[clang] [clang] Improve diagnostic on [[nodiscard]] attribute (PR #112521)
Yihe Li via cfe-commits
cfe-commits at lists.llvm.org
Fri Nov 15 07:08:36 PST 2024
https://github.com/Mick235711 updated https://github.com/llvm/llvm-project/pull/112521
>From 59f7dbdd8eed456b76e93f6260bf0e361242e9fd Mon Sep 17 00:00:00 2001
From: Yihe Li <winmikedows at hotmail.com>
Date: Wed, 16 Oct 2024 18:53:04 +0800
Subject: [PATCH 1/3] [clang] Improve diagnostic on [[nodiscard]] attribute
---
clang/docs/ReleaseNotes.rst | 3 +
clang/include/clang/AST/Expr.h | 8 +-
.../clang/Basic/DiagnosticSemaKinds.td | 6 +
clang/lib/AST/Expr.cpp | 18 +--
clang/lib/Sema/SemaStmt.cpp | 40 ++++---
.../dcl.attr/dcl.attr.nodiscard/p2.cpp | 28 ++---
.../dcl.attr/dcl.attr.nodiscard/p3.cpp | 2 +-
clang/test/Sema/c2x-nodiscard.c | 8 +-
clang/test/SemaCXX/warn-unused-result.cpp | 111 ++++++++++++++----
9 files changed, 159 insertions(+), 65 deletions(-)
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index dc5564b6db119f..5dd30569fad108 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -416,6 +416,9 @@ Improvements to Clang's diagnostics
name was a reserved name, which we improperly allowed to suppress the
diagnostic.
+- Clang now includes the return type of the function or constructor in the warning generated
+ when `[[nodiscard]]` is triggered by its placement on return types instead of function itself.
+
Improvements to Clang's time-trace
----------------------------------
diff --git a/clang/include/clang/AST/Expr.h b/clang/include/clang/AST/Expr.h
index cbe62411d11bff..592e1ef925796f 100644
--- a/clang/include/clang/AST/Expr.h
+++ b/clang/include/clang/AST/Expr.h
@@ -3181,12 +3181,14 @@ class CallExpr : public Expr {
QualType getCallReturnType(const ASTContext &Ctx) const;
/// Returns the WarnUnusedResultAttr that is either declared on the called
- /// function, or its return type declaration.
- const Attr *getUnusedResultAttr(const ASTContext &Ctx) const;
+ /// function, or its return type declaration, together with a NamedDecl that
+ /// refers to the declaration the attribute is attached onto.
+ std::pair<const NamedDecl *, const Attr *>
+ getUnusedResultAttr(const ASTContext &Ctx) const;
/// Returns true if this call expression should warn on unused results.
bool hasUnusedResultAttr(const ASTContext &Ctx) const {
- return getUnusedResultAttr(Ctx) != nullptr;
+ return getUnusedResultAttr(Ctx).second != nullptr;
}
SourceLocation getRParenLoc() const { return RParenLoc; }
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index c458a62d9be48c..d22ceb0920b2ab 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -9267,6 +9267,12 @@ def warn_unused_container_subscript_expr : Warning<
def warn_unused_call : Warning<
"ignoring return value of function declared with %0 attribute">,
InGroup<UnusedValue>;
+def warn_unused_return_type : Warning<
+ "ignoring %select{return value|temporary}0 of type %2 declared with %1 attribute">,
+ InGroup<UnusedValue>;
+def warn_unused_return_type_msg : Warning<
+ "ignoring %select{return value|temporary}0 of type %2 declared with %1 attribute: %3">,
+ InGroup<UnusedValue>;
def warn_unused_constructor : Warning<
"ignoring temporary created by a constructor declared with %0 attribute">,
InGroup<UnusedValue>;
diff --git a/clang/lib/AST/Expr.cpp b/clang/lib/AST/Expr.cpp
index 9ecbf121e3fc0d..bbb05ec48523f5 100644
--- a/clang/lib/AST/Expr.cpp
+++ b/clang/lib/AST/Expr.cpp
@@ -1616,22 +1616,24 @@ QualType CallExpr::getCallReturnType(const ASTContext &Ctx) const {
return FnType->getReturnType();
}
-const Attr *CallExpr::getUnusedResultAttr(const ASTContext &Ctx) const {
+std::pair<const NamedDecl *, const Attr *>
+CallExpr::getUnusedResultAttr(const ASTContext &Ctx) const {
+ // If the callee is marked nodiscard, return that attribute
+ const Decl *D = getCalleeDecl();
+ if (const auto *A = D->getAttr<WarnUnusedResultAttr>())
+ return {nullptr, A};
+
// If the return type is a struct, union, or enum that is marked nodiscard,
// then return the return type attribute.
