Implement P1301R4, which allows specifying an optional message on the [[nodiscard]] attribute.

This also bumps the attribute feature test value and introduces the notion of a C++2a extension warning.

llvm-svn: 366626

clang/include/clang/Basic/Attr.td

@@ -2335,10 +2335,11 @@ def WarnUnused : InheritableAttr {
 }
 
 def WarnUnusedResult : InheritableAttr {
-  let Spellings = [CXX11<"", "nodiscard", 201603>, C2x<"", "nodiscard">,
+  let Spellings = [CXX11<"", "nodiscard", 201907>, C2x<"", "nodiscard">,
                    CXX11<"clang", "warn_unused_result">,
                    GCC<"warn_unused_result">];
   let Subjects = SubjectList<[ObjCMethod, Enum, Record, FunctionLike]>;
+  let Args = [StringArgument<"Message", 1>];
   let Documentation = [WarnUnusedResultsDocs];
 }
 

clang/include/clang/Basic/AttrDocs.td

@@ -1482,6 +1482,13 @@ generated when a function or its return type is marked with ``[[nodiscard]]``
 potentially-evaluated discarded-value expression that is not explicitly cast to
 `void`.
 
+A string literal may optionally be provided to the attribute, which will be
+reproduced in any resulting diagnostics. Redeclarations using different forms
+of the attribute (with or without the string literal or with different string
+literal contents) are allowed. If there are redeclarations of the entity with
+differing string literals, it is unspecified which one will be used by Clang
+in any resulting diagnostics.
+
 .. code-block: c++
   struct [[nodiscard]] error_info { /*...*/ };
   error_info enable_missile_safety_mode();

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -7436,6 +7436,9 @@ def warn_side_effects_typeid : Warning<
 def warn_unused_result : Warning<
   "ignoring return value of function declared with %0 attribute">,
   InGroup<UnusedResult>;
+def warn_unused_result_msg : Warning<
+  "ignoring return value of function declared with %0 attribute: %1">,
+  InGroup<UnusedResult>;
 def warn_unused_volatile : Warning<
   "expression result unused; assign into a variable to force a volatile load">,
   InGroup<DiagGroup<"unused-volatile-lvalue">>;
@@ -7444,6 +7447,8 @@ def ext_cxx14_attr : Extension<
   "use of the %0 attribute is a C++14 extension">, InGroup<CXX14>;
 def ext_cxx17_attr : Extension<
   "use of the %0 attribute is a C++17 extension">, InGroup<CXX17>;
+def ext_cxx2a_attr : Extension<
+  "use of the %0 attribute is a C++2a extension">, InGroup<CXX2a>;
 
 def warn_unused_comparison : Warning<
   "%select{equality|inequality|relational|three-way}0 comparison result unused">,

clang/lib/Sema/SemaDeclAttr.cpp

@@ -2841,14 +2841,30 @@ static void handleWarnUnusedResult(Sema &S, Decl *D, const ParsedAttr &AL) {
       return;
     }
 
-  // If this is spelled as the standard C++17 attribute, but not in C++17, warn
-  // about using it as an extension.
-  if (!S.getLangOpts().CPlusPlus17 && AL.isCXX11Attribute() &&
-      !AL.getScopeName())
-    S.Diag(AL.getLoc(), diag::ext_cxx17_attr) << AL;
+  StringRef Str;
+  if ((AL.isCXX11Attribute() || AL.isC2xAttribute()) && !AL.getScopeName()) {
+    // If this is spelled as the standard C++17 attribute, but not in C++17,
+    // warn about using it as an extension. If there are attribute arguments,
+    // then claim it's a C++2a extension instead.
+    // FIXME: If WG14 does not seem likely to adopt the same feature, add an
+    // extension warning for C2x mode.
+    const LangOptions &LO = S.getLangOpts();
+    if (AL.getNumArgs() == 1) {
+      if (LO.CPlusPlus && !LO.CPlusPlus2a)
+        S.Diag(AL.getLoc(), diag::ext_cxx2a_attr) << AL;
+
+      // Since this this is spelled [[nodiscard]], get the optional string
+      // literal. If in C++ mode, but not in C++2a mode, diagnose as an
+      // extension.
+      // FIXME: C2x should support this feature as well, even as an extension.
+      if (!S.checkStringLiteralArgumentAttr(AL, 0, Str, nullptr))
+        return;
+    } else if (LO.CPlusPlus && !LO.CPlusPlus17)
+      S.Diag(AL.getLoc(), diag::ext_cxx17_attr) << AL;
+  }
 
