Implement C++17 [[nodiscard]] attribute.

	PR c++/38172
	PR c++/54379
gcc/c-family/
	* c-lex.c (c_common_has_attribute): Handle nodiscard.
gcc/cp/
	* parser.c (cp_parser_std_attribute): Handle [[nodiscard]].
	* tree.c (handle_nodiscard_attribute): New.
	(cxx_attribute_table): Add [[nodiscard]].
	* cvt.c (cp_get_fndecl_from_callee, cp_get_callee_fndecl): New.
	(maybe_warn_nodiscard): New.
	(convert_to_void): Call it.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@235597 138bc75d-0d04-0410-961f-82ee72b054a4

6 files changed, 251 insertions, 1 deletions

diff --git a/gcc/testsuite/g++.dg/cpp1z/feat-cxx1z.C b/gcc/testsuite/g++.dg/cpp1z/feat-cxx1z.C
index fe7a4c23290..f8a87a8ddc3 100644
--- a/gcc/testsuite/g++.dg/cpp1z/feat-cxx1z.C
+++ b/gcc/testsuite/g++.dg/cpp1z/feat-cxx1z.C
@@ -345,11 +345,19 @@
 #endif
 
 #ifdef __has_cpp_attribute
+
 #  if ! __has_cpp_attribute(maybe_unused)
 #    error "__has_cpp_attribute(maybe_unused)"
 #  elif __has_cpp_attribute(maybe_unused) != 201603
 #    error "__has_cpp_attribute(maybe_unused) != 201603"
 #  endif
+
+#  if ! __has_cpp_attribute(nodiscard)
+#    error "__has_cpp_attribute(nodiscard)"
+#  elif __has_cpp_attribute(nodiscard) != 201603
+#    error "__has_cpp_attribute(nodiscard) != 201603"
+#  endif
+
 #else
 #  error "__has_cpp_attribute"
 #endif
diff --git a/gcc/testsuite/g++.dg/cpp1z/nodiscard1.C b/gcc/testsuite/g++.dg/cpp1z/nodiscard1.C
new file mode 100644
index 00000000000..5f2345a3407
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/nodiscard1.C
@@ -0,0 +1,8 @@
+// { dg-do compile { target c++11 } }
+
+[[nodiscard]] int f();
+
+int main()
+{
+  f();				// { dg-warning "" }
+}
diff --git a/gcc/testsuite/g++.dg/cpp1z/nodiscard2.C b/gcc/testsuite/g++.dg/cpp1z/nodiscard2.C
new file mode 100644
index 00000000000..56022c3157a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/nodiscard2.C
@@ -0,0 +1,10 @@
+// { dg-do compile { target c++11 } }
+
+struct [[nodiscard]] A { };	// { dg-message "" }
+
+A f();				// { dg-message "" }
+
+int main()
+{
+  f();				// { dg-warning "Wunused-result" }
+}
diff --git a/gcc/testsuite/g++.dg/cpp1z/nodiscard3.C b/gcc/testsuite/g++.dg/cpp1z/nodiscard3.C
new file mode 100644
index 00000000000..bc2a032ecb9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/nodiscard3.C
@@ -0,0 +1,203 @@
+/* nodiscard attribute tests, adapted from gcc.dg/attr-warn-unused-result.c.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c++1z -O -ftrack-macro-expansion=0" } */
+
+#define WUR [[nodiscard]]
+#define WURAI [[nodiscard, gnu::always_inline]] inline
+enum [[nodiscard]] E { e };
+typedef E (*fnt) (void);
+
+typedef struct { long i; } A;
+typedef struct { long i; long j; } B;
+typedef struct { char big[1024]; fnt fn; } C;
+struct [[nodiscard]] D { int i; D(); ~D(); };
+
+WUR E check1 (void);
+WUR void check2 (void); /* { dg-warning "nodiscard" } */
+WUR int foo;		/* { dg-warning "nodiscard" } */
+int bar (void);
+WURAI E check3 (void) { return (E)bar (); }
+WUR A check4 (void);
+WUR B check5 (void);
+WUR C check6 (void);
+A bar7 (void);
