C++ PATCH for c++/81676 - bogus -Wunused warnings in constexpr if

[Marek Polacek <polacek at redhat dot com>]

This patch is an attempt to fix the annoying -Wunused-but-set-* warnings that
tend to occur with constexpr if.  When we have something like

  template < typename T >
  int f(T v){
      if constexpr(sizeof(T) == sizeof(int)){
	  return v;
      }else{
	  return 0;
      }
  }

and call f('a'), then the condition is false, meaning that we won't instantiate
the then-branch, as per tsubst_expr/IF_STMT:
17284       if (IF_STMT_CONSTEXPR_P (t) && integer_zerop (tmp))
17285         /* Don't instantiate the THEN_CLAUSE. */;
so we'll never get round to mark_exp_read-ing the decls used in the
then-branch, causing finish_function to emit "parameter set but not used"
warnings.

It's unclear how to best deal with this.  Marking the decls DECL_READ_P while
parsing doesn't seem like a viable approach, and since in this testcase the
decl is type-dependent, doing something similiar to the fix for c++/85827 is
out too.

One option is the below, still don't instantiate anything in such a dead
clause, but mark any use of a decl in it as a read.  Which means that "set but
unused" won't be warned about, but that's still better than bogus warnings.
(Clang doesn't seem to handle that, either.)
retrieve_local_specialization is there because we need to set the flag on the
aready instantiated PARM/VAR_DECL, not the template itself.

The good news is that when a param/decl is really unused, we still get the
warning.  And it works with lambdas too.  Better ideas, anyone?
(If anyone's wondering, constexpr function templates don't have this problem.)

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2019-08-16  Marek Polacek  <polacek@redhat.com>

	PR c++/81676 - bogus -Wunused warnings in constexpr if.
	* pt.c (maybe_mark_exp_read_r): New function.
	(tsubst_expr) <case IF_STMT>: Call it for an uninstantiated THEN_CLAUSE
	and ELSE_CLAUSE.

	* g++.dg/cpp1z/constexpr-if30.C: New test.
	* g++.dg/cpp1z/constexpr-if31.C: New test.

diff --git gcc/cp/pt.c gcc/cp/pt.c
index 17585119bce..e146ad38443 100644
--- gcc/cp/pt.c
+++ gcc/cp/pt.c
@@ -16995,6 +16995,23 @@ lookup_init_capture_pack (tree decl)
   return r;
 }
 
+/* Callback for cp_walk_tree to mark all instantiations of {VAR,PARM}_DECLs
+   in a tree as read.  The point is to mark the parameters or local variables
+   of a template function as read to prevent various -Wunused warnings.  */
+
+static tree
+maybe_mark_exp_read_r (tree *tp, int *, void *)
+{
+  tree t = *tp;
+  if (VAR_P (t) || TREE_CODE (t) == PARM_DECL)
+    {
+      tree s = retrieve_local_specialization (t);
+      if (s)
+	mark_exp_read (s);
+    }
+  return NULL_TREE;
+}
+
 /* Like tsubst_copy for expressions, etc. but also does semantic
    processing.  */
 
@@ -17282,7 +17299,10 @@ tsubst_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl,
 	  break;
 	}
       if (IF_STMT_CONSTEXPR_P (t) && integer_zerop (tmp))
-	/* Don't instantiate the THEN_CLAUSE. */;
+	/* Don't instantiate the THEN_CLAUSE.  But mark the PARM_DECLs and
+	   VAR_DECLs used in the THEN_CLAUSE as read, so as to suppress
+	   -Wunused-but-set-{variable,parameter} warnings.  */
+	cp_walk_tree (&THEN_CLAUSE (t), maybe_mark_exp_read_r, NULL, NULL);
       else
 	{
 	  tree folded = fold_non_dependent_expr (tmp, complain);
@@ -17296,7 +17316,10 @@ tsubst_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl,
       finish_then_clause (stmt);
 
       if (IF_STMT_CONSTEXPR_P (t) && integer_nonzerop (tmp))
-	/* Don't instantiate the ELSE_CLAUSE. */;
+	/* Don't instantiate the ELSE_CLAUSE.  But mark the PARM_DECLs and
+	   VAR_DECLs used in the ELSE_CLAUSE as read, so as to suppress
+	   -Wunused-but-set-{variable,parameter} warnings.  */
+	cp_walk_tree (&ELSE_CLAUSE (t), maybe_mark_exp_read_r, NULL, NULL);
       else if (ELSE_CLAUSE (t))
 	{
 	  tree folded = fold_non_dependent_expr (tmp, complain);
diff --git gcc/testsuite/g++.dg/cpp1z/constexpr-if30.C gcc/testsuite/g++.dg/cpp1z/constexpr-if30.C
new file mode 100644
index 00000000000..26e0c6f39a6
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp1z/constexpr-if30.C
@@ -0,0 +1,79 @@
+// PR c++/81676 - bogus -Wunused warnings in constexpr if.
+// { dg-do compile { target c++17 } }
+// { dg-options "-Wunused-variable -Wunused-parameter" }
+
+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 gcc/testsuite/g++.dg/cpp1z/constexpr-if31.C gcc/testsuite/g++.dg/cpp1z/constexpr-if31.C
new file mode 100644
index 00000000000..a1db91a5b40
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp1z/constexpr-if31.C
@@ -0,0 +1,16 @@
+// PR c++/81676 - bogus -Wunused warnings in constexpr if.
+// { dg-do compile { target c++17 } }
+// { dg-options "-Wunused-variable -Wunused-parameter" }
+
+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');
+}