Implement C++20 P1301 [[nodiscard("should have a reason")]].

2019-10-17  JeanHeyd Meneide  <phdofthehouse@gmail.com>

gcc/
        * escaped_string.h (escaped_string): New header.
        * tree.c (escaped_string): Remove escaped_string class.

gcc/c-family
        * c-lex.c (c_common_has_attribute): Update nodiscard value.

gcc/cp/
        * tree.c (handle_nodiscard_attribute) Added C++2a nodiscard
	string message.
        (std_attribute_table) Increase nodiscard argument handling
	max_length from 0 to 1.
        * parser.c (cp_parser_check_std_attribute): Add requirement
	that nodiscard only be seen once in attribute-list.
        (cp_parser_std_attribute): Check that empty parenthesis lists are
        not specified for attributes that have max_length > 0 (e.g.
	[[attr()]]).
        * cvt.c (maybe_warn_nodiscard): Add nodiscard message to
	output, if applicable.
	(convert_to_void): Allow constructors to be nodiscard-able (P1771).

gcc/testsuite/g++.dg/cpp0x
        * gen-attrs-67.C: Test new error message for empty-parenthesis-list.

gcc/testsuite/g++.dg/cpp2a
        * nodiscard-construct.C: New test.
        * nodiscard-once.C: New test.
        * nodiscard-reason-nonstring.C: New test.
        * nodiscard-reason-only-one.C: New test.
        * nodiscard-reason.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>

git-svn-id: https://gcc.gnu.org/svn/gcc/trunk@277200 138bc75d-0d04-0410-961f-82ee72b054a4

8 files changed, 256 insertions, 4 deletions

diff --git a/gcc/testsuite/g++.dg/cpp0x/gen-attrs-67.C b/gcc/testsuite/g++.dg/cpp0x/gen-attrs-67.C
index 3dd3a1fdc7e..c195dfe7b80 100644
--- a/gcc/testsuite/g++.dg/cpp0x/gen-attrs-67.C
+++ b/gcc/testsuite/g++.dg/cpp0x/gen-attrs-67.C
@@ -5,7 +5,7 @@
 [[noreturn(1)]] void f1 (); // { dg-error ".noreturn. attribute does not take any arguments" }
 [[noreturn(1, 2)]] void f2 (); // { dg-error ".noreturn. attribute does not take any arguments" }
 [[maybe_unused()]] int f3(); // { dg-error ".maybe_unused. attribute does not take any arguments" }
-[[nodiscard()]] int f4(); // { dg-error ".nodiscard. attribute does not take any arguments" }
+[[nodiscard()]] int f4(); // { dg-error "parentheses must be omitted if .nodiscard. attribute argument list is empty" }
 [[gnu::noinline()]] int f5(); // { dg-error ".noinline. attribute does not take any arguments" }
 [[gnu::constructor]] int f6();
 [[gnu::constructor(101)]] int f7(); // { dg-error "constructor priorities are not supported" "" { target { ! init_priority } } }
diff --git a/gcc/testsuite/g++.dg/cpp1z/feat-cxx1z.C b/gcc/testsuite/g++.dg/cpp1z/feat-cxx1z.C
index a5d94462c87..a18d6aea985 100644
--- a/gcc/testsuite/g++.dg/cpp1z/feat-cxx1z.C
+++ b/gcc/testsuite/g++.dg/cpp1z/feat-cxx1z.C
@@ -439,7 +439,7 @@
 
 #  if ! __has_cpp_attribute(nodiscard)
 #    error "__has_cpp_attribute(nodiscard)"
-#  elif __has_cpp_attribute(nodiscard) != 201603
+#  elif (__has_cpp_attribute(nodiscard) != 201603 && __has_cpp_attribute(nodiscard) != 201907)
 #    error "__has_cpp_attribute(nodiscard) != 201603"
 #  endif
 
diff --git a/gcc/testsuite/g++.dg/cpp2a/feat-cxx2a.C b/gcc/testsuite/g++.dg/cpp2a/feat-cxx2a.C
index 2b9cfd0effc..95251c2f5c6 100644
--- a/gcc/testsuite/g++.dg/cpp2a/feat-cxx2a.C
+++ b/gcc/testsuite/g++.dg/cpp2a/feat-cxx2a.C
@@ -470,8 +470,8 @@
 