if (const TagDecl *TD = getCallReturnType(Ctx)->getAsTagDecl())
if (const auto *A = TD->getAttr<WarnUnusedResultAttr>())
- return A;
+ return {TD, A};
for (const auto *TD = getCallReturnType(Ctx)->getAs<TypedefType>(); TD;
TD = TD->desugar()->getAs<TypedefType>())
if (const auto *A = TD->getDecl()->getAttr<WarnUnusedResultAttr>())
- return A;
-
- // Otherwise, see if the callee is marked nodiscard and return that attribute
- // instead.
- const Decl *D = getCalleeDecl();
- return D ? D->getAttr<WarnUnusedResultAttr>() : nullptr;
+ return {TD->getDecl(), A};
+ return {nullptr, nullptr};
}
SourceLocation CallExpr::getBeginLoc() const {
diff --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
index 9e235a46707cd4..5895da9daaf22d 100644
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/clang/lib/Sema/SemaStmt.cpp
@@ -204,22 +204,28 @@ static bool DiagnoseUnusedComparison(Sema &S, const Expr *E) {
return true;
}
-static bool DiagnoseNoDiscard(Sema &S, const WarnUnusedResultAttr *A,
- SourceLocation Loc, SourceRange R1,
- SourceRange R2, bool IsCtor) {
+static bool DiagnoseNoDiscard(Sema &S, const NamedDecl *OffendingDecl,
+ const WarnUnusedResultAttr *A, SourceLocation Loc,
+ SourceRange R1, SourceRange R2, bool IsCtor) {
if (!A)
return false;
StringRef Msg = A->getMessage();
if (Msg.empty()) {
+ if (OffendingDecl)
+ return S.Diag(Loc, diag::warn_unused_return_type)
+ << IsCtor << A << OffendingDecl << R1 << R2;
if (IsCtor)
return S.Diag(Loc, diag::warn_unused_constructor) << A << R1 << R2;
return S.Diag(Loc, diag::warn_unused_result) << A << R1 << R2;
}
+ if (OffendingDecl)
+ return S.Diag(Loc, diag::warn_unused_return_type_msg)
+ << IsCtor << A << OffendingDecl << Msg << R1 << R2;
if (IsCtor)
- return S.Diag(Loc, diag::warn_unused_constructor_msg) << A << Msg << R1
- << R2;
+ return S.Diag(Loc, diag::warn_unused_constructor_msg)
+ << A << Msg << R1 << R2;
return S.Diag(Loc, diag::warn_unused_result_msg) << A << Msg << R1 << R2;
}
@@ -290,9 +296,10 @@ void Sema::DiagnoseUnusedExprResult(const Stmt *S, unsigned DiagID) {
if (E->getType()->isVoidType())
return;
- if (DiagnoseNoDiscard(*this, cast_or_null<WarnUnusedResultAttr>(
- CE->getUnusedResultAttr(Context)),
- Loc, R1, R2, /*isCtor=*/false))
+ auto [OffendingDecl, A] = CE->getUnusedResultAttr(Context);
+ if (DiagnoseNoDiscard(*this, OffendingDecl,
+ cast_or_null<WarnUnusedResultAttr>(A), Loc, R1, R2,
+ /*isCtor=*/false))
return;
// If the callee has attribute pure, const, or warn_unused_result, warn with
@@ -313,16 +320,21 @@ void Sema::DiagnoseUnusedExprResult(const Stmt *S, unsigned DiagID) {
}
} else if (const auto *CE = dyn_cast<CXXConstructExpr>(E)) {
if (const CXXConstructorDecl *Ctor = CE->getConstructor()) {
+ const NamedDecl *OffendingDecl = nullptr;
const auto *A = Ctor->getAttr<WarnUnusedResultAttr>();
