On Mon, 19 Dec 2022, Jason Merrill wrote:
> On 12/6/22 13:35, Patrick Palka wrote:
> > This patch fixes some issues with substitution into a C++20 template
> > parameter object wrapper:
> >
> > * The first testcase demonstrates a situation where the same_type_p
> > assert in relevant case of tsubst_copy doesn't hold, because (partial)
> > substitution of {int,} into the VIEW_CONVERT_EXPR wrapper yields
> > A<int> but substitution into the underlying TEMPLATE_PARM_INDEX is a
> > nop and yields A<T> due to tsubst's level == 1 early exit test.
>
> We exit in that case because continuing would reduce the level to an
> impossible 0. Why doesn't the preceding code find a binding for T?
Whoops, I misspoke. The problem is that there's no binding for V since
only T=int was explicitly specified, so when substituting into the
TEMPLATE_PARM_INDEX for V in 'void f(B<V>)', we hit that early exit and
never get a chance to substitute T=int into the tpi's TREE_TYPE (which
would yield A<int> as desired). So the TREE_TYPE of the wrapped tpi
remains A<T> whereas the substituted TREE_TYPE of the wrapper is A<int>,
a mismatch.
>
> > So
> > this patch just gets rid of the assert; the type mismatch doesn't
> > seem to be a problem in practice, I suppose because the coercion is
> > from one dependent type to another.
> >
> > * In the second testcase, dependent substitution into the underlying
> > TEMPLATE_PARM_INDEX yields a CALL_EXPR with empty TREE_TYPE, which
> > tsubst_copy doesn't expect. This patch fixes this by handling empty
> > TREE_TYPE the same way as a non-const TREE_TYPE. Moreover, after
> > this substitution we're left with a VIEW_CONVERT_EXPR wrapping a
> > CALL_EXPR instead of a TEMPLATE_PARM_INDEX, which during the subsequent
> > non-dependent substitution tsubst_copy doesn't expect either. So
> > this patch also relaxes tsubst_copy to accept such VIEW_CONVERT_EXPR
> > too.
> >
> > * In the third testcase, we end up never resolving the call to
> > f.modify() since tsubst_copy doesn't do overload resolution.
> > This patch fixes this by moving the handling of these
> > VIEW_CONVERT_EXPR wrappers from tsubst_copy to tsubst_copy_and_build.
> > For good measure tsubst_copy_and_build should also handle
> > REF_PARENTHESIZED_P wrappers instead of delegating to tsubst_copy.
> >
> > After this patch, VIEW_CONVERT_EXPR substitution is ultimately just
> > moved from tsubst_copy to tsubst_copy_and_build and made more
> > permissive.
> >
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> > trunk?
> >
> > PR c++/103346
> > PR c++/104278
> > PR c++/102553
> >
> > gcc/cp/ChangeLog:
> >
> > * pt.cc (tsubst_copy) <case VIEW_CONVERT_EXPR>: In the handling
> > of C++20 template parameter object wrappers: Remove same_type_p
> > assert. Accept non-TEMPLATE_PARM_INDEX inner operand. Handle
> > empty TREE_TYPE on substituted inner operand. Move it to ...
> > (tsubst_copy_and_build): ... here. Also handle REF_PARENTHESIZED_P
> > VIEW_CONVERT_EXPRs.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/cpp2a/nontype-class52a.C: New test.
> > * g++.dg/cpp2a/nontype-class53.C: New test.
> > * g++.dg/cpp2a/nontype-class54.C: New test.
> > * g++.dg/cpp2a/nontype-class55.C: New test.
