PR c++/91428 - warn about std::is_constant_evaluated in if constexpr.

	* cp-tree.h (decl_in_std_namespace_p): Declare.
	* semantics.c (is_std_constant_evaluated_p): New.
	(finish_if_stmt_cond): Warn about "std::is_constant_evaluated ()" in
	an if-constexpr.
	* typeck.c (decl_in_std_namespace_p): No longer static.

	* g++.dg/cpp2a/is-constant-evaluated9.C: New test.

From-SVN: r274981

2 files changed, 54 insertions, 0 deletions

diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index cd6fb7f..ade1a69 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2019-08-27  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/91428 - warn about std::is_constant_evaluated in if constexpr.
+	* g++.dg/cpp2a/is-constant-evaluated9.C: New test.
+
 2019-08-27  Martin Sebor  <msebor@redhat.com>
 
 	PR tree-optimization/91567
diff --git a/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated9.C b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated9.C
new file mode 100644
index 0000000..3783369
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated9.C
@@ -0,0 +1,49 @@
+// PR c++/91428 - warn about std::is_constant_evaluated in if constexpr.
+// { dg-do compile { target c++2a } }
+// { dg-options "-Wtautological-compare" }
+
+namespace std {
+  constexpr inline bool
+  is_constant_evaluated () noexcept
+  {
+    return __builtin_is_constant_evaluated (); 
+  }
+}
+
+constexpr int
+foo(int i)
+{
+  if constexpr (std::is_constant_evaluated ()) // { dg-warning ".std::is_constant_evaluated. always evaluates to true in .if constexpr." }
+    return 42;
+  else
+    return i;
+}
+
+constexpr int
+foo2(int i)
+{
+  if constexpr (__builtin_is_constant_evaluated ()) // { dg-warning ".std::is_constant_evaluated. always evaluates to true in .if constexpr." }
+    return 42;
+  else
+    return i;
+}
+
+constexpr int
+foo3(int i)
+{
+  // I is not a constant expression but we short-circuit it.
+  if constexpr (__builtin_is_constant_evaluated () || i)
+    return 42;
+  else
+    return i;
+}
+
+constexpr int
+foo4(int i)
+{
+  const int j = 0;
+  if constexpr (j && __builtin_is_constant_evaluated ())
+    return 42;
+  else
+    return i;
+}