On Tue, Oct 20, 2026 at 06:08:54PM -0400, Jason Merrill wrote:
> On 10/8/26 5:26 PM, Marek Polacek wrote:
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> >
> > -- >8 --
> > [expr.prim.splice]/2 doesn't actually allow templates in
> > a splice-expression of the form splice-specifier, but we are not
> > currently detecting that in check_splice_expr, which can mean
> > that we crash if the user attempts to use such a splice.
> >
> > As an extension, we allow function templates, and assume that the
> > user forgot to type 'template'.
>
> Do we still give a diagnostic by default in that case?
No, the missing "expected template keyword" is subject of PR127802.
> > mangle3.C was testing invalid cases so remove that.
> >
> > PR c++/125631
> >
> > gcc/cp/ChangeLog:
> >
> > * reflect.cc (check_splice_expr): Reject (non-function) templates.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/reflect/crash6.C: Adjust dg-error.
> > * g++.dg/reflect/mangle3.C: Remove some invalid tests.
> > * g++.dg/reflect/error15.C: New test.
> > ---
> > gcc/cp/reflect.cc | 15 +++++
> > gcc/testsuite/g++.dg/reflect/crash6.C | 4 +-
> > gcc/testsuite/g++.dg/reflect/error15.C | 84 ++++++++++++++++++++++++++
> > gcc/testsuite/g++.dg/reflect/mangle3.C | 9 ---
> > 4 files changed, 101 insertions(+), 11 deletions(-)
> > create mode 100644 gcc/testsuite/g++.dg/reflect/error15.C
> >
> > diff --git a/gcc/cp/reflect.cc b/gcc/cp/reflect.cc
> > index 8bf518df64f..62c4e168108 100644
> > --- a/gcc/cp/reflect.cc
> > +++ b/gcc/cp/reflect.cc
> > @@ -9362,6 +9362,21 @@ check_splice_expr (location_t loc, location_t
> > start_loc, tree t,
> > return false;
> > }
> > + /* [expr.prim.splice]/2 doesn't allow for the splice-expression of the
> > form
> > + splice-specifier to designate a template. But we accept a function
> > + template here as an extension, because it would be valid if the
> > 'template'
> > + keyword were present. */
> > + if (!template_p
> > + && !targs_p
> > + && !member_access_p
> > + && TREE_CODE (t) == TEMPLATE_DECL
> > + && !DECL_FUNCTION_TEMPLATE_P (t))
> > + {
> > + if (complain_p)
> > + error_at (loc, "cannot use template %qE in a splice expression", t);
> > + return false;
> > + }
> > +
> > /* [expr.prim.splice]/2: "The expression is ill-formed if S [the
> > construct
> > designated by splice-specifier] is
> > -- a local entity such that there is a lambda scope that intervenes
> > diff --git a/gcc/testsuite/g++.dg/reflect/crash6.C
> > b/gcc/testsuite/g++.dg/reflect/crash6.C
> > index a455a61cd65..4dd2f5fc7b8 100644
> > --- a/gcc/testsuite/g++.dg/reflect/crash6.C
> > +++ b/gcc/testsuite/g++.dg/reflect/crash6.C
> > @@ -10,8 +10,8 @@ template<info R>
> > void
> > f ()
> > {
> > - [:R:] r; // { dg-error "expected" }
> > - [:R:]<int> r; // { dg-error "expected" }
> > + [:R:] r; // { dg-error "expected|cannot use template" }
> > + [:R:]<int> r; // { dg-error "expected|cannot use template" }
> > }
> > void
> > diff --git a/gcc/testsuite/g++.dg/reflect/error15.C
> > b/gcc/testsuite/g++.dg/reflect/error15.C
> > new file mode 100644
> > index 00000000000..a2fdf4a52ed
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/reflect/error15.C
> > @@ -0,0 +1,84 @@
> > +// PR c++/125631
> > +// { dg-do compile { target c++26 } }
> > +// { dg-additional-options "-freflection" }
> > +
> > +#include <tuple>
> > +
> > +template<auto Templ, class Tuple>
> > +const auto
> > +make_from_tuple (Tuple&& t)