   D->addAttr(::new (S.Context)
-             WarnUnusedResultAttr(AL.getRange(), S.Context,
+             WarnUnusedResultAttr(AL.getRange(), S.Context, Str,
                                   AL.getAttributeSpellingListIndex()));
 }
 

clang/lib/Sema/SemaStmt.cpp

@@ -258,8 +258,13 @@ void Sema::DiagnoseUnusedExprResult(const Stmt *S) {
     if (E->getType()->isVoidType())
       return;
 
-    if (const Attr *A = CE->getUnusedResultAttr(Context)) {
-      Diag(Loc, diag::warn_unused_result) << A << R1 << R2;
+    if (const auto *A = cast_or_null<WarnUnusedResultAttr>(
+            CE->getUnusedResultAttr(Context))) {
+      StringRef Msg = A->getMessage();
+      if (!Msg.empty())
+        Diag(Loc, diag::warn_unused_result_msg) << A << Msg << R1 << R2;
+      else
+        Diag(Loc, diag::warn_unused_result) << A << R1 << R2;
       return;
     }
 
@@ -290,7 +295,11 @@ void Sema::DiagnoseUnusedExprResult(const Stmt *S) {
     const ObjCMethodDecl *MD = ME->getMethodDecl();
     if (MD) {
       if (const auto *A = MD->getAttr<WarnUnusedResultAttr>()) {
-        Diag(Loc, diag::warn_unused_result) << A << R1 << R2;
+        StringRef Msg = A->getMessage();
+        if (!Msg.empty())
+          Diag(Loc, diag::warn_unused_result_msg) << A << Msg << R1 << R2;
+        else
+          Diag(Loc, diag::warn_unused_result) << A << R1 << R2;
         return;
       }
     }

clang/test/CXX/dcl.dcl/dcl.attr/dcl.attr.nodiscard/p1.cpp

@@ -1,8 +1,8 @@
-// RUN: %clang_cc1 -fsyntax-only -std=c++1z -verify %s
+// RUN: %clang_cc1 -fsyntax-only -std=c++2a -verify %s
 
 struct [[nodiscard]] S1 {}; // ok
 struct [[nodiscard nodiscard]] S2 {}; // expected-error {{attribute 'nodiscard' cannot appear multiple times in an attribute specifier}}
-struct [[nodiscard("Wrong")]] S3 {}; // expected-error {{'nodiscard' cannot have an argument list}}
+struct [[nodiscard("Wrong")]] S3 {};
 
 [[nodiscard]] int f();
 enum [[nodiscard]] E {};

clang/test/CXX/dcl.dcl/dcl.attr/dcl.attr.nodiscard/p2.cpp

@@ -1,5 +1,6 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++2a -verify -Wc++2a-extensions %s
 // RUN: %clang_cc1 -fsyntax-only -std=c++17 -verify -Wc++17-extensions %s
-// RUN: %clang_cc1 -fsyntax-only -std=c++11 -verify -DEXT -Wc++17-extensions %s
+// RUN: %clang_cc1 -fsyntax-only -std=c++11 -verify -DEXT -Wc++17-extensions -Wc++2a-extensions %s
 
 struct [[nodiscard]] S {};
 S get_s();
@@ -61,10 +62,33 @@ void f() {
 }
 } // namespace PR31526
 
