On Thu, May 28, 2026 at 03:21:35PM -0400, Jason Merrill wrote:
> On 5/28/26 11:42 AM, Marek Polacek wrote:
> > 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.
>
> Ah, because arg is still INTEGER_CST. This suggests that we want the full
> convert_nontype_argument, which gives
>
> > initializing ‘const int&’ with ‘int’ in converted constant expression does
> > not bind directly
>
> then we should be able to drop the separate strip_typedefs_expr.
So like this? I'm not certain about the forced argument. But the
dependent_implicit_conv_p is necessary to avoid crashing on dependent
trees.
-- >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) <case TEMPLATE_PARM_INDEX>: Call
convert_nontype_argument and return unify_invalid if it failed.
Don't call strip_typedefs_expr.
gcc/testsuite/ChangeLog:
* g++.dg/template/deduce11.C: New test.
* g++.dg/template/deduce12.C: New test.
---
gcc/cp/pt.cc | 12 ++++++++----
gcc/testsuite/g++.dg/template/deduce11.C | 15 +++++++++++++++
gcc/testsuite/g++.dg/template/deduce12.C | 12 ++++++++++++
3 files changed, 35 insertions(+), 4 deletions(-)
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..a70a758c61f 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -26530,10 +26530,14 @@ 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);
- {
- bool removed_attr = false;
- arg = strip_typedefs_expr (arg, &removed_attr);
- }
+ if (!dependent_implicit_conv_p (tparm, arg, /*forced=*/false))
+ {
+ arg = convert_from_reference (arg);
+ arg = convert_nontype_argument (tparm, arg, complain);
+ if (!arg || arg == error_mark_node)
+ return unify_invalid (explain_p);
+ }
+
TREE_VEC_ELT (INNERMOST_TEMPLATE_ARGS (targs), idx) = arg;
return unify_success (explain_p);
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..0bd2762edd4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/deduce12.C
@@ -0,0 +1,12 @@
+// 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|does not bind directly|could not convert)"
"candidate note" { target c++17 } .-1 }
+// { dg-message "(candidate|not a valid template argument)" "candidate note" {
target c++14_down } .-2 }
base-commit: 4f6979f50c39860578c8762e0822661bf058fb6a
--
2.54.0
