Re: [PATCH] c++: recalculating local specs via build_extra_args [PR114303]

[Patrick Palka <ppalka@redhat.com>]

On Tue, 12 Mar 2024, Jason Merrill wrote:

> On 3/11/24 12:53, Patrick Palka wrote:
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look
> > OK for trunk and release branches?
> > 
> > -- >8 --
> > 
> > r13-6452-g341e6cd8d603a3 made build_extra_args walk evaluated contexts
> > first so that we prefer processing a local specialization in an evaluated
> > context even if its first use is in an unevaluated context.  But this
> > means we need to avoid walking a tree that already has extra args/specs
> > saved because the list of saved specs appears to be an evaluated
> > context.  It seems then that we should be calculating the saved specs
> > from scratch each time, rather than potentially walking the saved specs
> > list from an earlier partial instantiation when calling build_extra_args
> > a second time around.
> 
> Makes sense, but I wonder if we want to approach that by avoiding walking into
> *_EXTRA_ARGS in extract_locals_r?  Or do we still want to walk into any nested
> extra args?  And if so, will we run into this same problem then?

I'm not sure totally but I'd expect a nested extra-args tree to always
have empty *_EXTRA_ARGS since the outer extra-args tree should intercept
any substitution before the inner extra-args tree can see it?

> 
> > 	PR c++/114303
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	* constraint.cc (tsubst_requires_expr): Clear
> > 	REQUIRES_EXPR_EXTRA_ARGS before calling build_extra_args.
> > 	* pt.cc (tsubst_stmt) <case IF_STMT>: Call build_extra_args
> > 	on the new IF_STMT instead of t which might already have
> > 	IF_STMT_EXTRA_ARGS.
> > 
> > gcc/testsuite/ChangeLog:
> > 
> > 	* g++.dg/cpp1z/constexpr-if-lambda6.C: New test.
> > ---
> >   gcc/cp/constraint.cc                             |  1 +
> >   gcc/cp/pt.cc                                     |  2 +-
> >   .../g++.dg/cpp1z/constexpr-if-lambda6.C          | 16 ++++++++++++++++
> >   3 files changed, 18 insertions(+), 1 deletion(-)
> >   create mode 100644 gcc/testsuite/g++.dg/cpp1z/constexpr-if-lambda6.C
> > 
> > diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
> > index 49de3211d4c..8a3b5d80ba7 100644
> > --- a/gcc/cp/constraint.cc
> > +++ b/gcc/cp/constraint.cc
> > @@ -2362,6 +2362,7 @@ tsubst_requires_expr (tree t, tree args, sat_info
> > info)
> >   	 matching or dguide constraint rewriting), in which case we need
> >   	 to partially substitute.  */
> >         t = copy_node (t);
> > +      REQUIRES_EXPR_EXTRA_ARGS (t) = NULL_TREE;
> >         REQUIRES_EXPR_EXTRA_ARGS (t) = build_extra_args (t, args,
> > info.complain);
> >         return t;
> >       }
> > diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> > index 8cf0d5b7a8d..37f2392d035 100644
> > --- a/gcc/cp/pt.cc
> > +++ b/gcc/cp/pt.cc
> > @@ -18718,7 +18718,7 @@ tsubst_stmt (tree t, tree args, tsubst_flags_t
> > complain, tree in_decl)
> >   	  IF_COND (stmt) = IF_COND (t);
> >   	  THEN_CLAUSE (stmt) = THEN_CLAUSE (t);
> >   	  ELSE_CLAUSE (stmt) = ELSE_CLAUSE (t);
> > -	  IF_STMT_EXTRA_ARGS (stmt) = build_extra_args (t, args, complain);
> > +	  IF_STMT_EXTRA_ARGS (stmt) = build_extra_args (stmt, args, complain);
> >   	  add_stmt (stmt);
> >   	  break;
> >   	}
> > diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-if-lambda6.C
> > b/gcc/testsuite/g++.dg/cpp1z/constexpr-if-lambda6.C
> > new file mode 100644
> > index 00000000000..038c2a41210
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-if-lambda6.C
> > @@ -0,0 +1,16 @@
> > +// PR c++/114303
> > +// { dg-do compile { target c++17 } }
> > +
> > +struct A { static constexpr bool value = true; };
> > +
> > +int main() {
> > +  [](auto x1) {
> > +    return [&](auto) {
> > +      return [&](auto x3) {
> > +        if constexpr (decltype(x3)::value) {
> > +          static_assert(decltype(x1)::value);
> > +        }
> > +      }(A{});
> > +    }(0);
> > +  }(A{});
> > +}
> 
>