> > +{
> > + return [:Templ:]{}; // { dg-error "expected|cannot use template
> > .std::tuple. in a splice expression" }
> > +}
> > +auto p = make_from_tuple<^^std::tuple>(std::tuple());
> > +
> > +template<auto N> int tv;
> > +template<auto N> concept C = requires { true; };
> > +template<typename T> struct S { };
> > +template<typename T>
> > +using A = S<T>;
> > +template<typename> using I = int;
> > +
> > +template<auto R>
> > +void
> > +g1 ()
> > +{
> > + auto a = [:R:]; // { dg-error "cannot use template .tv<N>. in a splice
> > expression" }
> > +}
> > +
> > +template<auto R>
> > +void
> > +g2 ()
> > +{
> > + auto a = [:R:]; // { dg-error "cannot use template .C. in a splice
> > expression" }
> > +}
> > +
> > +template<auto R>
> > +void
> > +g3 ()
> > +{
> > + auto a = [:R:]; // { dg-error "cannot use template .S. in a splice
> > expression" }
> > +}
> > +
> > +template<auto R>
> > +void
> > +g4 ()
> > +{
> > + auto a = [:R:]; // { dg-error "cannot use template .A. in a splice
> > expression" }
> > +}
> > +
> > +template<auto R>
> > +void
> > +g5 ()
> > +{
> > + auto a = [:R:]; // { dg-error "cannot use template .I. in a splice
> > expression" }
> > +}
> > +
> > +template<template<class> class TT>
> > +void
> > +g6 ()
> > +{
> > + auto x = [:^^TT:]; // { dg-error "cannot use template .S. in a splice
> > expression" }
> > +}
> > +
> > +template<auto R>
> > +void
> > +g7 ()
> > +{
> > + &[:R:]; // { dg-error "cannot use template .S. in a splice expression" }
> > +}
> > +
> > +template<auto R> auto g8 () -> decltype([:R:]); // { dg-error "cannot
> > use template .tv<N>. in a splice expression" }
> > +
> > +void
> > +h ()
> > +{
> > + g1<^^tv>();
> > + g2<^^C>();
> > + g3<^^S>();
> > + g4<^^A>();
> > + g5<^^I>();
> > + g6<S>();
> > + g7<^^S>();
> > + g8<^^tv>(); // { dg-error "no matching function" }
> > +}
> > diff --git a/gcc/testsuite/g++.dg/reflect/mangle3.C
> > b/gcc/testsuite/g++.dg/reflect/mangle3.C
> > index 390f249fb0c..1e1302d1377 100644
> > --- a/gcc/testsuite/g++.dg/reflect/mangle3.C
> > +++ b/gcc/testsuite/g++.dg/reflect/mangle3.C
> > @@ -15,8 +15,6 @@ using Alias = int;
> > template<auto>
> > struct TCls { };
> > -template <auto> int TVar;
> > -
> > template<typename T>
> > struct B {
> > T t;
> > @@ -41,7 +39,6 @@ namespace N {
> > }
> > namespace NSAlias = N;
> > -template <auto> concept Concept = requires { true; };
> > constexpr auto ctx = std::meta::access_context::current ();
> > template<typename T>
> > @@ -160,18 +157,12 @@ g (int p)
> > // { dg-final { scan-assembler "_Z2f4ILDmsb2a1EE4TClsIXszDST_EEEv" } }
> > f4<^^A>();
> > // { dg-final { scan-assembler "_Z2f4ILDmen4Enum1AEE4TClsIXszDST_EEEv" } }
> > - f4<^^TVar>();
> > -// { dg-final { scan-assembler "_Z2f4ILDmvt4TVarEE4TClsIXszDST_EEEv" } }
> > f5<^^i>();
> > // { dg-final { scan-assembler "_Z2f5ILDmvr1iEEDtDST_EEv" } }
> > f5<^^a1>();
> > // { dg-final { scan-assembler "_Z2f5ILDmsb2a1EEDtDST_EEv" } }
> > f5<^^A>();
> > // { dg-final { scan-assembler "_Z2f5ILDmen4Enum1AEEDtDST_EEv" } }
> > - f5<^^TVar>();
> > -// { dg-final { scan-assembler "_Z2f5ILDmvt4TVarEEDtDST_EEv" } }
> > - f5<^^Concept>();
> > -// { dg-final { scan-assembler "_Z2f5ILDmco7ConceptEEDtDST_EEv" } }
> > f5<^^p>();
> > // { dg-final { scan-assembler "_Z2f5ILDmvrZ1giE1pEEDtDST_EEv" } }
> > f5<std::meta::reflect_constant (42)>();
> >
> > base-commit: 9b2bbe9104aef2e6bd9e8d7ace5fed3e2726f4d3
>
Marek
