On Wed, May 27, 2026 at 06:15:04PM -0400, Jason Merrill wrote:
> On 5/27/26 4:32 PM, Marek Polacek wrote:
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> >
> > -- >8 --
> > [temp.arg.nontype] tells us that a temporary object is not an acceptable
> > template-argument when the corresponding template parameter has reference
> > type. So
> >
> > template<const int &> struct B {};
> > B<1> b;
> >
> > is ill-formed. In the test below we have a tparm `const int &I` and we
> > are trying to deduce `I` from `A<0>`. Since a temporary would be required
> > for the template argument, this should be a deduction failure.
> >
> > PR c++/107124
> >
> > gcc/cp/ChangeLog:
> >
> > * pt.cc (unify) <case TEMPLATE_PARM_INDEX>: Return
> > unify_type_mismatch if a temporary would be required for the
> > template argument.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/template/deduce11.C: New test.
> > * g++.dg/template/deduce12.C: New test.
> > ---
> > gcc/cp/pt.cc | 7 ++++++-
> > gcc/testsuite/g++.dg/template/deduce11.C | 15 +++++++++++++++
> > gcc/testsuite/g++.dg/template/deduce12.C | 11 +++++++++++
> > 3 files changed, 32 insertions(+), 1 deletion(-)
> > create mode 100644 gcc/testsuite/g++.dg/template/deduce11.C
> > create mode 100644 gcc/testsuite/g++.dg/template/deduce12.C
> >
> > diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> > index ada5f53a9f1..41b86a4e4f7 100644
> > --- a/gcc/cp/pt.cc
> > +++ b/gcc/cp/pt.cc
> > @@ -26485,7 +26485,12 @@ unify (tree tparms, tree targs, tree parm, tree
> > arg, int strict,
> > ;
> > else if (same_type_ignoring_top_level_qualifiers_p
> > (non_reference (TREE_TYPE (arg)),
> > - non_reference (tparm)))
> > + non_reference (tparm))
> > + /* A temporary object is not an acceptable template-argument
> > + when the corresponding template parm has reference type. */
> > + && !(TYPE_REF_P (tparm)
> > + && ref_conv_binds_to_temporary
> > + (tparm, convert_from_reference (arg)).is_true ()))
>
> This seems like an awkward place for this check since it isn't a type
> mismatch, which leads to the unhelpful diagnostic
>
> > • mismatched types ‘const int&’ and ‘int’
>
> I think better would be to after checking for pack mismatch, check
> invalid_tparm_referent_p, get the diagnostic from there, and return plain
> unify_invalid if it fails.
I put it there because it seemed simple to fall back on unify_type_mismatch
(and it's what clang++ says too). But fair enough; I agree with your point.
I can't use invalid_tparm_referent_p because it wouldn't detect the problem
(it's the first thing I tried). So I propose this version.
dg.exp passed so far.
-- >8 --
[temp.arg.nontype] tells us that a temporary object is not an acceptable
template-argument when the corresponding template parameter has reference
type. So
template<const int& CRI> struct B { };
B<1> b;
is ill-formed. In the test below we have a tparm `const int &I` and we
are trying to deduce `I` from `A<0>`. Since a temporary would be required
for the template argument, this should be a deduction failure.
PR c++/107124
gcc/cp/ChangeLog:
* pt.cc (unify_temporary): New.
(unify) <case TEMPLATE_PARM_INDEX>: Return unify_temporary
if a temporary would be required for the template argument.
gcc/testsuite/ChangeLog:
* g++.dg/template/deduce11.C: New test.
* g++.dg/template/deduce12.C: New test.
---
gcc/cp/pt.cc | 17 +++++++++++++++++
gcc/testsuite/g++.dg/template/deduce11.C | 15 +++++++++++++++
gcc/testsuite/g++.dg/template/deduce12.C | 11 +++++++++++
3 files changed, 43 insertions(+)
create mode 100644 gcc/testsuite/g++.dg/template/deduce11.C
create mode 100644 gcc/testsuite/g++.dg/template/deduce12.C
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index ada5f53a9f1..659c07f9800 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -7369,6 +7369,16 @@ unify_template_argument_mismatch (bool explain_p, tree
parm, tree arg)
return unify_invalid (explain_p);
}
+static int
+unify_temporary (bool explain_p, tree parm, tree arg)
+{
+ if (explain_p)
+ inform (input_location,
+ " template argument %qE for %qT would require a temporary",
+ arg, parm);
+ return unify_invalid (explain_p);
+}
+
/* Subroutine of convert_nontype_argument, to check whether EXPR, as an
argument for TYPE, points to an unsuitable object.
@@ -26530,6 +26540,13 @@ unify (tree tparms, tree targs, tree parm, tree arg,
int strict,
&& !TEMPLATE_PARM_PARAMETER_PACK (parm))
return unify_parameter_pack_mismatch (explain_p, parm, arg);
+ /* A temporary object is not an acceptable template-argument
+ when the corresponding template parm has reference type. */
+ if (TYPE_REF_P (tparm)
+ && ref_conv_binds_to_temporary
+ (tparm, convert_from_reference (arg)).is_true ())
+ return unify_temporary (explain_p, tparm, arg);
+
{
bool removed_attr = false;
arg = strip_typedefs_expr (arg, &removed_attr);
diff --git a/gcc/testsuite/g++.dg/template/deduce11.C
b/gcc/testsuite/g++.dg/template/deduce11.C
new file mode 100644
index 00000000000..c7671c04397
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/deduce11.C
@@ -0,0 +1,15 @@
+// PR c++/107124
+// { dg-do compile { target c++11 } }
+
+template<int>
+struct A {};
+
+template<const int &I>
+constexpr int f (A<I>) { return 0; }
+
+template<typename T>
+constexpr int f (T) { return 1; }
+
+const int a = 42;
+static_assert (f (A<0>{}) == 1, "");
+static_assert (f (A<a>{}) == 1, "");
diff --git a/gcc/testsuite/g++.dg/template/deduce12.C
b/gcc/testsuite/g++.dg/template/deduce12.C
new file mode 100644
index 00000000000..11ab9e3b865
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/deduce12.C
@@ -0,0 +1,11 @@
+// PR c++/107124
+// { dg-do compile { target c++11 } }
+
+template<int>
+struct A {};
+
+template<const int &I>
+constexpr int f (A<I>) { return 0; }
+
+constexpr int i = f (A<0>{}); // { dg-error "no matching function for call" }
+// { dg-message "(candidate|require a temporary)" "candidate note" { target
*-*-* } .-1 }
base-commit: b64700dec403f9b2b605df295e43c73b3185d0f5
--
2.54.0
