Re: [PATCH] c++: ICE w/ maybe_warn_nodiscard on non-dep call [PR123495, PR126860]

On Fri, Aug 14, 2026 at 12:22:55PM -0400, Patrick Palka wrote:
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK
> for trunk? It's a checking-only ICE so no big need to backport,
> maybe to 16 at most.
> 
> -- >8 --
> 
> Another latent issue uncovered by the NON_DEPENDENT_EXPR removal.
> cp_get_fndecl_from_callee, called from maybe_warn_nodiscard, tries to
> fold the templated callee via constant evaluation, but constant
> evaluation expects non-templated trees.  So use fold_non_dependent_expr
> which instantiates templated trees beforehand.
> 
> 	PR c++/123495
> 	PR c++/126860
> 
> gcc/cp/ChangeLog:
> 
> 	* cvt.cc (cp_get_fndecl_from_callee): Use fold_non_dependent_expr
> 	instead of maybe_constant_init.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/template/non-dependent36.C: New test.
> 	* g++.dg/template/non-dependent37.C: New test.
> ---
>  gcc/cp/cvt.cc                                   | 2 +-
>  gcc/testsuite/g++.dg/template/non-dependent36.C | 4 ++++
>  gcc/testsuite/g++.dg/template/non-dependent37.C | 9 +++++++++
>  3 files changed, 14 insertions(+), 1 deletion(-)
>  create mode 100644 gcc/testsuite/g++.dg/template/non-dependent36.C
>  create mode 100644 gcc/testsuite/g++.dg/template/non-dependent37.C
> 
> diff --git a/gcc/cp/cvt.cc b/gcc/cp/cvt.cc
> index a378b51a2bed..33d528b3c324 100644
> --- a/gcc/cp/cvt.cc
> +++ b/gcc/cp/cvt.cc
> @@ -1069,7 +1069,7 @@ cp_get_fndecl_from_callee (tree fn, bool fold /* = true */)
>    if (type == NULL_TREE || !INDIRECT_TYPE_P (type))
>      return NULL_TREE;
>    if (fold)
> -    fn = maybe_constant_init (fn);
> +    fn = fold_non_dependent_expr (fn);

Since this is _init, I would expect fold_non_dependent_init.
Was _expr a deliberate choice?

>    STRIP_NOPS (fn);
>    if (TREE_CODE (fn) == ADDR_EXPR
>        || TREE_CODE (fn) == FDESC_EXPR)
> diff --git a/gcc/testsuite/g++.dg/template/non-dependent36.C b/gcc/testsuite/g++.dg/template/non-dependent36.C
> new file mode 100644
> index 000000000000..a4bf06974028
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/non-dependent36.C
> @@ -0,0 +1,4 @@
> +// PR c++/123495
> +
> +template <int N>
> +void foo () { ((int (*) ()) 0) (); }
> diff --git a/gcc/testsuite/g++.dg/template/non-dependent37.C b/gcc/testsuite/g++.dg/template/non-dependent37.C
> new file mode 100644
> index 000000000000..ad58cf6f10ce
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/non-dependent37.C
> @@ -0,0 +1,9 @@
> +// PR c++/126860
> +
> +struct A {
> +  int (*foo)();
> +};
> +
> +template <typename T> struct S : public A {
> +  void bar() { A::foo(); }
> +};
> -- 
> 2.55.0.559.g11c6700f10
> 

Marek