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

gcc/cp/ChangeLog

@@ -1,5 +1,9 @@
 2019-08-27  Marek Polacek  <polacek@redhat.com>
 
+	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.
+
 	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.

gcc/cp/semantics.c

@@ -810,6 +810,18 @@ finish_else_clause (tree if_stmt)
   ELSE_CLAUSE (if_stmt) = pop_stmt_list (ELSE_CLAUSE (if_stmt));
 }
 
+/* Callback for cp_walk_tree to mark all {VAR,PARM}_DECLs in a tree as
+   read.  */
+
+static tree
+maybe_mark_exp_read_r (tree *tp, int *, void *)
+{
+  tree t = *tp;
+  if (VAR_P (t) || TREE_CODE (t) == PARM_DECL)
+    mark_exp_read (t);
+  return NULL_TREE;
+}
+
 /* Finish an if-statement.  */
 
 void
@@ -817,6 +829,16 @@ finish_if_stmt (tree if_stmt)
 {
   tree scope = IF_SCOPE (if_stmt);
   IF_SCOPE (if_stmt) = NULL;
+  if (IF_STMT_CONSTEXPR_P (if_stmt))
+    {
+      /* Prevent various -Wunused warnings.  We might not instantiate
+	 either of these branches, so we would not mark the variables
+	 used in that branch as read.  */
+      cp_walk_tree_without_duplicates (&THEN_CLAUSE (if_stmt),
+				       maybe_mark_exp_read_r, NULL);
+      cp_walk_tree_without_duplicates (&ELSE_CLAUSE (if_stmt),
+				       maybe_mark_exp_read_r, NULL);
+    }
   add_stmt (do_poplevel (scope));
 }
 

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.
 

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');
+}

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');
+}