
> call, the errors are gone.
> 
> That the errors don't occur without -fchecking=2 is a regression caused
> by r12-7264-gc19f317a78c0e4 and is the subject of PR104620.  As described
> in comment #5, I think it was only an accident that we were diagnosing
> these two calls correctly before r12-7264, so perhaps we can live
> without them in GCC 12.  To that end this patch just XFAILs the two tests.
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?

OK.

> 	PR c++/104620
> 
> gcc/cp/ChangeLog:
> 
> 	* call.cc (build_over_call): Use cxx_constant_value_sfinae
> 	instead of cxx_constant_value to evaluate a consteval call.
> 	* constexpr.cc (cxx_constant_value_sfinae): Add decl parameter
> 	and pass it to cxx_eval_outermost_constant_expr.
> 	* cp-tree.h (cxx_constant_value_sfinae): Add decl parameter.
> 	* pt.cc (fold_targs_r): Pass NULL_TREE as decl to
> 	cxx_constant_value_sfinae.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp23/consteval-if2.C: XFAIL two dg-error tests where
> 	the argument to the non-dependent consteval call is wrapped in
> 	NON_DEPENDENT_EXPR.
> 	* g++.dg/cpp2a/consteval30.C: New test.
> ---
>   gcc/cp/call.cc                             |  2 +-
>   gcc/cp/constexpr.cc                        |  4 ++--
>   gcc/cp/cp-tree.h                           |  2 +-
>   gcc/cp/pt.cc                               |  2 +-
>   gcc/testsuite/g++.dg/cpp23/consteval-if2.C |  4 ++--
>   gcc/testsuite/g++.dg/cpp2a/consteval30.C   | 12 ++++++++++++
>   6 files changed, 19 insertions(+), 7 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/consteval30.C
> 
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index 23d3fc496b8..ec6c5d5baa2 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -9939,7 +9939,7 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
>   		    obj_arg = TREE_OPERAND (addr, 0);
>   		}
>   	    }
> -	  call = cxx_constant_value (call, obj_arg);
> +	  call = cxx_constant_value_sfinae (call, obj_arg, complain);
>   	  if (obj_arg && !error_operand_p (call))
>   	    call = build2 (INIT_EXPR, void_type_node, obj_arg, call);
>   	  call = convert_from_reference (call);
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index a3136ce819d..2163a328e74 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -7959,10 +7959,10 @@ cxx_constant_value (tree t, tree decl)
>   /* As above, but respect SFINAE.  */
>   
>   tree
> -cxx_constant_value_sfinae (tree t, tsubst_flags_t complain)
> +cxx_constant_value_sfinae (tree t, tree decl, tsubst_flags_t complain)
>   {
>     bool sfinae = !(complain & tf_error);
> -  tree r = cxx_eval_outermost_constant_expr (t, sfinae, true, true);
> +  tree r = cxx_eval_outermost_constant_expr (t, sfinae, true, true, false, decl);
>     if (sfinae && !TREE_CONSTANT (r))
>       r = error_mark_node;
>     return r;
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index 1bd7bc6fca2..2f718852ac1 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -8414,7 +8414,7 @@ extern bool require_constant_expression (tree);
>   extern bool require_rvalue_constant_expression (tree);
>   extern bool require_potential_rvalue_constant_expression (tree);
>   extern tree cxx_constant_value			(tree, tree = NULL_TREE);
> -extern tree cxx_constant_value_sfinae		(tree, tsubst_flags_t);
> +extern tree cxx_constant_value_sfinae		(tree, tree, tsubst_flags_t);
>   extern void cxx_constant_dtor			(tree, tree);
>   extern tree cxx_constant_init			(tree, tree = NULL_TREE);
>   extern tree maybe_constant_value		(tree, tree = NULL_TREE, bool = false);
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 715eea27577..173bc3a8c7f 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -19811,7 +19811,7 @@ fold_targs_r (tree targs, tsubst_flags_t complain)
>   	       && !glvalue_p (elt)
>   	       && !TREE_CONSTANT (elt))
>   	{
> -	  elt = cxx_constant_value_sfinae (elt, complain);
> +	  elt = cxx_constant_value_sfinae (elt, NULL_TREE, complain);
>   	  if (elt == error_mark_node)
>   	    return false;
>   	}
> diff --git a/gcc/testsuite/g++.dg/cpp23/consteval-if2.C b/gcc/testsuite/g++.dg/cpp23/consteval-if2.C
> index f7053b91c3c..d1845da9e58 100644
> --- a/gcc/testsuite/g++.dg/cpp23/consteval-if2.C
> +++ b/gcc/testsuite/g++.dg/cpp23/consteval-if2.C
> @@ -77,11 +77,11 @@ qux (int x)
>       }
>     else
>       {
> -      r += foo (8 * x);	// { dg-error "is not a constant expression" }
> +      r += foo (8 * x);	// { dg-error "is not a constant expression" "" { xfail *-*-* } }
>       }
>     if ! consteval	// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
>       {
> -      r += foo (32 * x);// { dg-error "is not a constant expression" }
> +      r += foo (32 * x);// { dg-error "is not a constant expression" "" { xfail *-*-* } }
>       }
>     if consteval		// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
>       {
> diff --git a/gcc/testsuite/g++.dg/cpp2a/consteval30.C b/gcc/testsuite/g++.dg/cpp2a/consteval30.C
> new file mode 100644
> index 00000000000..d393dcc0857
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/consteval30.C
> @@ -0,0 +1,12 @@
> +// Test SFINAE behavior for an invalid consteval call.
> +// { dg-do compile { target c++20 } }
> +
> +consteval int deref(const int* x) { return *x; }
> +
> +template<const int* P, int = deref(P)> // { dg-bogus "null pointer" }
> +constexpr int f(int) { return 0; }
> +
> +template<const int* P>
> +constexpr int f(...) { return 1; }
> +
> +static_assert(f<nullptr>(0) == 1);