From 0077c0fb19981c108a01cd15af9b2d6d478c183b Mon Sep 17 00:00:00 2001
From: Patrick Palka <ppalka@redhat.com>
Date: Thu, 16 Nov 2023 09:32:07 -0500
Subject: [PATCH] c++: constantness of call to function pointer [PR111703]

potential_constant_expression for CALL_EXPR tests FUNCTION_POINTER_TYPE_P
on the callee rather than on the type of the callee, which means we
always pass want_rval=any when recursing and so may fail to identify a
non-constant function pointer callee as such.  Fixing this turns out to
further work around PR111703.

	PR c++/111703
	PR c++/107939

gcc/cp/ChangeLog:

	* constexpr.cc (potential_constant_expression_1) <case CALL_EXPR>:
	Fix FUNCTION_POINTER_TYPE_P test.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-fn8.C: Extend test.
	* g++.dg/diagnostic/constexpr4.C: New test.
---
 gcc/cp/constexpr.cc                          | 7 ++++++-
 gcc/testsuite/g++.dg/cpp2a/concepts-fn8.C    | 2 ++
 gcc/testsuite/g++.dg/diagnostic/constexpr4.C | 9 +++++++++
 3 files changed, 17 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/diagnostic/constexpr4.C

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 8a6b210144a..344107d494b 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -9547,7 +9547,12 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
 	  }
 	else if (fun)
           {
-	    if (RECUR (fun, FUNCTION_POINTER_TYPE_P (fun) ? rval : any))
+	    if (TREE_TYPE (fun)
+		&& FUNCTION_POINTER_TYPE_P (TREE_TYPE (fun)))
+	      want_rval = rval;
+	    else
+	      want_rval = any;
+	    if (RECUR (fun, want_rval))
 	      /* Might end up being a constant function pointer.  But it
 		 could also be a function object with constexpr op(), so
 		 we pass 'any' so that the underlying VAR_DECL is deemed
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-fn8.C b/gcc/testsuite/g++.dg/cpp2a/concepts-fn8.C
index 3f63a5b28d7..c63d26c931d 100644
--- a/gcc/testsuite/g++.dg/cpp2a/concepts-fn8.C
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-fn8.C
@@ -15,10 +15,12 @@ struct P {
 };
 
 void (*f)(P);
+P (*h)(P);
 
 template<class T>
 constexpr bool g() {
   P x;
   f(x); // { dg-bogus "from here" }
+  f(h(x)); // { dg-bogus "from here" }
   return true;
 }
diff --git a/gcc/testsuite/g++.dg/diagnostic/constexpr4.C b/gcc/testsuite/g++.dg/diagnostic/constexpr4.C
new file mode 100644
index 00000000000..f37c01cac55
--- /dev/null
+++ b/gcc/testsuite/g++.dg/diagnostic/constexpr4.C
@@ -0,0 +1,9 @@
+// Verify we diagnose a call to a non-constant function pointer ahead of time.
+// { dg-do compile { target c++11 } }
+
+bool (*f)(int);
+
+template<int N>
+void g() {
+  static_assert(f(N), ""); // { dg-error "non-constant|'f' is not usable" }
+}
-- 
2.11.4.GIT