PR c++/81676 - bogus -Wunused warnings in constexpr if.

	* semantics.c (maybe_mark_exp_read_r): New function.
	(finish_if_stmt): Call it on THEN_CLAUSE and ELSE_CLAUSE.

	* g++.dg/cpp1z/constexpr-if31.C: New test.
	* g++.dg/cpp1z/constexpr-if32.C: New test.

From-SVN: r274982

3 files changed, 99 insertions, 0 deletions

diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index ade1a69..4c58780 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,5 +1,9 @@
 2019-08-27  Marek Polacek  <polacek@redhat.com>
 
+	PR c++/81676 - bogus -Wunused warnings in constexpr if.
+	* g++.dg/cpp1z/constexpr-if31.C: New test.
+	* g++.dg/cpp1z/constexpr-if32.C: New test.
+
 	PR c++/91428 - warn about std::is_constant_evaluated in if constexpr.
 	* g++.dg/cpp2a/is-constant-evaluated9.C: New test.
 
diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-if31.C b/gcc/testsuite/g++.dg/cpp1z/constexpr-if31.C
new file mode 100644
index 0000000..02140cf
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-if31.C
@@ -0,0 +1,79 @@
+// PR c++/81676 - bogus -Wunused warnings in constexpr if.
+// { dg-do compile { target c++17 } }
+// { dg-options "-Wall -Wextra" }
+
+template <typename T> int
+f1 (T v)
+{
+  T x = 0;
+  if constexpr(sizeof(T) == sizeof(int))
+    return v + x;
+  else
+    return 0;
+}
+
+template <typename T> int
+f2 (T v) // { dg-warning "unused parameter .v." }
+{
+  T x = 0;
+  if constexpr(sizeof(T) == sizeof(int))
+    return x;
+  else
+    return 0;
+}
+
+template <typename T> int
+f3 (T v)
+{
+  T x = 0; // { dg-warning "unused variable .x." }
+  if constexpr(sizeof(T) == sizeof(int))
+    return v;
+  else
+    return 0;
+}
+
+template <typename T> int
+f4 (T v)
+{
+  T x = 0;
+  if constexpr(sizeof(T) == sizeof(int))
+    return 0;
+  else
+    return v + x;
+}
+
+template <typename T> int
+f5 (T v) // { dg-warning "unused parameter .v." }
+{
+  T x = 0;
+  if constexpr(sizeof(T) == sizeof(int))
+    return 0;
+  else
+    return x;
+}
+
+template <typename T> int
+f6 (T v)
+{
+  T x = 0; // { dg-warning "unused variable .x." }
+  if constexpr(sizeof(T) == sizeof(int))
+    return 0;
+  else
+    return v;
+}
+
+int main()
+{
+  f1(0);
+  f1('a');
+  f2(0);
+  f2('a');
+  f3(0);
+  f3('a');
+  f4(0);
+  f4('a');
+  f5(0);
+  f5('a');
+  f6(0);
+  f6('a');
+}
diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-if32.C b/gcc/testsuite/g++.dg/cpp1z/constexpr-if32.C
new file mode 100644
index 0000000..13a6039
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-if32.C
@@ -0,0 +1,16 @@
+// PR c++/81676 - bogus -Wunused warnings in constexpr if.
+// { dg-do compile { target c++17 } }
+// { dg-options "-Wall -Wextra" }
+
+int main()
+{
+    auto f = [](auto a, auto b) {
+        if constexpr (sizeof(b) == 1) {
+            return a;
+        } else {
+            return b;
+        }
+    };
+
+    return f(1, 1) + f(1, 'a');
+}