- A = A ? A : Ctor->getParent()->getAttr<WarnUnusedResultAttr>();
- if (DiagnoseNoDiscard(*this, A, Loc, R1, R2, /*isCtor=*/true))
+ if (!A) {
+ OffendingDecl = Ctor->getParent();
+ A = OffendingDecl->getAttr<WarnUnusedResultAttr>();
+ }
+ if (DiagnoseNoDiscard(*this, OffendingDecl, A, Loc, R1, R2,
+ /*isCtor=*/true))
return;
}
} else if (const auto *ILE = dyn_cast<InitListExpr>(E)) {
if (const TagDecl *TD = ILE->getType()->getAsTagDecl()) {
- if (DiagnoseNoDiscard(*this, TD->getAttr<WarnUnusedResultAttr>(), Loc, R1,
- R2, /*isCtor=*/false))
+ if (DiagnoseNoDiscard(*this, TD, TD->getAttr<WarnUnusedResultAttr>(), Loc,
+ R1, R2, /*isCtor=*/false))
return;
}
} else if (ShouldSuppress)
@@ -336,8 +348,8 @@ void Sema::DiagnoseUnusedExprResult(const Stmt *S, unsigned DiagID) {
}
const ObjCMethodDecl *MD = ME->getMethodDecl();
if (MD) {
- if (DiagnoseNoDiscard(*this, MD->getAttr<WarnUnusedResultAttr>(), Loc, R1,
- R2, /*isCtor=*/false))
+ if (DiagnoseNoDiscard(*this, nullptr, MD->getAttr<WarnUnusedResultAttr>(),
+ Loc, R1, R2, /*isCtor=*/false))
return;
}
} else if (const PseudoObjectExpr *POE = dyn_cast<PseudoObjectExpr>(E)) {
diff --git a/clang/test/CXX/dcl.dcl/dcl.attr/dcl.attr.nodiscard/p2.cpp b/clang/test/CXX/dcl.dcl/dcl.attr/dcl.attr.nodiscard/p2.cpp
index 693ca29370cf3f..da1f8201f55dcc 100644
--- a/clang/test/CXX/dcl.dcl/dcl.attr/dcl.attr.nodiscard/p2.cpp
+++ b/clang/test/CXX/dcl.dcl/dcl.attr/dcl.attr.nodiscard/p2.cpp
@@ -17,10 +17,10 @@ E get_e();
// cxx11-warning at -1 {{use of the 'nodiscard' attribute is a C++17 extension}}
void f() {
- get_s(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ get_s(); // expected-warning {{ignoring return value of type 'S' declared with 'nodiscard' attribute}}
get_i(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
get_vi(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
- get_e(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ get_e(); // expected-warning {{ignoring return value of type 'E' declared with 'nodiscard' attribute}}
// Okay, warnings are not encouraged
get_s_ref();
@@ -54,10 +54,10 @@ void f() {
fp3 three;
fp2_alias four;
- one(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
- two(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
- three(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
- four(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ one(); // expected-warning {{ignoring return value of type 'E' declared with 'nodiscard' attribute}}
+ two(); // expected-warning {{ignoring return value of type 'S' declared with 'nodiscard' attribute}}
+ three(); // expected-warning {{ignoring return value of type 'S' declared with 'nodiscard' attribute}}
+ four(); // expected-warning {{ignoring return value of type 'S' declared with 'nodiscard' attribute}}
// These are all okay because of the explicit cast to void.