 #  if ! __has_cpp_attribute(nodiscard)
 #    error "__has_cpp_attribute(nodiscard)"
-#  elif __has_cpp_attribute(nodiscard) != 201603
-#    error "__has_cpp_attribute(nodiscard) != 201603"
+#  elif __has_cpp_attribute(nodiscard) != 201907
+#    error "__has_cpp_attribute(nodiscard) != 201907"
 #  endif
 
 #  if ! __has_cpp_attribute(fallthrough)
diff --git a/gcc/testsuite/g++.dg/cpp2a/nodiscard-constructor.c b/gcc/testsuite/g++.dg/cpp2a/nodiscard-constructor.c
new file mode 100644
index 00000000000..a5c2c6514cf
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/nodiscard-constructor.c
@@ -0,0 +1,13 @@
+/* nodiscard attribute tests  */
+/* { dg-do compile { target c++2a } } */
+/* { dg-options "-O -ftrack-macro-expansion=0" } */
+
+struct A { [[nodiscard("bad constructor")]] A() {} };
+struct B { [[nodiscard]] B() {} };
+
+void
+test (void)
+{
+  A{}; /* { dg-warning "(?n)nodiscard.*bad constructor" } */
+  B{}; /* { dg-warning "(?n)nodiscard" } */
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/nodiscard-once.C b/gcc/testsuite/g++.dg/cpp2a/nodiscard-once.C
new file mode 100644
index 00000000000..4518a11c970
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/nodiscard-once.C
@@ -0,0 +1,12 @@
+/* nodiscard attribute tests  */
+/* { dg-do compile { target c++2a } } */
+/* { dg-options "-O -ftrack-macro-expansion=0" } */
+
+[[nodiscard, nodiscard]] int check1 (void); /* { dg-error "(?n)nodiscard.*can appear at most once" } */
+
+void
+test (void)
+{
+  check1 ();
+  (void) check1 ();
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/nodiscard-reason-nonstring.C b/gcc/testsuite/g++.dg/cpp2a/nodiscard-reason-nonstring.C
new file mode 100644
index 00000000000..af715e2d6b6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/nodiscard-reason-nonstring.C
@@ -0,0 +1,12 @@
+/* nodiscard attribute tests  */
+/* { dg-do compile { target c++2a } } */
+/* { dg-options "-O -ftrack-macro-expansion=0" } */
+
+[[nodiscard(123)]] int check1 (void); /* { dg-error "(?n)nodiscard.*must be a string constant" } */
+
+void
+test (void)
+{
+  check1 ();
+  (void) check1 ();
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/nodiscard-reason-only-one.C b/gcc/testsuite/g++.dg/cpp2a/nodiscard-reason-only-one.C
new file mode 100644
index 00000000000..602a4e273f5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/nodiscard-reason-only-one.C
@@ -0,0 +1,12 @@
+/* nodiscard attribute tests  */
+/* { dg-do compile { target c++2a } } */
+/* { dg-options "-O -ftrack-macro-expansion=0" } */
+
+[[nodiscard("not", "allowed")]] int check1 (void); /* { dg-error "(?n)wrong number of arguments..*nodiscard" } */
+
+void
+test (void)
+{
+  check1 ();
+  (void) check1 ();
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/nodiscard-reason.C b/gcc/testsuite/g++.dg/cpp2a/nodiscard-reason.C
new file mode 100644
index 00000000000..0113c904260
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/nodiscard-reason.C
@@ -0,0 +1,203 @@
+/* nodiscard attribute tests, adapted from gcc.dg/attr-warn-unused-result.c.  */
+/* { dg-do compile { target c++2a } } */
+/* { dg-options "-O -ftrack-macro-expansion=0" } */
