On Wed, 13 May 2020, Jason Merrill wrote:
> On 5/11/20 6:43 PM, Patrick Palka wrote:
> > In the testcase below we're prematurely folding away the
> > requires-expression to 'true' after substituting in the function's
> > template arguments, but before substituting in the lambda's deduced
> > template arguments.
> >
> > This happens because during the first tsubst_requires_expr,
> > processing_template_decl is 1 but 'args' is just {void} and therefore
> > non-dependent, so we end up folding away the requires-expression to
> > boolean_true_node before we could substitute in the lambda's template
> > arguments and determine that '*v' is ill-formed.
> >
> > This patch removes the uses_template_parms check when deciding in
> > tsubst_requires_expr whether to keep around a new requires-expression.
> > Regardless of whether the template arguments are dependent, there still
> > might be more template parameters to later substitute in -- as in the
> > testcase below -- and even if not, tsubst_expr doesn't perform full
> > semantic processing unless !processing_template_decl, so it seems we
> > should wait until then to fold away the requires-expression.
> >
> > Passes 'make check-c++', does this look OK to commit after a full
> > bootstrap/regtest?
>
> OK.
Would the same patch be OK to backport to the GCC 10 branch?
>
> > gcc/cp/ChangeLog:
> >
> > PR c++/95020
> > * constraint.c (tsubst_requires_expr): Produce a new
> > requires-expression when processing_template_decl, even if
> > template arguments are not dependent.
> >
> > gcc/testsuite/ChangeLog:
> >
> > PR c++/95020
> > * g++/cpp2a/concepts-lambda7.C: New test.
> > ---
> > gcc/cp/constraint.cc | 4 +---
> > gcc/testsuite/g++.dg/cpp2a/concepts-lambda7.C | 14 ++++++++++++++
> > 2 files changed, 15 insertions(+), 3 deletions(-)
> > create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-lambda7.C
> >
> > diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
> > index 4ad17f3b7d8..8ee347cae60 100644
> > --- a/gcc/cp/constraint.cc
> > +++ b/gcc/cp/constraint.cc
> > @@ -2173,9 +2173,7 @@ tsubst_requires_expr (tree t, tree args,
> > if (reqs == error_mark_node)
> > return boolean_false_node;
> > - /* In certain cases, produce a new requires-expression.
> > - Otherwise the value of the expression is true. */
> > - if (processing_template_decl && uses_template_parms (args))
> > + if (processing_template_decl)
> > return finish_requires_expr (cp_expr_location (t), parms, reqs);
> > return boolean_true_node;
> > diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-lambda7.C
> > b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda7.C
> > new file mode 100644
> > index 00000000000..50746b777a3
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda7.C
> > @@ -0,0 +1,14 @@
> > +// PR c++/95020
> > +// { dg-do compile { target c++2a } }
> > +
> > +template<typename>
> > +void foo() {
> > + auto t = [](auto v) {
> > + static_assert(requires { *v; }); // { dg-error "static assertion
> > failed" }
> > + };
> > + t(0);
> > +}
> > +
> > +void bar() {
> > + foo<void>();
> > +}
> >
>
>