(void)one();
@@ -84,8 +84,8 @@ LaterReason get_later_reason();
// cxx11-17-warning at -1 {{use of the 'nodiscard' attribute is a C++20 extension}}
void cxx20_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}}
+ get_reason(); // expected-warning {{ignoring return value of type 'ReasonStruct' declared with 'nodiscard' attribute: reason}}
+ get_later_reason(); // expected-warning {{ignoring return value of type 'LaterReason' 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}}
}
@@ -115,20 +115,20 @@ void usage() {
S('A'); // expected-warning {{ignoring temporary created by a constructor declared with 'nodiscard' attribute: Don't let that S-Char go!}}
S(1);
S(2.2);
- Y(); // expected-warning {{ignoring temporary created by a constructor declared with 'nodiscard' attribute: Don't throw me away either!}}
+ Y(); // expected-warning {{ignoring temporary of type 'Y' declared with 'nodiscard' attribute: Don't throw me away either!}}
S s;
- ConvertTo{}; // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute: Don't throw me away!}}
+ ConvertTo{}; // expected-warning {{ignoring return value of type 'ConvertTo' declared with 'nodiscard' attribute: Don't throw me away!}}
// AST is different in C++17 mode. Before, a move ctor for ConvertTo is there
// as well, hence the constructor warning.
- // since-cxx17-warning at +2 {{ignoring return value of function declared with 'nodiscard' attribute: Don't throw me away!}}
- // cxx11-warning at +1 {{ignoring temporary created by a constructor declared with 'nodiscard' attribute: Don't throw me away!}}
+ // since-cxx17-warning at +2 {{ignoring return value of type 'ConvertTo' declared with 'nodiscard' attribute: Don't throw me away!}}
+ // cxx11-warning at +1 {{ignoring temporary of type 'ConvertTo' declared with 'nodiscard' attribute: Don't throw me away!}}
(ConvertTo) s;
(int)s; // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
(S)'c'; // expected-warning {{ignoring temporary created by a constructor declared with 'nodiscard' attribute: Don't let that S-Char go!}}
- // since-cxx17-warning at +2 {{ignoring return value of function declared with 'nodiscard' attribute: Don't throw me away!}}
- // cxx11-warning at +1 {{ignoring temporary created by a constructor declared with 'nodiscard' attribute: Don't throw me away!}}
+ // since-cxx17-warning at +2 {{ignoring return value of type 'ConvertTo' declared with 'nodiscard' attribute: Don't throw me away!}}
+ // cxx11-warning at +1 {{ignoring temporary of type 'ConvertTo' declared with 'nodiscard' attribute: Don't throw me away!}}
static_cast<ConvertTo>(s);
static_cast<int>(s); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
static_cast<double>(s); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute: Don't throw away as a double}}
diff --git a/clang/test/CXX/dcl.dcl/dcl.attr/dcl.attr.nodiscard/p3.cpp b/clang/test/CXX/dcl.dcl/dcl.attr/dcl.attr.nodiscard/p3.cpp
index a3543cff7d2c92..b37517921b1ca4 100644
--- a/clang/test/CXX/dcl.dcl/dcl.attr/dcl.attr.nodiscard/p3.cpp
+++ b/clang/test/CXX/dcl.dcl/dcl.attr/dcl.attr.nodiscard/p3.cpp
@@ -8,7 +8,7 @@ namespace std_example {
error_info enable_missile_safety_mode();
void launch_missiles();
void test_missiles() {
- enable_missile_safety_mode(); // expected-warning {{ignoring return value of function declared with 'nodiscard'}}
+ enable_missile_safety_mode(); // expected-warning {{ignoring return value of type 'error_info' declared with 'nodiscard'}}
launch_missiles();
}
diff --git a/clang/test/Sema/c2x-nodiscard.c b/clang/test/Sema/c2x-nodiscard.c
index f8b0567366465d..e2537bcf1d29d6 100644
