PR c++/89070 - bogus [[nodiscard]] warning in SFINAE.

This is a complaint that we issue a [[nodiscard]] warning even in SFINAE
contexts.  Here 'complain' is tf_decltype, but not tf_warning so I guess
we can fix it as below.

	* cvt.c (convert_to_void): Guard maybe_warn_nodiscard calls with
	tf_warning.

	* g++.dg/cpp1z/nodiscard7.C: New test.

From-SVN: r278147

gcc/cp/ChangeLog

+2019-11-13  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/89070 - bogus [[nodiscard]] warning in SFINAE.
+	* cvt.c (convert_to_void): Guard maybe_warn_nodiscard calls with
+	tf_warning.
+
 2019-11-13  Richard Sandiford  <richard.sandiford@arm.com>
 
 	PR c++/92206

gcc/cp/cvt.c

@@ -1201,7 +1201,8 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
 	if (DECL_DESTRUCTOR_P (fn))
 	  return expr;
 
-      maybe_warn_nodiscard (expr, implicit);
+      if (complain & tf_warning)
+	maybe_warn_nodiscard (expr, implicit);
       break;
 
     case INDIRECT_REF:
@@ -1357,7 +1358,8 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
                 && !is_reference)
               warning_at (loc, OPT_Wunused_value, "value computed is not used");
             expr = TREE_OPERAND (expr, 0);
-	    if (TREE_CODE (expr) == CALL_EXPR)
+	    if (TREE_CODE (expr) == CALL_EXPR
+		&& (complain & tf_warning))
 	      maybe_warn_nodiscard (expr, implicit);
           }
 
@@ -1435,7 +1437,8 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
 					   AGGR_INIT_EXPR_ARGP (init));
 	    }
 	}
-      maybe_warn_nodiscard (expr, implicit);
+      if (complain & tf_warning)
+	maybe_warn_nodiscard (expr, implicit);
       break;
 
     default:;

gcc/testsuite/ChangeLog

+2019-11-13  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/89070 - bogus [[nodiscard]] warning in SFINAE.
+	* g++.dg/cpp1z/nodiscard7.C: New test.
+
 2019-11-13  Joseph Myers  <joseph@codesourcery.com>
 
 	* gcc.dg/c11-float-3.c, gcc.dg/c2x-float-1.c: New tests.

gcc/testsuite/g++.dg/cpp1z/nodiscard7.C

+// PR c++/89070 - bogus [[nodiscard]] warning in SFINAE.
+// { dg-do compile { target c++11 } }
+
+struct A
+{
+    [[nodiscard]] static int match() { return 42; }
+};
+
+template<typename T>
+auto g() -> decltype( T::match(), bool() )
+{
+    return T::match();
+}
+
+int main()
+{
+    g<A>();
+}