[C++ Patch] PR 80896 ("[[nodiscard]] is ignored for functions returning references")

[Paolo Carlini <paolo.carlini@oracle.com>]

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Hi,

this one appears to be a rather simple case of missing diagnostic: in 
convert_to_void we aren't calling maybe_warn_nodiscard when we strip an 
INDIRECT_REF wrapping a CALL_EXPR thus we don't issue the diagnostic 
that we normally provide for plain CALL_EXPRs (eg, for a func returning 
a plain int). Tested x86_64-linux.

Thanks, Paolo.

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/cp
2017-05-31  Paolo Carlini  <paolo.carlini@oracle.com>

	PR c++/80896
	* cvt.c (convert_to_void): Possibly call maybe_warn_nodiscard
	for case INDIRECT_REF too in the main switch.

/testsuite
2017-05-31  Paolo Carlini  <paolo.carlini@oracle.com>

	PR c++/80896
	* g++.dg/cpp1z/nodiscard5.C: New.

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Index: testsuite/g++.dg/cpp1z/nodiscard5.C
===================================================================
--- testsuite/g++.dg/cpp1z/nodiscard5.C	(revision 0)
+++ testsuite/g++.dg/cpp1z/nodiscard5.C	(working copy)
@@ -0,0 +1,7 @@
+// PR c++/80896
+// { dg-do compile { target c++11 } }
+
+int x = 42;
+[[nodiscard]] int& func() { return x; }
+
+int main() { func(); }  // { dg-warning "ignoring return value" }
Index: cp/cvt.c
===================================================================
--- cp/cvt.c	(revision 248728)
+++ cp/cvt.c	(working copy)
@@ -1296,6 +1296,8 @@ convert_to_void (tree expr, impl_conv_void implici
                 && !is_reference)
               warning_at (loc, OPT_Wunused_value, "value computed is not used");
             expr = TREE_OPERAND (expr, 0);
+	    if (TREE_CODE (expr) == CALL_EXPR)
+	      maybe_warn_nodiscard (expr, implicit);
           }
 
 	break;