[PATCH] c++: missing SFINAE with pointer subtraction [PR78173]

[Patrick Palka via Gcc-patches]

This fixes a missed SFINAE when subtracting pointers to an incomplete
type.

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

gcc/cp/ChangeLog:

        PR c++/78173
        * typeck.c (pointer_diff): Use complete_type_or_maybe_complain
        instead of complete_type_or_else.

gcc/testsuite/ChangeLog:

        PR c++/78173
        * g++.dg/cpp2a/concepts-pr78173.C: New test.
---
 gcc/cp/typeck.c                               | 2 +-
 gcc/testsuite/g++.dg/cpp2a/concepts-pr78173.C | 9 +++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-pr78173.C

diff --git a/gcc/cp/typeck.c b/gcc/cp/typeck.c
index dd84674a5e6..c41457bcce8 100644
--- a/gcc/cp/typeck.c
+++ b/gcc/cp/typeck.c
@@ -5985,7 +5985,7 @@ pointer_diff (location_t loc, tree op0, tree op1, tree 
ptrtype,
   tree restype = ptrdiff_type_node;
   tree target_type = TREE_TYPE (ptrtype);
 
-  if (!complete_type_or_else (target_type, NULL_TREE))
+  if (!complete_type_or_maybe_complain (target_type, NULL_TREE, complain))
     return error_mark_node;
 
   if (VOID_TYPE_P (target_type))
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-pr78173.C 
b/gcc/testsuite/g++.dg/cpp2a/concepts-pr78173.C
new file mode 100644
index 00000000000..50f561abcce
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-pr78173.C
@@ -0,0 +1,9 @@
+// PR c++/78173
+// { dg-do compile { target c++20 } }
+
+template <class T>
+concept CanDifference = requires(T x, T y) {
+    x - y;
+};
+
+static_assert(!CanDifference<void*>);
-- 
2.29.2.540.g3cf59784d4