--- a/clang/test/Sema/c2x-nodiscard.c
+++ b/clang/test/Sema/c2x-nodiscard.c
@@ -31,10 +31,10 @@ enum E2 get_e(void);
[[nodiscard]] int get_i(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_s(); // expected-warning {{ignoring return value of type 'S4' declared with 'nodiscard' attribute}}
+ get_s3(); // expected-warning {{ignoring return value of type 'S3' 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}}
+ get_e(); // expected-warning {{ignoring return value of type 'E2' declared with 'nodiscard' attribute}}
// Okay, warnings are not encouraged
(void)get_s();
@@ -50,7 +50,7 @@ struct [[nodiscard]] error_info{
struct error_info enable_missile_safety_mode(void);
void launch_missiles(void);
void test_missiles(void) {
- enable_missile_safety_mode(); // expected-warning {{ignoring return value of function declared with 'nodiscard'}}
+ enable_missile_safety_mode(); // expected-warning {{ignoring return value of type 'error_info' declared with 'nodiscard'}}
launch_missiles();
}
diff --git a/clang/test/SemaCXX/warn-unused-result.cpp b/clang/test/SemaCXX/warn-unused-result.cpp
index 4b7a2503ecc0dd..682c500dc1d96d 100644
--- a/clang/test/SemaCXX/warn-unused-result.cpp
+++ b/clang/test/SemaCXX/warn-unused-result.cpp
@@ -108,7 +108,7 @@ void lazy() {
(void)DoAnotherThing();
(void)DoYetAnotherThing();
- DoSomething(); // expected-warning {{ignoring return value}}
+ DoSomething(); // expected-warning {{ignoring return value of type 'Status' declared with 'warn_unused_result'}}
DoSomethingElse();
DoAnotherThing();
DoYetAnotherThing();
@@ -120,11 +120,11 @@ class [[clang::warn_unused_result]] StatusOr {
StatusOr<int> doit();
void test() {
Foo f;
- f.doStuff(); // expected-warning {{ignoring return value}}
- doit(); // expected-warning {{ignoring return value}}
+ f.doStuff(); // expected-warning {{ignoring return value of type 'Status' declared with 'warn_unused_result'}}
+ doit(); // expected-warning {{ignoring return value of type 'StatusOr<int>' declared with 'warn_unused_result'}}
auto func = []() { return Status(); };
- func(); // expected-warning {{ignoring return value}}
+ func(); // expected-warning {{ignoring return value of type 'Status' declared with 'warn_unused_result'}}
}
}
@@ -139,7 +139,7 @@ struct Status {};
void Bar() {
Foo f;
- f.Bar(); // expected-warning {{ignoring return value}}
+ f.Bar(); // expected-warning {{ignoring return value of type 'Status' declared with 'warn_unused_result'}}
};
}
@@ -215,18 +215,18 @@ P operator--(const P &) { return {}; };
void f() {
S s;
P p;
- s.DoThing(); // expected-warning {{ignoring return value}}
- p.DoThing(); // expected-warning {{ignoring return value}}
+ s.DoThing(); // expected-warning {{ignoring return value of type 'S' declared with 'warn_unused_result'}}
+ p.DoThing(); // expected-warning {{ignoring return value of type 'P' declared with 'warn_unused_result'}}
// Only postfix is expected to warn when written correctly.
- s++; // expected-warning {{ignoring return value}}
- s--; // expected-warning {{ignoring return value}}
- p++; // expected-warning {{ignoring return value}}
- p--; // expected-warning {{ignoring return value}}
+ s++; // expected-warning {{ignoring return value of type 'S' declared with 'warn_unused_result'}}
+ s--; // expected-warning {{ignoring return value of type 'S' declared with 'warn_unused_result'}}
+ p++; // expected-warning {{ignoring return value of type 'P' declared with 'warn_unused_result'}}
+ p--; // expected-warning {{ignoring return value of type 'P' declared with 'warn_unused_result'}}
// Improperly written prefix operators should still warn.
