[pushed] c++: Fix NaN as C++20 template argument

[Jason Merrill]

C++20 allows floating-point types for non-type template parameters;
floating-point values are considered to be equivalent template arguments if
they are "identical", which conveniently seems to map onto an existing GCC
predicate.

Tested x86_64-pc-linux-gnu, applying to trunk.

gcc/cp/ChangeLog:

	* tree.c (cp_tree_equal): Use real_identical.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/nontype-float1.C: New test.
---
 gcc/cp/tree.c                               |  2 +-
 gcc/testsuite/g++.dg/cpp2a/nontype-float1.C | 12 ++++++++++++
 2 files changed, 13 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-float1.C

diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
index 3c469750e9d..3acb6433b64 100644
--- a/gcc/cp/tree.c
+++ b/gcc/cp/tree.c
@@ -3740,7 +3740,7 @@ cp_tree_equal (tree t1, tree t2)
       return tree_int_cst_equal (t1, t2);
 
     case REAL_CST:
-      return real_equal (&TREE_REAL_CST (t1), &TREE_REAL_CST (t2));
+      return real_identical (&TREE_REAL_CST (t1), &TREE_REAL_CST (t2));
 
     case STRING_CST:
       return TREE_STRING_LENGTH (t1) == TREE_STRING_LENGTH (t2)
diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-float1.C b/gcc/testsuite/g++.dg/cpp2a/nontype-float1.C
new file mode 100644
index 00000000000..4fafac13793
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/nontype-float1.C
@@ -0,0 +1,12 @@
+// { dg-do compile { target c++20 } }
+
+#include <cmath>
+
+template<auto> class MyClass { };
+
+static_assert(__is_same(MyClass<NAN>, MyClass<NAN>));
+
+constexpr auto mynan = NAN;
+static_assert(__is_same(MyClass<mynan>, MyClass<mynan>));
+
+static_assert(__is_same(MyClass<NAN>, MyClass<mynan>));

base-commit: 0251051db64f13c9a31a05c8133c31dc50b2b235
-- 
2.27.0