+B bar8 (void);
+C bar9 (void);
+WURAI A check7 (void) { return bar7 (); }
+WURAI B check8 (void) { return bar8 (); }
+WURAI C check9 (void) { return bar9 (); }
+/* This is useful for checking whether return value of statement
+   expressions (returning int in this case) is used.  */
+WURAI int check_int_result (int res) { return res; }
+#define GU(v) ({ int e = 0; (v) = bar (); if ((v) < 23) e = 14; e; })
+fnt fnptr;
+WUR E check10 (void);
+int baz (void);
+WURAI E check11 (void) { return (E)baz (); }
+int k;
+
+D check12();
+
+void
+test (void)
+{
+  int i = 0, j;
+  const fnt pcheck1 = check1;
+  const fnt pcheck3 = check3;
+  A a;
+  B b;
+  C c;
+  D d;
+  if (check1 ())
+    return;
+  i += check1 ();
+  i += ({ check1 (); });
+  check1 ();		/* { dg-warning "nodiscard" } */
+  (void) check1 ();
+  check1 (), bar ();	/* { dg-warning "nodiscard" } */
+  check2 ();
+  (void) check2 ();
+  check2 (), bar ();
+  if (check3 ())
+    return;
+  i += check3 ();
+  i += ({ check3 (); });
+  check3 ();		/* { dg-warning "nodiscard" } */
+  (void) check3 ();
+  check3 (), bar ();	/* { dg-warning "nodiscard" } */
+  a = check4 ();
+  if (a.i)
+    return;
+  if (check4 ().i)
+    return;
+  if (({ check4 (); }).i)
+    return;
+  check4 ();		/* { dg-warning "nodiscard" } */
+  (void) check4 ();
+  check4 (), bar ();	/* { dg-warning "nodiscard" } */
+  b = check5 ();
+  if (b.i + b.j)
+    return;
+  if (check5 ().j)
+    return;
+  if (({ check5 (); }).j)
+    return;
+  check5 ();		/* { dg-warning "nodiscard" } */
+  (void) check5 ();
+  check5 (), bar ();	/* { dg-warning "nodiscard" } */
+  c = check6 ();
+  if (c.big[12] + c.big[29])
+    return;
+  if (check6 ().big[27])
+    return;
+  if (({ check6 (); }).big[0])
+    return;
+  check6 ();		/* { dg-warning "nodiscard" } */
+  (void) check6 ();
+  check6 (), bar ();	/* { dg-warning "nodiscard" } */
+  a = check7 ();
+  if (a.i)
+    return;
+  if (check7 ().i)
+    return;
+  if (({ check7 (); }).i)
+    return;
+  check7 ();		/* { dg-warning "nodiscard" } */
+  (void) check7 ();
+  check7 (), bar ();	/* { dg-warning "nodiscard" } */
+  b = check8 ();
+  if (b.i + b.j)
+    return;
+  if (check8 ().j)
+    return;
+  if (({ check8 (); }).j)
+    return;
+  check8 ();		/* { dg-warning "nodiscard" } */
+  (void) check8 ();
+  check8 (), bar ();	/* { dg-warning "nodiscard" } */
+  c = check9 ();
+  if (c.big[12] + c.big[29])
+    return;
+  if (check9 ().big[27])
+    return;
+  if (({ check9 (); }).big[0])
+    return;
+  check9 ();		/* { dg-warning "nodiscard" } */
+  (void) check9 ();
+  check9 (), bar ();	/* { dg-warning "nodiscard" } */
+  if (check_int_result (GU (j)))
+    return;
+  i += check_int_result (GU (j));
+  i += ({ check_int_result (GU (j)); });
+  check_int_result (GU (j)); /* { dg-warning "nodiscard" } */
+  (void) check_int_result (GU (j));
+  check_int_result (GU (j)), bar (); /* { dg-warning "nodiscard" } */
+  if (fnptr ())
+    return;
+  i += fnptr ();
+  i += ({ fnptr (); });
+  fnptr ();		/* { dg-warning "nodiscard" } */
+  (void) fnptr ();
