re PR c++/67980 (left shift count is negative [-Wshift-count-negative] generated…

re PR c++/67980 (left shift count is negative [-Wshift-count-negative] generated for unreachable code)

/cp
2016-11-04  Paolo Carlini  <paolo.carlini@oracle.com>

	PR c++/67980
	* pt.c (tsubst_expr, case IF_STMT): Use fold_non_dependent_expr
	to suppress unwanted warnings.

/testsuite
2016-11-04  Paolo Carlini  <paolo.carlini@oracle.com>

	PR c++/67980
	* g++.dg/cpp1y/pr67980.C: New.

From-SVN: r241858

gcc/cp/ChangeLog

+2016-11-04  Paolo Carlini  <paolo.carlini@oracle.com>
+
+	PR c++/67980
+	* pt.c (tsubst_expr, case IF_STMT): Use fold_non_dependent_expr
+	to suppress unwanted warnings.
+
 2016-11-03  Jason Merrill  <jason@redhat.com>
 
 	PR c++/78198

gcc/cp/pt.c

@@ -15437,15 +15437,27 @@ tsubst_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl,
       if (IF_STMT_CONSTEXPR_P (t) && integer_zerop (tmp))
 	/* Don't instantiate the THEN_CLAUSE. */;
       else
-	RECUR (THEN_CLAUSE (t));
+	{
+	  bool inhibit = integer_zerop (fold_non_dependent_expr (tmp));
+	  if (inhibit)
+	    ++c_inhibit_evaluation_warnings;
+	  RECUR (THEN_CLAUSE (t));
+	  if (inhibit)
+	    --c_inhibit_evaluation_warnings;
+	}
       finish_then_clause (stmt);
 
       if (IF_STMT_CONSTEXPR_P (t) && integer_nonzerop (tmp))
 	/* Don't instantiate the ELSE_CLAUSE. */;
       else if (ELSE_CLAUSE (t))
 	{
+	  bool inhibit = integer_nonzerop (fold_non_dependent_expr (tmp));
 	  begin_else_clause (stmt);
+	  if (inhibit)
+	    ++c_inhibit_evaluation_warnings;
 	  RECUR (ELSE_CLAUSE (t));
+	  if (inhibit)
+	    --c_inhibit_evaluation_warnings;
 	  finish_else_clause (stmt);
 	}
 

gcc/testsuite/ChangeLog

+2016-11-04  Paolo Carlini  <paolo.carlini@oracle.com>
+
+	PR c++/67980
+	* g++.dg/cpp1y/pr67980.C: New.
+
 2016-11-04  Bill Schmidt  <wschmidt@linux.vnet.ibm.com>
 
 	* gcc.target/powerpc/fold-vec-add-1.c: New.

gcc/testsuite/g++.dg/cpp1y/pr67980.C

+// { dg-do compile { target c++14 } }
+
+template <int Y, class T>
+constexpr T cpp14_constexpr_then(T value) {
+  if (Y < 0)
+    return (value << -Y);
+  else
+    return 0;
+}
+
+template <int Y, class T>
+constexpr T cpp14_constexpr_else(T value) {
+  if (Y > 0)
+    return 0;
+  else
+    return (value << -Y);
+}
+
+int main()
+{
+  cpp14_constexpr_then<1>(0);
+  cpp14_constexpr_else<1>(0);
+}