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

On Fri, 14 Aug 2026, Marek Polacek wrote:

> 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?

Ah I forgot about fold_non_dependent_init, so I defaulted to _expr,
but if I knew about it I would use _init for sake of consistency.
Testing that now.  I'm not sure why we use _init here in the
first place though, maybe Jason knows?

> 
> >    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
> 
>