- ++s; // expected-warning {{ignoring return value}}
- --s; // expected-warning {{ignoring return value}}
- ++p; // expected-warning {{ignoring return value}}
- --p; // expected-warning {{ignoring return value}}
+ ++s; // expected-warning {{ignoring return value of type 'S' declared with 'warn_unused_result'}}
+ --s; // expected-warning {{ignoring return value of type 'S' declared with 'warn_unused_result'}}
+ ++p; // expected-warning {{ignoring return value of type 'P' declared with 'warn_unused_result'}}
+ --p; // expected-warning {{ignoring return value of type 'P' declared with 'warn_unused_result'}}
// Silencing the warning by cast to void still works.
(void)s.DoThing();
@@ -243,7 +243,7 @@ namespace PR39837 {
void g() {
int a[2];
for (int b : a)
- f(b); // expected-warning {{ignoring return value}}
+ f(b); // expected-warning {{ignoring return value of function declared with 'warn_unused_result'}}
}
} // namespace PR39837
@@ -261,12 +261,12 @@ typedef a indirect;
a af1();
indirect indirectf1();
void af2() {
- af1(); // expected-warning {{ignoring return value}}
+ af1(); // expected-warning {{ignoring return value of type 'a' declared with 'warn_unused_result'}}
void *(*a1)();
a1(); // no warning
a (*a2)();
- a2(); // expected-warning {{ignoring return value}}
- indirectf1(); // expected-warning {{ignoring return value}}
+ a2(); // expected-warning {{ignoring return value of type 'a' declared with 'warn_unused_result'}}
+ indirectf1(); // expected-warning {{ignoring return value of type 'a' declared with 'warn_unused_result'}}
}
[[nodiscard]] typedef void *b1; // expected-warning {{'[[nodiscard]]' attribute ignored when applied to a typedef; consider using '__attribute__((warn_unused_result))' or '[[clang::warn_unused_result]]' instead}}
[[gnu::warn_unused_result]] typedef void *b2; // expected-warning {{'[[gnu::warn_unused_result]]' attribute ignored when applied to a typedef; consider using '__attribute__((warn_unused_result))' or '[[clang::warn_unused_result]]' instead}}
@@ -279,10 +279,79 @@ void bf2() {
__attribute__((warn_unused_result)) typedef void *c;
c cf1();
void cf2() {
- cf1(); // expected-warning {{ignoring return value}}
+ cf1(); // expected-warning {{ignoring return value of type 'c' declared with 'warn_unused_result'}}
void *(*c1)();
c1();
c (*c2)();
- c2(); // expected-warning {{ignoring return value}}
+ c2(); // expected-warning {{ignoring return value of type 'c' declared with 'warn_unused_result'}}
}
}
+
+namespace nodiscard_specialization {
+// Test to only mark a specialization of class template as nodiscard
+template<typename T> struct S { S(int) {} };
+template<> struct [[nodiscard]] S<int> { S(int) {} };
+template<typename T> struct [[clang::warn_unused_result]] S<const T> { S(int) {} };
+
+template<typename T>
+S<T> obtain(const T&) { return {2}; }
+
+template<typename T>
+[[nodiscard]] S<T> obtain2(const T&) { return {2}; }
+
+template<typename T>
+__attribute__((warn_unused_result)) S<T> obtain3(const T&) { return {2}; }
+
+void use() {
+ obtain(1.0); // no warning
+ obtain(1); // expected-warning {{ignoring return value of type 'S<int>' declared with 'nodiscard'}}
+ obtain<const double>(1); // expected-warning {{ignoring return value of type 'S<const double>' declared with 'warn_unused_result'}}
+
+ S<double>(2); // no warning
+ S<int>(2); // expected-warning {{ignoring temporary of type 'S<int>' declared with 'nodiscard'}}
+ S<const char>(2); // no warning (warn_unused_result does not diagnose constructor temporaries)
+
+ // function should take precedence over type
+ obtain2(1.0); // expected-warning {{ignoring return value of function declared with 'nodiscard'}}
+ obtain2(1); // expected-warning {{ignoring return value of function declared with 'nodiscard'}}