> > ---
> > gcc/cp/pt.cc | 73 ++++++++++---------
> > gcc/testsuite/g++.dg/cpp2a/nontype-class52a.C | 15 ++++
> > gcc/testsuite/g++.dg/cpp2a/nontype-class53.C | 25 +++++++
> > gcc/testsuite/g++.dg/cpp2a/nontype-class54.C | 23 ++++++
> > gcc/testsuite/g++.dg/cpp2a/nontype-class55.C | 15 ++++
> > 5 files changed, 116 insertions(+), 35 deletions(-)
> > create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class52a.C
> > create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class53.C
> > create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class54.C
> > create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class55.C
> >
> > diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> > index 2d8e4fdd4b5..0a196f069ad 100644
> > --- a/gcc/cp/pt.cc
> > +++ b/gcc/cp/pt.cc
> > @@ -17271,42 +17271,16 @@ tsubst_copy (tree t, tree args, tsubst_flags_t
> > complain, tree in_decl)
> > return maybe_wrap_with_location (op0, EXPR_LOCATION (t));
> > }
> > tree op = TREE_OPERAND (t, 0);
> > - if (code == VIEW_CONVERT_EXPR
> > - && TREE_CODE (op) == TEMPLATE_PARM_INDEX)
> > - {
> > - /* Wrapper to make a C++20 template parameter object const. */
> > - op = tsubst_copy (op, args, complain, in_decl);
> > - if (!CP_TYPE_CONST_P (TREE_TYPE (op)))
> > - {
> > - /* The template argument is not const, presumably because
> > - it is still dependent, and so not the const template parm
> > - object. */
> > - tree type = tsubst (TREE_TYPE (t), args, complain, in_decl);
> > - gcc_checking_assert
> > (same_type_ignoring_top_level_qualifiers_p
> > - (type, TREE_TYPE (op)));
> > - if (TREE_CODE (op) == CONSTRUCTOR
> > - || TREE_CODE (op) == IMPLICIT_CONV_EXPR)
> > - {
> > - /* Don't add a wrapper to these. */
> > - op = copy_node (op);
> > - TREE_TYPE (op) = type;
> > - }
> > - else
> > - /* Do add a wrapper otherwise (in particular, if op is
> > - another TEMPLATE_PARM_INDEX). */
> > - op = build1 (code, type, op);
> > - }
> > - return op;
> > - }
> > /* force_paren_expr can also create a VIEW_CONVERT_EXPR. */
> > - else if (code == VIEW_CONVERT_EXPR && REF_PARENTHESIZED_P (t))
> > + if (code == VIEW_CONVERT_EXPR && REF_PARENTHESIZED_P (t))
> > {
> > op = tsubst_copy (op, args, complain, in_decl);
> > op = build1 (code, TREE_TYPE (op), op);
> > REF_PARENTHESIZED_P (op) = true;
> > return op;
> > }
> > - /* We shouldn't see any other uses of these in templates. */
> > + /* We shouldn't see any other uses of these in templates
> > + (tsubst_copy_and_build handles C++20 tparm object wrappers). */
> > gcc_unreachable ();
> > }
> > @@ -21569,12 +21543,41 @@ tsubst_copy_and_build (tree t,
> > case NON_LVALUE_EXPR:
> > case VIEW_CONVERT_EXPR:
> > - if (location_wrapper_p (t))
> > - /* We need to do this here as well as in tsubst_copy so we get the
> > - other tsubst_copy_and_build semantics for a PARM_DECL operand. */
> > - RETURN (maybe_wrap_with_location (RECUR (TREE_OPERAND (t, 0)),
> > - EXPR_LOCATION (t)));
> > - /* fallthrough. */
> > + {
> > + tree op = RECUR (TREE_OPERAND (t, 0));
> > + if (location_wrapper_p (t))
> > + /* We need to do this here as well as in tsubst_copy so we get the
> > + other tsubst_copy_and_build semantics for a PARM_DECL operand.
