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.
Jason
/* OK. Ignore top-level quals here because a class-type template
parameter object is const. */;
else if ((strict & UNIFY_ALLOW_INTEGER)
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..a7f589a5f73
--- /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|mismatched types)" "candidate note" { target *-*-*
} .-1 }
base-commit: b5248505eb5276c2e71264e641cafeb1d826faed
