c++: Distinguish alignof and __alignof__ in cp_tree_equal [PR97273]

cp_tree_equal currently considers alignof the same as __alignof__, but
these operators are semantically different ever since r8-7957.  In the
testcase below, this causes the second static_assert to fail on targets
where alignof(double) != __alignof__(double) because the specialization
table (which uses cp_tree_equal as its equality predicate) conflates the
two dependent specializations integral_constant<__alignof__(T)> and
integral_constant<alignof(T)>.

This patch makes cp_tree_equal distinguish between these two operators
by inspecting the ALIGNOF_EXPR_STD_P flag.

gcc/cp/ChangeLog:

	PR c++/88115
	PR libstdc++/97273
	* tree.c (cp_tree_equal) <case ALIGNOF_EXPR>: Return false if
	ALIGNOF_EXPR_STD_P differ.

gcc/testsuite/ChangeLog:

	PR c++/88115
	PR libstdc++/97273
	* g++.dg/template/alignof3.C: New test.

2 files changed, 15 insertions, 0 deletions

diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
index 074fa0c..9bc37ac 100644
--- a/gcc/cp/tree.c
+++ b/gcc/cp/tree.c
@@ -3803,6 +3803,8 @@ cp_tree_equal (tree t1, tree t2)
 	    if (SIZEOF_EXPR_TYPE_P (t2))
 	      o2 = TREE_TYPE (o2);
 	  }
+	else if (ALIGNOF_EXPR_STD_P (t1) != ALIGNOF_EXPR_STD_P (t2))
+	  return false;
 
 	if (TREE_CODE (o1) != TREE_CODE (o2))
 	  return false;
diff --git a/gcc/testsuite/g++.dg/template/alignof3.C b/gcc/testsuite/g++.dg/template/alignof3.C
new file mode 100644
index 0000000..e573727
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/alignof3.C
@@ -0,0 +1,13 @@
+// PR c++/88115
+// { dg-do compile { target c++11 } }
+
+template<int __v>
+struct integral_constant {
+  static constexpr int value = __v;
+};
+
+template <class T> using StdAlignOf = integral_constant<alignof(T)>;
+template <class T> using GCCAlignOf = integral_constant<__alignof__(T)>;
+
+static_assert(StdAlignOf<double>::value == alignof(double), "");
+static_assert(GCCAlignOf<double>::value == __alignof__(double), "");