> > */
> > + RETURN (maybe_wrap_with_location (op, EXPR_LOCATION (t)));
> > +
> > + gcc_checking_assert (TREE_CODE (t) == VIEW_CONVERT_EXPR);
> > + if (REF_PARENTHESIZED_P (t))
> > + /* force_paren_expr can also create a VIEW_CONVERT_EXPR. */
> > + RETURN (finish_parenthesized_expr (op));
> > +
> > + /* Otherwise, we're dealing with a wrapper to make a C++20 template
> > + parameter object const. */
> > + if (TREE_TYPE (op) == NULL_TREE
> > + || !CP_TYPE_CONST_P (TREE_TYPE (op)))
> > + {
> > + /* The template argument is not const, presumably because
> > + it is still dependent, and so not the const template parm
> > + object. */
> > + tree type = tsubst (TREE_TYPE (t), args, complain, in_decl);
> > + if (TREE_CODE (op) == CONSTRUCTOR
> > + || TREE_CODE (op) == IMPLICIT_CONV_EXPR)
> > + {
> > + /* Don't add a wrapper to these. */
> > + op = copy_node (op);
> > + TREE_TYPE (op) = type;
> > + }
> > + else
> > + /* Do add a wrapper otherwise (in particular, if op is
> > + another TEMPLATE_PARM_INDEX). */
> > + op = build1 (VIEW_CONVERT_EXPR, type, op);
> > + }
> > + RETURN (op);
> > + }
> > default:
> > /* Handle Objective-C++ constructs, if appropriate. */
> > diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class52a.C
> > b/gcc/testsuite/g++.dg/cpp2a/nontype-class52a.C
> > new file mode 100644
> > index 00000000000..ae5d5df70ac
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class52a.C
> > @@ -0,0 +1,15 @@
> > +// A version of nontype-class52.C where explicit template arguments are
> > +// given in the call to f (which during deduction need to be partially
> > +// substituted into the NTTP object V in f's signature).
> > +// { dg-do compile { target c++20 } }
> > +
> > +template<class> struct A { };
> > +
> > +template<auto> struct B { };
> > +
> > +template<class T, A<T> V> void f(B<V>);
> > +
> > +int main() {
> > + constexpr A<int> a;
> > + f<int>(B<a>{});
> > +}
> > diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class53.C
> > b/gcc/testsuite/g++.dg/cpp2a/nontype-class53.C
> > new file mode 100644
> > index 00000000000..9a6398c5f57
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class53.C
> > @@ -0,0 +1,25 @@
> > +// PR c++/103346
> > +// { dg-do compile { target c++20 } }
> > +
> > +struct Item {};
> > +
> > +template<class T, T... ts>
> > +struct Sequence { };
> > +
> > +template<Item... items>
> > +using ItemSequence = Sequence<Item, items...>;
> > +
> > +template<Item... items>
> > +constexpr auto f() {
> > + constexpr auto l = [](Item item) { return item; };
> > + return ItemSequence<l(items)...>{};
> > +}
> > +
> > +using ty0 = decltype(f<>());
> > +using ty0 = ItemSequence<>;
> > +
> > +using ty1 = decltype(f<{}>());
> > +using ty1 = ItemSequence<{}>;
> > +
> > +using ty3 = decltype(f<{}, {}, {}>());
> > +using ty3 = ItemSequence<{}, {}, {}>;
> > diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class54.C
> > b/gcc/testsuite/g++.dg/cpp2a/nontype-class54.C
> > new file mode 100644
> > index 00000000000..8127b1f5426
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class54.C
> > @@ -0,0 +1,23 @@
> > +// PR c++/104278
> > +// { dg-do compile { target c++20 } }
> > +
> > +struct foo {
> > + int value;
> > + constexpr foo modify() const { return { value + 1 }; }
> > +};
> > +
> > +template<foo f, bool Enable = f.value & 1>
> > +struct bar {
> > + static void run() { }
> > +};
> > +
> > +template<foo f>
> > +struct qux {
> > + static void run() {
> > + bar<f.modify()>::run();
> > + }
> > +};
> > +
> > +int main() {
> > + qux<foo{}>::run();
> > +}
> > diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class55.C
> > b/gcc/testsuite/g++.dg/cpp2a/nontype-class55.C
> > new file mode 100644
> > index 00000000000..afcb3d64495
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class55.C
> > @@ -0,0 +1,15 @@
> > +// PR c++/102553
> > +// { dg-do compile { target c++20 } }
> > +
> > +struct s1{};
> > +template<int> inline constexpr s1 ch{};
> > +
> > +template<s1 f> struct s2{};
> > +template<s1 f> using alias1 = s2<f>;
> > +
> > +template<class T>
> > +void general(int n) {
> > + alias1<ch<1>>{};
> > +}
> > +
> > +template void general<int>(int);
>
>
