On Fri, May 29, 2026 at 04:37:55PM -0400, Jason Merrill wrote:
> On 5/29/26 4:21 PM, Marek Polacek wrote:
> > 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.
>
> I think forced false is correct, we shouldn't be dealing with aliases or
> concepts here.
>
> > -- >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);
> > + }
>
> We probably want the else from convert_template_argument, too:
>
> > if (!dependent_implicit_conv_p (t, orig_arg, force_conv))
> > /* We used to call digest_init here. However, digest_init
> > will report errors, which we don't want when complain
> > is zero. More importantly, digest_init will try too
> > hard to convert things: for example, `0' should not be
> > converted to pointer type at this point according to
> > the standard. Accepting this is not merely an
> > extension, since deciding whether or not these
> > conversions can occur is part of determining which
> > function template to call, or whether a given explicit
> > argument specification is valid. */
> > val = convert_nontype_argument (t, orig_arg, complain);
> > else
> > {
> > val = canonicalize_expr_argument (orig_arg, complain);
> > val = maybe_convert_nontype_argument (t, val, force_conv);
> > }
>
> ...maybe factoring that out into another function.
Sounds good, done here.
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& 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 (coerce_nontype_argument): New, factored out of...
(convert_template_argument): ...here.
* pt.cc (unify) <case TEMPLATE_PARM_INDEX>: Call
coerce_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 | 52 ++++++++++++++----------
gcc/testsuite/g++.dg/template/deduce11.C | 15 +++++++
gcc/testsuite/g++.dg/template/deduce12.C | 12 ++++++
3 files changed, 58 insertions(+), 21 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 da45a4971a7..4dbec36316b 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -8780,6 +8780,32 @@ dependent_implicit_conv_p (tree type, tree expr, bool
forced)
&& value_dependent_expression_p (expr)));
}
+/* Convert the non-type template parameter ARG to the indicated TYPE.
+ If one of them is dependent, create an appropriate conversion.
+ FORCE_CONV is true in a forced context (i.e. alias or concept). */
+
+static tree
+coerce_nontype_argument (tree type, tree arg, bool force_conv,
+ tsubst_flags_t complain)
+{
+ if (dependent_implicit_conv_p (type, arg, force_conv))
+ {
+ tree val = canonicalize_expr_argument (arg, complain);
+ return maybe_convert_nontype_argument (type, val, force_conv);
+ }
+
+ /* We used to call digest_init here. However, digest_init will report
+ errors, which we don't want when complain is zero. More importantly,
+ digest_init will try too hard to convert things: for example,
+ `0' should not be converted to pointer type at this point according to
+ the standard. Accepting this is not merely an extension, since
+ deciding whether or not these conversions can occur is part of
+ determining which function template to call, or whether a given
+ explicit argument specification is valid. */
+ return convert_nontype_argument (type, convert_from_reference (arg),
+ complain);
+}
+
/* Convert the indicated template ARG as necessary to match the
indicated template PARM. Returns the converted ARG, or
error_mark_node if the conversion was unsuccessful. Error and
@@ -9060,23 +9086,7 @@ convert_template_argument (tree parm,
&& same_type_p (TREE_TYPE (orig_arg), t))
orig_arg = TREE_OPERAND (orig_arg, 0);
- if (!dependent_implicit_conv_p (t, orig_arg, force_conv))
- /* We used to call digest_init here. However, digest_init
- will report errors, which we don't want when complain
- is zero. More importantly, digest_init will try too
- hard to convert things: for example, `0' should not be
- converted to pointer type at this point according to
- the standard. Accepting this is not merely an
- extension, since deciding whether or not these
- conversions can occur is part of determining which
- function template to call, or whether a given explicit
- argument specification is valid. */
- val = convert_nontype_argument (t, orig_arg, complain);
- else
- {
- val = canonicalize_expr_argument (orig_arg, complain);
- val = maybe_convert_nontype_argument (t, val, force_conv);
- }
+ val = coerce_nontype_argument (t, orig_arg, force_conv, complain);
if (val == NULL_TREE)
val = error_mark_node;
@@ -26530,10 +26540,10 @@ 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);
- }
+ arg = coerce_nontype_argument (tparm, arg, /*forced=*/false, 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: bee542c6b13fb08607dc03aadf5cd5eef5fe6642
--
2.54.0