+struct [[nodiscard("reason")]] ReasonStruct {};
+struct LaterReason;
+struct [[nodiscard("later reason")]] LaterReason {};
+
+ReasonStruct get_reason();
+LaterReason get_later_reason();
+[[nodiscard("another reason")]] int another_reason();
+
+[[nodiscard("conflicting reason")]] int conflicting_reason();
+[[nodiscard("special reason")]] int conflicting_reason();
+
+void cxx2a_use() {
+  get_reason(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute: reason}}
+  get_later_reason(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute: later reason}}
+  another_reason(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute: another reason}}
+  conflicting_reason(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute: special reason}}
+}
+
 #ifdef EXT
-// expected-warning@4 {{use of the 'nodiscard' attribute is a C++17 extension}}
-// expected-warning@8 {{use of the 'nodiscard' attribute is a C++17 extension}}
-// expected-warning@11 {{use of the 'nodiscard' attribute is a C++17 extension}}
+// expected-warning@5 {{use of the 'nodiscard' attribute is a C++17 extension}}
+// expected-warning@9 {{use of the 'nodiscard' attribute is a C++17 extension}}
 // expected-warning@12 {{use of the 'nodiscard' attribute is a C++17 extension}}
-// expected-warning@28 {{use of the 'nodiscard' attribute is a C++17 extension}}
+// expected-warning@13 {{use of the 'nodiscard' attribute is a C++17 extension}}
+// expected-warning@29 {{use of the 'nodiscard' attribute is a C++17 extension}}
+// expected-warning@65 {{use of the 'nodiscard' attribute is a C++2a extension}}
+// expected-warning@67 {{use of the 'nodiscard' attribute is a C++2a extension}}
+// expected-warning@71 {{use of the 'nodiscard' attribute is a C++2a extension}}
+// expected-warning@73 {{use of the 'nodiscard' attribute is a C++2a extension}}
+// expected-warning@74 {{use of the 'nodiscard' attribute is a C++2a extension}}
 #endif

clang/test/Preprocessor/has_attribute.cpp

@@ -63,7 +63,7 @@ CXX11(unlikely)
 // CHECK: maybe_unused: 201603L
 // ITANIUM: no_unique_address: 201803L
 // WINDOWS: no_unique_address: 0
-// CHECK: nodiscard: 201603L
+// CHECK: nodiscard: 201907L
 // CHECK: noreturn: 200809L
 // FIXME(201803L) CHECK: unlikely: 0
 

clang/test/Sema/c2x-nodiscard.c

@@ -6,10 +6,12 @@ struct [[nodiscard]] S1 { // ok
 struct [[nodiscard nodiscard]] S2 { // expected-error {{attribute 'nodiscard' cannot appear multiple times in an attribute specifier}}
   int i;
 };
-struct [[nodiscard("Wrong")]] S3 { // expected-error {{'nodiscard' cannot have an argument list}}
+struct [[nodiscard("Wrong")]] S3 { // FIXME: may need an extension warning.
   int i;
 };
 
+struct S3 get_s3(void);
+
 [[nodiscard]] int f1(void);
 enum [[nodiscard]] E1 { One };
 
@@ -27,11 +29,13 @@ enum E2 get_e(void);
 
 void f2(void) {
   get_s(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  get_s3(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute: Wrong}}
   get_i(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
   get_e(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
 
   // Okay, warnings are not encouraged
   (void)get_s();
+  (void)get_s3();
   (void)get_i();
   (void)get_e();
 }

clang/test/SemaCXX/cxx11-attr-print.cpp

@@ -37,13 +37,15 @@ void foo() __attribute__((const));
 // CHECK: void bar() __attribute__((__const));
 void bar() __attribute__((__const));
 
-// CHECK: int f1() __attribute__((warn_unused_result));
+// FIXME: It's unfortunate that the string literal prints with the below three
+// cases given that the string is only exposed via the [[nodiscard]] spelling.
+// CHECK: int f1() __attribute__((warn_unused_result("")));
 int f1() __attribute__((warn_unused_result));
 
-// CHECK: {{\[}}[clang::warn_unused_result]];
+// CHECK: {{\[}}[clang::warn_unused_result("")]];
 int f2 [[clang::warn_unused_result]] ();
 
-// CHECK: {{\[}}[gnu::warn_unused_result]];
+// CHECK: {{\[}}[gnu::warn_unused_result("")]];
 int f3 [[gnu::warn_unused_result]] ();
 
 // FIXME: ast-print need to print C++11