Re: [PATCH] c++: fix ICE with -Wduplicated-cond [PR107593]

[Patrick Palka via Gcc-patches]

On Fri, 27 Jan 2023, Marek Polacek wrote:

> On Fri, Jan 27, 2023 at 05:15:00PM -0500, Patrick Palka wrote:
> > On Thu, 26 Jan 2023, Marek Polacek via Gcc-patches wrote:
> > 
> > > Here we crash because a CAST_EXPR, representing T(), doesn't have
> > > its operand, and operand_equal_p's STRIP_ANY_LOCATION_WRAPPER doesn't
> > > expect that.  (o_e_p is called from warn_duplicated_cond_add_or_warn.)
> > > 
> > > In the past we've adjusted o_e_p to better cope with template codes,
> > > but in this case I think we just want to avoid attempting to warn
> > > about inst-dependent expressions; I don't think I've ever envisioned
> > > -Wduplicated-cond to warn about them.
> > > 
> > > The ICE started with r12-6022, two-stage name lookup for overloaded
> > > operators, which gave dependent operators a TREE_TYPE (in particular,
> > > DEPENDENT_OPERATOR_TYPE), so we no longer bail out here in o_e_p:
> > > 
> > >   /* Similar, if either does not have a type (like a template id),
> > >      they aren't equal.  */
> > >   if (!TREE_TYPE (arg0) || !TREE_TYPE (arg1))
> > >     return false;
> > > 
> > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > > 
> > >   PR c++/107593
> > > 
> > > gcc/cp/ChangeLog:
> > > 
> > >   * parser.cc (cp_parser_selection_statement): Don't do
> > >   -Wduplicated-cond when the condition is dependent.
> > > 
> > > gcc/testsuite/ChangeLog:
> > > 
> > >   * g++.dg/warn/Wduplicated-cond3.C: New test.
> > > ---
> > >  gcc/cp/parser.cc                              |  3 +-
> > >  gcc/testsuite/g++.dg/warn/Wduplicated-cond3.C | 38 +++++++++++++++++++
> > >  2 files changed, 40 insertions(+), 1 deletion(-)
> > >  create mode 100644 gcc/testsuite/g++.dg/warn/Wduplicated-cond3.C
> > > 
> > > diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
> > > index 4cdc1cd472f..3df85d49e16 100644
> > > --- a/gcc/cp/parser.cc
> > > +++ b/gcc/cp/parser.cc
> > > @@ -13209,7 +13209,8 @@ cp_parser_selection_statement (cp_parser* parser, 
> > > bool *if_p,
> > >       /* Add the condition.  */
> > >       condition = finish_if_stmt_cond (condition, statement);
> > >  
> > > -     if (warn_duplicated_cond)
> > > +     if (warn_duplicated_cond
> > > +         && !instantiation_dependent_expression_p (condition))
> > >         warn_duplicated_cond_add_or_warn (token->location, condition,
> > >                                           &chain);
> > 
> > I noticed warn_duplicated_cond_add_or_warn already has logic to handle
> > TREE_SIDE_EFFECTS conditions by invaliding the entire chain.  I wonder
> > if we'd want to do the same for instantiation-dep conditions?
> 
> warn_duplicated_cond_add_or_warn lives in c-family/c-warn.cc so I can't
> use instantiation_dependent_expression_p there.  Sure, I could write a
> C++ wrapper but with my patch we just won't add CONDITION to the chain
> which I thought would work just as well.

Ah that's unfortunate :( ISTM desirable to conservatively assume an
inst-dep cond has side effects though (possibly directly from
cp_parser_selection_statement), to avoid false positives as in:

  int n;

  template<class T> bool g() { n = 42; }

  template<class T>
  void f() {
    if (n)
      ;
    else if (g<T>())
      ;
    else if (n)
      ;
  }