+
+#define NODIS [[nodiscard("exact_message")]]
+#define NODISAI [[nodiscard("exact_inline_message"), gnu::always_inline]] inline
+enum [[nodiscard("exact_E_message")]] 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("exact_D_message")]] D { int i; D(); ~D(); };
+
+NODIS E check1 (void);
+NODIS void check2 (void); /* { dg-warning "(?n)10:.nodiscard.*exact_message" } */
+NODIS int foo;         /* { dg-warning "(?n)9:.nodiscard.*exact_message" } */
+int bar (void);
+NODISAI E check3 (void) { return (E)bar (); }
+NODIS A check4 (void);
+NODIS B check5 (void);
+NODIS C check6 (void);
+A bar7 (void);
+B bar8 (void);
+C bar9 (void);
+NODISAI A check7 (void) { return bar7 (); }
+NODISAI B check8 (void) { return bar8 (); }
+NODISAI C check9 (void) { return bar9 (); }
+/* This is useful for checking whether return value of statement
+   expressions (returning int in this case) is used.  */
+NODISAI int check_int_result (int res) { return res; }
+#define GU(v) ({ int e = 0; (v) = bar (); if ((v) < 23) e = 14; e; })
+fnt fnptr;
+NODIS E check10 (void);
+int baz (void);
+NODISAI 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 "(?n)nodiscard.*exact_message" } */
+  (void) check1 ();
+  check1 (), bar ();   /* { dg-warning "(?n)nodiscard.*exact_message" } */
+  check2 ();
+  (void) check2 ();
+  check2 (), bar ();
+  if (check3 ())
+    return;
+  i += check3 ();
+  i += ({ check3 (); });
+  check3 ();           /* { dg-warning "(?n)nodiscard.*exact_inline_message" } */
+  (void) check3 ();
+  check3 (), bar ();   /* { dg-warning "(?n)nodiscard.*exact_inline_message" } */
+  a = check4 ();
+  if (a.i)
+    return;
+  if (check4 ().i)
+    return;
+  if (({ check4 (); }).i)
+    return;
+  check4 ();           /* { dg-warning "(?n)nodiscard.*exact_message" } */
+  (void) check4 ();
+  check4 (), bar ();   /* { dg-warning "(?n)nodiscard.*exact_message" } */
+  b = check5 ();
+  if (b.i + b.j)
+    return;
+  if (check5 ().j)
+    return;
+  if (({ check5 (); }).j)
+    return;
+  check5 ();           /* { dg-warning "(?n)nodiscard.*exact_message" } */
+  (void) check5 ();
+  check5 (), bar ();   /* { dg-warning "(?n)nodiscard.*exact_message" } */
+  c = check6 ();
+  if (c.big[12] + c.big[29])
+    return;
+  if (check6 ().big[27])
+    return;
+  if (({ check6 (); }).big[0])
+    return;
+  check6 ();           /* { dg-warning "(?n)nodiscard.*exact_message" } */
+  (void) check6 ();
+  check6 (), bar ();   /* { dg-warning "(?n)nodiscard.*exact_message" } */
+  a = check7 ();
+  if (a.i)
+    return;
+  if (check7 ().i)
+    return;
+  if (({ check7 (); }).i)
+    return;
+  check7 ();           /* { dg-warning "(?n)nodiscard.*exact_inline_message" } */
+  (void) check7 ();
+  check7 (), bar ();   /* { dg-warning "(?n)nodiscard.*exact_inline_message" } */
+  b = check8 ();
+  if (b.i + b.j)
+    return;
+  if (check8 ().j)
+    return;
+  if (({ check8 (); }).j)
+    return;
+  check8 ();           /* { dg-warning "(?n)nodiscard.*exact_inline_message" } */
+  (void) check8 ();
+  check8 (), bar ();   /* { dg-warning "(?n)nodiscard.*exact_inline_message" } */
