From 124f2f62e01c6f110279608ad09e0f1d378e4899 Mon Sep 17 00:00:00 2001
From: Patrick Palka <ppalka@redhat.com>
Date: Tue, 5 Nov 2024 15:18:26 -0500
Subject: [PATCH] c++: reference variable as default targ [PR101463]

Here during default template argument substitution we wrongly consider
the (substituted) default arguments v and vt<int> as value-dependent
which ultimately leads to deduction failure for the calls.

The bogus value_dependent_expression_p result aside, I noticed
type_unification_real during default targ substitution keeps track of
whether all previous targs are known and non-dependent, as is the case
for these calls.  And in such cases it should be safe to avoid checking
dependence of the substituted default targ and just assume it's not.
This patch implements this optimization for GCC 14, which lets us accept
both testcases by sidestepping the value_dependent_expression_p issue
altogether.  (Note that for GCC 15 we fixed this differently, see
r15-3038-g5348e3cb9bc99d.)

	PR c++/101463

gcc/cp/ChangeLog:

	* pt.cc (type_unification_real): Avoid checking dependence of
	a substituted default template argument if we can assume it's
	non-dependent.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/nontype6.C: New test.
	* g++.dg/cpp1z/nontype6a.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>
---
 gcc/cp/pt.cc                           |  4 +++-
 gcc/testsuite/g++.dg/cpp1z/nontype6.C  | 24 ++++++++++++++++++++++++
 gcc/testsuite/g++.dg/cpp1z/nontype6a.C | 25 +++++++++++++++++++++++++
 3 files changed, 52 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp1z/nontype6.C
 create mode 100644 gcc/testsuite/g++.dg/cpp1z/nontype6a.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 818d5b65edce..4f83426fb56c 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -23651,12 +23651,14 @@ type_unification_real (tree tparms,
 		    /* We replaced all the tparms, substitute again out of
 		       template context.  */
 		    substed = NULL_TREE;
+		  else
+		    processing_template_decl = 1;
 		}
 	      if (!substed)
 		substed = tsubst_template_arg (arg, full_targs, complain,
 					       NULL_TREE);
 
-	      if (!uses_template_parms (substed))
+	      if (!processing_template_decl || !uses_template_parms (substed))
 		arg = convert_template_argument (parm, substed, full_targs,
 						 complain, i, NULL_TREE);
 	      else if (saw_undeduced == 1)
diff --git a/gcc/testsuite/g++.dg/cpp1z/nontype6.C b/gcc/testsuite/g++.dg/cpp1z/nontype6.C
new file mode 100644
index 000000000000..06cd234cc61a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/nontype6.C
@@ -0,0 +1,24 @@
+// PR c++/101463
+// { dg-do compile { target c++17 } }
+
+int a;
+
+int& v = a;
+
+template<const int& = v>
+void f(int) { }
+
+template<class T, int& = v>
+void g(T) { }
+
+template<class T>
+int& vt = a;
+
+template<class T, int& = vt<T>>
+void h(T) { }
+
+int main() {
+  f(0);
+  g(0);
+  h(0);
+}
diff --git a/gcc/testsuite/g++.dg/cpp1z/nontype6a.C b/gcc/testsuite/g++.dg/cpp1z/nontype6a.C
new file mode 100644
index 000000000000..8bc40a0505c2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/nontype6a.C
@@ -0,0 +1,25 @@
+// PR c++/101463
+// A version of nontype6.C where v and vt are constexpr.
+// { dg-do compile { target c++17 } }
+
+int a;
+
+constexpr int& v = a;
+
+template<const int& = v>
+void f(int) { }
+
+template<class T, const int& = v>
+void g(T) { }
+
+template<class T>
+constexpr int& vt = a;
+
+template<class T, const int& = vt<T>>
+void h(T) { }
+
+int main() {
+  f(0);
+  g(0);
+  h(0);
+}
-- 
2.47.2