+  fnptr (), bar ();	/* { dg-warning "nodiscard" } */
+  fnptr = check1;
+  if (fnptr ())
+    return;
+  i += fnptr ();
+  i += ({ fnptr (); });
+  fnptr ();		/* { dg-warning "nodiscard" } */
+  (void) fnptr ();
+  fnptr (), bar ();	/* { dg-warning "nodiscard" } */
+  fnptr = check3;
+  if (fnptr ())
+    return;
+  i += fnptr ();
+  i += ({ fnptr (); });
+  fnptr ();		/* { dg-warning "nodiscard" } */
+  (void) fnptr ();
+  fnptr (), bar ();	/* { dg-warning "nodiscard" } */
+  if (bar9 ().fn ())
+    return;
+  i += bar9 ().fn ();
+  i += ({ bar9 ().fn (); });
+  bar9 ().fn ();	/* { dg-warning "nodiscard" } */
+  (void) bar9 ().fn ();
+  bar9 ().fn (), bar (); /* { dg-warning "nodiscard" } */
+  if ((k ? check1 : check10) ())
+    return;
+  i += (k ? check1 : check10) ();
+  i += ({ (k ? check1 : check10) (); });
+  (k ? check1 : check10) (); /* { dg-warning "nodiscard" } */
+  (void) (k ? check1 : check10) ();
+  (k ? check1 : check10) (), bar (); /* { dg-warning "nodiscard" } */
+  if ((k ? check3 : check11) ())
+    return;
+  i += (k ? check3 : check11) ();
+  i += ({ (k ? check3 : check11) (); });
+  (k ? check3 : check11) (); /* { dg-warning "nodiscard" } */
+  (void) (k ? check3 : check11) ();
+  (k ? check3 : check11) (), bar (); /* { dg-warning "nodiscard" } */
+  if (pcheck1 ())
+    return;
+  i += pcheck1 ();
+  i += ({ pcheck1 (); });
+  pcheck1 ();		/* { dg-warning "nodiscard" } */
+  (void) pcheck1 ();
+  pcheck1 (), bar ();	/* { dg-warning "nodiscard" } */
+  if (pcheck3 ())
+    return;
+  i += pcheck3 ();
+  i += ({ pcheck3 (); });
+  pcheck3 ();		/* { dg-warning "nodiscard" } */
+  (void) pcheck3 ();
+  pcheck3 (), bar ();	/* { dg-warning "nodiscard" } */
+  d = check12 ();
+  if (d.i)
+    return;
+  if (check12 ().i)
+    return;
+  if (({ check12 (); }).i)
+    return;
+  check12 ();		/* { dg-warning "nodiscard" } */
+  (void) check12 ();
+  check12 (), bar ();	/* { dg-warning "nodiscard" } */
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wunused-result-2.C b/gcc/testsuite/g++.dg/warn/Wunused-result-2.C
new file mode 100644
index 00000000000..56ed17a6abe
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wunused-result-2.C
@@ -0,0 +1,21 @@
+// PR c++/66177
+
+struct QSize
+{
+    QSize(int w, int h) : wd(w), ht(h) {}
+
+    QSize expandedTo() const __attribute__ ((__warn_unused_result__))
+    {
+        return QSize(2, 3);
+    }
+
+private:
+    int wd;
+    int ht;
+};
+
+void foo()
+{
+    QSize sz(2, 2);
+    sz.expandedTo();		// { dg-warning "warn_unused_result" }
+}
diff --git a/gcc/testsuite/g++.dg/warn/unused-result1.C b/gcc/testsuite/g++.dg/warn/unused-result1.C
index 466c99e7d97..1b9ef8af786 100644
--- a/gcc/testsuite/g++.dg/warn/unused-result1.C
+++ b/gcc/testsuite/g++.dg/warn/unused-result1.C
@@ -6,5 +6,5 @@ public:
 };
 class QString {
   QByteArray toLocal8Bit() const __attribute__ ((warn_unused_result));
-  void fooWarnHere() const { toLocal8Bit(); } // { dg-warning "ignoring" "" { xfail *-*-* } }
+  void fooWarnHere() const { toLocal8Bit(); } // { dg-warning "ignoring" }
 };