On Tue, 2 Jun 2026, Jason Merrill wrote:
> On 6/2/26 2:57 PM, Patrick Palka wrote:
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look
> > OK for trunk/16?
> >
> > -- >8 --
> >
> > Here the nonsensical constraint-expression f() == 42 isn't getting
> > rejected ahead of time ultimately because we pass no_toplevel_fold_p=true
> > to cp_parser_binary_expression which makes it use build_min instead of
> > build_x_binary_op to avoid undesirable folding/canonicalization for sake
> > of OpenMP for loop parsing (r145014). But this certainly doesn't happen
> > during templated parsing, and constraint-expressions are always
> > templated, so we might as well pass no_toplevel_fold_p=false here.
>
> In this testcase the concept is never checked for satisfaction. In the
> testcase in the PR there is a similarly nonsensical concept that is used, and
> is diagnosed at that point. So this patch is about diagnosing more IFNDR
> code, yes?
It's mainly about IFNDR, but there's also name lookup implications:
struct A {
constexpr operator int() { return 0; };
constexpr friend bool operator+(A, int) { return true; } // #1
};
template<class T>
concept C = A{} + A{}; // resolves to #1 with patch, #2 without
constexpr bool operator+(A, A) { return false; } // #2
static_assert(C<int>); // succeeds only with patch
>
> With that clarification to the commit message, OK for trunk but not 16.
>
> > PR c++/125490
> >
> > gcc/cp/ChangeLog:
> > that is used
> > * parser.cc (cp_parser_constraint_expression): Pass
> > no_toplevel_fold_p=false instead of =true to
> > cp_parser_binary_expression.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/cpp2a/concepts-pr125490.C: New test.
> > ---
> > gcc/cp/parser.cc | 2 +-
> > gcc/testsuite/g++.dg/cpp2a/concepts-pr125490.C | 10 ++++++++++
> > 2 files changed, 11 insertions(+), 1 deletion(-)
> > create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-pr125490.C
> >
> > diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
> > index b5c8c62118cc..c1797191b833 100644
> > --- a/gcc/cp/parser.cc
> > +++ b/gcc/cp/parser.cc
> > @@ -34761,7 +34761,7 @@ cp_parser_constraint_expression (cp_parser *parser)
> > {
> > processing_constraint_expression_sentinel parsing_constraint;
> > ++processing_template_decl;
> > - cp_expr expr = cp_parser_binary_expression (parser, false, true,
> > + cp_expr expr = cp_parser_binary_expression (parser, false, false,
> > PREC_NOT_OPERATOR, NULL);
> > --processing_template_decl;
> > if (check_for_bare_parameter_packs (expr))
> > diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-pr125490.C
> > b/gcc/testsuite/g++.dg/cpp2a/concepts-pr125490.C
> > new file mode 100644
> > index 000000000000..a33416b232d1
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-pr125490.C
> > @@ -0,0 +1,10 @@
> > +// PR c++/125490
> > +// { dg-do compile { target c++20 } }
> > +
> > +void f();
> > +
> > +template<class T>
> > +concept C = f() == 42; // { dg-error "invalid operands" }
> > +
> > +template<class T>
> > +concept D = requires { requires f() == 42; }; // { dg-error "invalid
> > operands" }
>
>
