From 6d1556ecfae3eff010ef7dc15f3da998403fa196 Mon Sep 17 00:00:00 2001
From: Patrick Palka <ppalka@redhat.com>
Date: Sat, 23 May 2020 14:39:28 -0400
Subject: [PATCH] c++: Avoid concept evaluation when uid-sensitive [PR94038]

Concept evaluation may entail DECL_UID generation and/or template
instantiation, so in general we can't perform it during uid-sensitive
constexpr evaluation.

gcc/cp/ChangeLog:

	PR c++/94038
	* constexpr.c (cxx_eval_constant_expression)
	<case TEMPLATE_ID_EXPR>: Don't evaluate the concept when
	constexpr evaluation is uid-sensitive.

gcc/testsuite/ChangeLog:

	PR c++/94038
	* g++.dg/warn/pr94038-3.C: New test.
---
 gcc/cp/ChangeLog                      |  7 +++++++
 gcc/cp/constexpr.c                    |  3 ++-
 gcc/testsuite/ChangeLog               |  5 +++++
 gcc/testsuite/g++.dg/warn/pr94038-3.C | 15 +++++++++++++++
 4 files changed, 29 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/warn/pr94038-3.C

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 385bfcd72db..2feeb12bcb0 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,10 @@
+2020-05-23  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/94038
+	* constexpr.c (cxx_eval_constant_expression)
+	<case TEMPLATE_ID_EXPR>: Don't evaluate the concept when
+	constexpr evaluation is uid-sensitive.
+
 2020-05-21  Patrick Palka  <ppalka@redhat.com>
 
 	PR c++/94038
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 98c974e657f..4e441ac8d2f 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -6486,7 +6486,8 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	    break;
 	  }
 
-	if (!processing_template_decl)
+	if (!processing_template_decl
+	    && !uid_sensitive_constexpr_evaluation_p ())
 	  r = evaluate_concept_check (t, tf_warning_or_error);
 	else
 	  *non_constant_p = true;
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 9711a27e29c..80fd33dac18 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2020-05-23  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/94038
+	* g++.dg/warn/pr94038-3.C: New test.
+
 2020-05-22  Mark Wielaard  <mark@klomp.org>
 
 	* gcc.dg/spellcheck-stdint.c: New test.
diff --git a/gcc/testsuite/g++.dg/warn/pr94038-3.C b/gcc/testsuite/g++.dg/warn/pr94038-3.C
new file mode 100644
index 00000000000..49b6d133f08
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/pr94038-3.C
@@ -0,0 +1,15 @@
+// PR c++/94038
+// { dg-do compile { target c++20 } }
+// { dg-additional-options "-Wall" }
+
+template<typename T>
+constexpr int foo() {
+  return T::x;
+}
+
+constexpr bool bar(bool a) { return a; }
+
+template<typename T>
+concept C = foo<T>() == 0;
+
+static_assert(decltype(bar(C<int>)){} == false);
-- 
2.30.2