+  c = check9 ();
+  if (c.big[12] + c.big[29])
+    return;
+  if (check9 ().big[27])
+    return;
+  if (({ check9 (); }).big[0])
+    return;
+  check9 ();           /* { dg-warning "(?n)nodiscard.*exact_inline_message" } */
+  (void) check9 ();
+  check9 (), bar ();   /* { dg-warning "(?n)nodiscard.*exact_inline_message" } */
+  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 "(?n)nodiscard.*exact_inline_message" } */
+  (void) check_int_result (GU (j));
+  check_int_result (GU (j)), bar (); /* { dg-warning "(?n)nodiscard.*exact_inline_message" } */
+  if (fnptr ())
+    return;
+  i += fnptr ();
+  i += ({ fnptr (); });
+  fnptr ();            /* { dg-warning "(?n)nodiscard.*exact_E_message" } */
+  (void) fnptr ();
+  fnptr (), bar ();    /* { dg-warning "(?n)nodiscard.*exact_E_message" } */
+  fnptr = check1;
+  if (fnptr ())
+    return;
+  i += fnptr ();
+  i += ({ fnptr (); });
+  fnptr ();            /* { dg-warning "(?n)nodiscard.*exact_E_message" } */
+  (void) fnptr ();
+  fnptr (), bar ();    /* { dg-warning "(?n)nodiscard.*exact_E_message" } */
+  fnptr = check3;
+  if (fnptr ())
+    return;
+  i += fnptr ();
+  i += ({ fnptr (); });
+  fnptr ();            /* { dg-warning "(?n)nodiscard.*exact_E_message" } */
+  (void) fnptr ();
+  fnptr (), bar ();    /* { dg-warning "(?n)nodiscard.*exact_E_message" } */
+  if (bar9 ().fn ())
+    return;
+  i += bar9 ().fn ();
+  i += ({ bar9 ().fn (); });
+  bar9 ().fn ();       /* { dg-warning "(?n)nodiscard.*exact_E_message" } */
+  (void) bar9 ().fn ();
+  bar9 ().fn (), bar (); /* { dg-warning "(?n)nodiscard.*exact_E_message" } */
+  if ((k ? check1 : check10) ())
+    return;
+  i += (k ? check1 : check10) ();
+  i += ({ (k ? check1 : check10) (); });
+  (k ? check1 : check10) (); /* { dg-warning "(?n)nodiscard.*exact_E_message" } */
+  (void) (k ? check1 : check10) ();
+  (k ? check1 : check10) (), bar (); /* { dg-warning "(?n)nodiscard.*exact_E_message" } */
+  if ((k ? check3 : check11) ())
+    return;
+  i += (k ? check3 : check11) ();
+  i += ({ (k ? check3 : check11) (); });
+  (k ? check3 : check11) (); /* { dg-warning "(?n)nodiscard.*exact_inline_message" } */
+  (void) (k ? check3 : check11) ();
+  (k ? check3 : check11) (), bar (); /* { dg-warning "(?n)nodiscard.*exact_inline_message" } */
+  if (pcheck1 ())
+    return;
+  i += pcheck1 ();
+  i += ({ pcheck1 (); });
+  pcheck1 ();          /* { dg-warning "(?n)nodiscard.*exact_E_message" } */
+  (void) pcheck1 ();
+  pcheck1 (), bar ();  /* { dg-warning "(?n)nodiscard.*exact_E_message" } */
+  if (pcheck3 ())
+    return;
+  i += pcheck3 ();
+  i += ({ pcheck3 (); });
+  pcheck3 ();          /* { dg-warning "(?n)nodiscard.*exact_E_message" } */
+  (void) pcheck3 ();
+  pcheck3 (), bar ();  /* { dg-warning "(?n)nodiscard.*exact_E_message" } */
+  d = check12 ();
+  if (d.i)
+    return;
+  if (check12 ().i)
+    return;
+  if (({ check12 (); }).i)
+    return;
+  check12 ();          /* { dg-warning "(?n)nodiscard.*exact_D_message" } */
+  (void) check12 ();
+  check12 (), bar ();  /* { dg-warning "(?n)nodiscard.*exact_D_message" } */
+}