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

[Marek Polacek]

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.

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

2019-11-12  Marek Polacek  <pola...@redhat.com>

        PR c++/89070 - bogus [[nodiscard]] warning in SFINAE.
        * cvt.c (convert_to_void): Guard maybe_warn_nodiscard calls with
        tf_warning.

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

diff --git gcc/cp/cvt.c gcc/cp/cvt.c
index d41aeb8f1fc..23facb77634 100644
--- gcc/cp/cvt.c
+++ 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:;
diff --git gcc/testsuite/g++.dg/cpp1z/nodiscard7.C 
gcc/testsuite/g++.dg/cpp1z/nodiscard7.C
new file mode 100644
index 00000000000..80dac63e41e
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp1z/nodiscard7.C
@@ -0,0 +1,18 @@
+// 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>();
+}