[PATCH] c++: Fix deduction with reference NTTP [PR83476]

[Patrick Palka via Gcc-patches]

In the testcase ref11.C below, during deduction for the call f(a),
uses_deducible_template_parms returns false for the dependent
specialization A<V> because V is wrapped in an implicit INDIRECT_REF
(formed from template_parm_to_arg), and this causes unify_one_argument
to exit early, causing deduction to fail.  This patch fixes this
by making deducible_expression look through such implicit INDIRECT_REFs.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?

gcc/cp/ChangeLog:

        PR c++/83476
        PR c++/99885
        * pt.c (deducible_expression): Look through implicit
        INDIRECT_REFs as well.

gcc/testsuite/ChangeLog:

        PR c++/83476
        PR c++/99885
        * g++.dg/cpp1z/class-deduction85.C: New test.
        * g++.dg/template/ref11.C: New test.
---
 gcc/cp/pt.c                                    |  6 ++++--
 gcc/testsuite/g++.dg/cpp1z/class-deduction85.C | 16 ++++++++++++++++
 gcc/testsuite/g++.dg/template/ref11.C          |  9 +++++++++
 3 files changed, 29 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction85.C
 create mode 100644 gcc/testsuite/g++.dg/template/ref11.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 59df79484bf..f488a5a8c12 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -21902,8 +21902,10 @@ static bool uses_deducible_template_parms (tree type);
 static bool
 deducible_expression (tree expr)
 {
-  /* Strip implicit conversions.  */
-  while (CONVERT_EXPR_P (expr) || TREE_CODE (expr) == VIEW_CONVERT_EXPR)
+  /* Strip implicit conversions and implicit INDIRECT_REFs.  */
+  while (CONVERT_EXPR_P (expr)
+        || TREE_CODE (expr) == VIEW_CONVERT_EXPR
+        || REFERENCE_REF_P (expr))
     expr = TREE_OPERAND (expr, 0);
   return (TREE_CODE (expr) == TEMPLATE_PARM_INDEX);
 }
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction85.C 
b/gcc/testsuite/g++.dg/cpp1z/class-deduction85.C
new file mode 100644
index 00000000000..0b22f8eb982
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction85.C
@@ -0,0 +1,16 @@
+// PR c++/99885
+// { dg-do compile { target c++17 } }
+
+template <auto const& A>
+struct Foo {};
+
+template <auto const& A>
+struct Bar {
+    constexpr auto foo() const -> Foo<A> {
+        return {};
+    }
+};
+
+constexpr int a = 1;
+constexpr Bar<a> bar;
+Foo foo = bar.foo(); // <-- CTAD failure
diff --git a/gcc/testsuite/g++.dg/template/ref11.C 
b/gcc/testsuite/g++.dg/template/ref11.C
new file mode 100644
index 00000000000..ef29fb72bef
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/ref11.C
@@ -0,0 +1,9 @@
+// PR c++/83476
+
+template <int& V> struct A {};
+template <int& V> void f(A<V>);
+int n;
+int main() {
+  A<n> a;
+  f(a);
+}
-- 
2.31.1.272.g89b43f80a5