[PATCH] c++: ICE in value_dependent_expression_p in C++98 mode [PR94938]

[Jason Merrill]

On 5/4/20 7:32 PM, Marek Polacek wrote:
> Here we ICE with -std=c++98 since the newly added call to uses_template_parms
> (r10-6357): we hit
> 26530             gcc_assert (cxx_dialect >= cxx11
> 26531                         || INTEGRAL_OR_ENUMERATION_TYPE_P (type));
> and TYPE is a record type.  The problem is that the argument to
> value_dependent_expression_p does not satisfy potential_constant_expression
> which it must, as the comment explains.  I thought about fixing this in
> uses_template_parms -- only call v_d_e_p if p_c_e is true, but in this
> case we want to also suppress the warnings if we don't have a constant
> expression.  I couldn't simply check TREE_CONSTANT as in
> compute_array_index_type_loc, because then we'd stop warning in the new
> Wtype-limits3.C test.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/10?
> 
> 	PR c++/94938
> 	* pt.c (tsubst_copy_and_build): Don't call value_dependent_expression_p
> 	if the expression is not potential_constant_expression.
> 
> 	* g++.dg/warn/template-2.C: New test.
> ---
>   gcc/cp/pt.c                            |  5 ++++-
>   gcc/testsuite/g++.dg/warn/template-2.C | 22 ++++++++++++++++++++++
>   2 files changed, 26 insertions(+), 1 deletion(-)
>   create mode 100644 gcc/testsuite/g++.dg/warn/template-2.C
> 
> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> index 61cb75bf801..691f6e4df36 100644
> --- a/gcc/cp/pt.c
> +++ b/gcc/cp/pt.c
> @@ -19423,7 +19423,10 @@ tsubst_copy_and_build (tree t,
>         {
>   	/* If T was type-dependent, suppress warnings that depend on the range
>   	   of the types involved.  */
> -	bool was_dep = uses_template_parms (t);
> +	++processing_template_decl;
> +	bool was_dep = (!potential_constant_expression (t)
> +			|| value_dependent_expression_p (t));
> +	--processing_template_decl;

Hmm, it seems weird to condition the warning on whether or not the 
expression is constant.

Do we want type_dependent_expression_p or 
instantiation_dependent_expression_p here?

I also wonder if we want to move the warning_sentinels after the RECURs.

>   	warning_sentinel s1(warn_type_limits, was_dep);
>   	warning_sentinel s2(warn_div_by_zero, was_dep);
>   	warning_sentinel s3(warn_logical_op, was_dep);
> diff --git a/gcc/testsuite/g++.dg/warn/template-2.C b/gcc/testsuite/g++.dg/warn/template-2.C
> new file mode 100644
> index 00000000000..1d29528b2ac
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/template-2.C
> @@ -0,0 +1,22 @@
> +// PR c++/94938 - ICE in value_dependent_expression_p in C++98 mode.
> +// { dg-do compile }
> +
> +template <typename> struct S { S(); S(bool); };
> +
> +struct C {
> +  bool operator()(S<float>);
> +};
> +
> +S<float> fn (bool);
> +
> +template<typename T> void
> +foo (T)
> +{
> +  S<float> s;
> +  S<float> x = fn(false || C()(s));
> +}
> +
> +int main ()
> +{
> +  foo(int());
> +}
> 
> base-commit: 5e681acd3587285cc3c8c6d603e4ce93cf6dacf2
>