+ obtain2<const double>(1); // expected-warning {{ignoring return value of function declared with 'nodiscard'}}
+ obtain3(1.0); // expected-warning {{ignoring return value of function declared with 'warn_unused_result'}}
+ obtain3(1); // expected-warning {{ignoring return value of function declared with 'warn_unused_result'}}
+ obtain3<const double>(1); // expected-warning {{ignoring return value of function declared with 'warn_unused_result'}}
+}
+
+// Test on constructor nodiscard
+struct H {
+ explicit H(int) {}
+ [[nodiscard]] explicit H(double) {}
+ __attribute__((warn_unused_result)) H(const char*) {}
+};
+
+struct [[nodiscard]] G {
+ explicit G(int) {}
+ [[nodiscard]] explicit G(double) {}
+ [[clang::warn_unused_result]] G(const char*) {}
+};
+
+void use2() {
+ H{2}; // no warning
+ H(2.0); // expected-warning {{ignoring temporary created by a constructor declared with 'nodiscard'}}
+ H("Hello"); // no warning (warn_unused_result does not diagnose constructor temporaries)
+
+ // no warning for explicit cast to void
+ (void)H(2);
+ (void)H{2.0};
+ (void)H{"Hello"};
+
+ // warns for all these invocations
+ // here, constructor/function should take precedence over type
+ G{2}; // expected-warning {{ignoring temporary of type 'G' declared with 'nodiscard'}}
+ G(2.0); // expected-warning {{ignoring temporary created by a constructor declared with 'nodiscard'}}
+ G("Hello"); // expected-warning {{ignoring temporary created by a constructor declared with 'warn_unused_result'}}
+
+ // no warning for explicit cast to void
+ (void)G(2);
+ (void)G{2.0};
+ (void)G{"Hello"};
+}
+} // namespace nodiscard_specialization
>From fc7f05172e2289bbda20f9a85591f87dc033c63d Mon Sep 17 00:00:00 2001
From: Yihe Li <winmikedows at hotmail.com>
Date: Wed, 23 Oct 2024 02:26:38 +0800
Subject: [PATCH 2/3] Make the release note more concise
---
clang/docs/ReleaseNotes.rst | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 5dd30569fad108..2fc48e9e1341d2 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -416,8 +416,8 @@ Improvements to Clang's diagnostics
name was a reserved name, which we improperly allowed to suppress the
diagnostic.
-- Clang now includes the return type of the function or constructor in the warning generated
- when `[[nodiscard]]` is triggered by its placement on return types instead of function itself.
+- When diagnosing an unused return value of a type declared ``[[nodiscard]]``, the type
+ itself is now included in the diagnostic.
Improvements to Clang's time-trace
----------------------------------
>From f1f116dbe30b6aad63032d01c2aaaa1b05151d17 Mon Sep 17 00:00:00 2001
From: Yihe Li <winmikedows at hotmail.com>
Date: Fri, 15 Nov 2024 23:08:09 +0800
Subject: [PATCH 3/3] Add release note
---
clang/docs/ReleaseNotes.rst | 13 +++++++++++++
1 file changed, 13 insertions(+)
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index b5993e5ab1e4bf..3f4e9285a5f5d3 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -148,6 +148,19 @@ C++ Specific Potentially Breaking Changes
// Now diagnoses with an error.
void f(int& i [[clang::lifetimebound]]);
+- Clang will now prefer the ``[[nodiscard]]`` declaration on function declarations over ``[[nodiscard]]``
+ declaration on the return type of a function. Previously, when both have a ``[[nodiscard]]`` declaration attached,
+ the one on the return type would be preferred. This may affect the generated warning message:
+
+ .. code-block:: c++
+
+ struct [[nodiscard("Reason 1")]] S {};
+ [[nodiscard("Reason 2")]] S getS();
+ void use()
+ {
+ getS(); // Now diagnoses "Reason 2", previously diagnoses "Reason 1"
+ }
+
ABI Changes in This Version
---------------------------
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