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Implement WG14 N2764 the [[noreturn]] attribute
This adds support for http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2764.pdf, which was adopted at the Feb 2022 WG14 meeting. That paper adds [[noreturn]] and [[_Noreturn]] to the list of supported attributes in C2x. These attributes have the same semantics as the [[noreturn]] attribute in C++. The [[_Noreturn]] attribute was added as a deprecated feature so that translation units which include <stdnoreturn.h> do not get an error on use of [[noreturn]] because the macro expands to _Noreturn. Users can use -Wno-deprecated-attributes to silence the diagnostic. Use of <stdnotreturn.h> or the noreturn macro were both deprecated. Users can define the _CLANG_DISABLE_CRT_DEPRECATION_WARNINGS macro to suppress the deprecation diagnostics coming from the header file.
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‎clang/docs/ReleaseNotes.rst

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@@ -92,6 +92,11 @@ Windows Support
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C Language Changes in Clang
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---------------------------
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C2x Feature Support
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-------------------
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- Implemented `WG14 N2674 The noreturn attribute <http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2764.pdf>`_.
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C++ Language Changes in Clang
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-----------------------------
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‎clang/include/clang/Basic/Attr.td

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@@ -1209,10 +1209,10 @@ def C11NoReturn : InheritableAttr {
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}
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def CXX11NoReturn : InheritableAttr {
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let Spellings = [CXX11<"", "noreturn", 200809>];
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let Spellings = [CXX11<"", "noreturn", 200809>,
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C2x<"", "noreturn", 202202>, C2x<"", "_Noreturn", 202202>];
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let Subjects = SubjectList<[Function], ErrorDiag>;
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let Documentation = [CXX11NoReturnDocs];
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let SimpleHandler = 1;
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}
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// Similar to CUDA, OpenCL attributes do not receive a [[]] spelling because

‎clang/include/clang/Basic/DiagnosticSemaKinds.td

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@@ -4027,6 +4027,9 @@ def warn_vector_mode_deprecated : Warning<
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"specifying vector types with the 'mode' attribute is deprecated; "
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"use the 'vector_size' attribute instead">,
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InGroup<DeprecatedAttributes>;
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def warn_deprecated_noreturn_spelling : Warning<
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"the '[[_Noreturn]]' attribute spelling is deprecated in C2x; use "
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"'[[noreturn]]' instead">, InGroup<DeprecatedAttributes>;
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def err_complex_mode_vector_type : Error<
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"type of machine mode does not support base vector types">;
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def err_enum_mode_vector_type : Error<

‎clang/lib/Headers/stdnoreturn.h

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@@ -13,4 +13,13 @@
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#define noreturn _Noreturn
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#define __noreturn_is_defined 1
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#if __STDC_VERSION__ > 201710L && \
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!defined(_CLANG_DISABLE_CRT_DEPRECATION_WARNINGS)
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/* The noreturn macro is deprecated in C2x. */
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#pragma clang deprecated(noreturn)
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/* Including the header file in C2x is also deprecated. */
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#warning "the '<stdnoreturn.h>' header is deprecated in C2x"
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#endif
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#endif /* __STDNORETURN_H */

‎clang/lib/Sema/SemaDeclAttr.cpp

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@@ -2175,6 +2175,16 @@ static void handleNoReturnAttr(Sema &S, Decl *D, const ParsedAttr &Attrs) {
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D->addAttr(::new (S.Context) NoReturnAttr(S.Context, Attrs));
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}
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static void handleStandardNoReturnAttr(Sema &S, Decl *D, const ParsedAttr &A) {
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// The [[_Noreturn]] spelling is deprecated in C2x, so if that was used,
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// issue an appropriate diagnostic.
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if (!S.getLangOpts().CPlusPlus &&
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A.getSemanticSpelling() == CXX11NoReturnAttr::C2x_Noreturn)
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S.Diag(A.getLoc(), diag::warn_deprecated_noreturn_spelling) << A.getRange();
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D->addAttr(::new (S.Context) CXX11NoReturnAttr(S.Context, A));
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}
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static void handleNoCfCheckAttr(Sema &S, Decl *D, const ParsedAttr &Attrs) {
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if (!S.getLangOpts().CFProtectionBranch)
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S.Diag(Attrs.getLoc(), diag::warn_nocf_check_attribute_ignored);
@@ -8432,6 +8442,9 @@ static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D,
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case ParsedAttr::AT_NoReturn:
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handleNoReturnAttr(S, D, AL);
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break;
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case ParsedAttr::AT_CXX11NoReturn:
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handleStandardNoReturnAttr(S, D, AL);
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break;
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case ParsedAttr::AT_AnyX86NoCfCheck:
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handleNoCfCheckAttr(S, D, AL);
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break;

‎clang/test/AST/ast-dump-lambda.cpp

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@@ -305,7 +305,7 @@ template <typename... Ts> void test(Ts... a) {
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// CHECK-NEXT: | | `-Destructor simple irrelevant trivial needs_implicit
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// CHECK-NEXT: | |-CXXMethodDecl {{.*}} <col:20, col:23> col:3{{( imported)?}} operator() 'auto () const' inline
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// CHECK-NEXT: | | |-CompoundStmt {{.*}} <col:22, col:23>
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// CHECK-NEXT: | | `-CXX11NoReturnAttr {{.*}} <col:8>
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// CHECK-NEXT: | | `-CXX11NoReturnAttr {{.*}} <col:8> noreturn
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// CHECK-NEXT: | |-CXXConversionDecl {{.*}} <col:3, col:23> col:3{{( imported)?}} implicit constexpr operator auto (*)() 'auto (*() const noexcept)()' inline
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// CHECK-NEXT: | `-CXXMethodDecl {{.*}} <col:3, col:23> col:3{{( imported)?}} implicit __invoke 'auto ()' static inline
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// CHECK-NEXT: `-CompoundStmt {{.*}} <col:22, col:23>

‎clang/test/Sema/c2x-noreturn.c

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// RUN: %clang_cc1 -verify=all,c2x -std=c2x -fsyntax-only %s
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// RUN: %clang_cc1 -verify=all -std=c17 -fdouble-square-bracket-attributes -fsyntax-only %s
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// RUN: %clang_cc1 -verify=none -Wno-deprecated-attributes -D_CLANG_DISABLE_CRT_DEPRECATION_WARNINGS -std=c2x -fsyntax-only %s
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// RUN: %clang_cc1 -verify=none -Wno-deprecated-attributes -D_CLANG_DISABLE_CRT_DEPRECATION_WARNINGS -std=c17 -fdouble-square-bracket-attributes -fsyntax-only %s
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// none-no-diagnostics
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// Test preprocessor functionality.
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#if !__has_c_attribute(noreturn)
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#error "No noreturn attribute support?"
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#endif
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#if !__has_c_attribute(_Noreturn)
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#error "No _Noreturn attribute support?"
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#endif
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#if __has_c_attribute(noreturn) != __has_c_attribute(_Noreturn) || \
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__has_c_attribute(noreturn) != 202202L
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#error "Wrong value for __has_c_attribute(noreturn)"
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#endif
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// If we're testings with deprecations disabled, we don't care about testing
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// the scenarios that trigger errors because we're only interested in the
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// deprecation warning behaviors.
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#ifndef _CLANG_DISABLE_CRT_DEPRECATION_WARNINGS
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// Test that the attribute accepts no args, applies to the correct subject, etc.
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[[noreturn(12)]] void func4(void); // all-error {{attribute 'noreturn' cannot have an argument list}}
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[[noreturn]] int not_a_func; // all-error {{'noreturn' attribute only applies to functions}}
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void func5(void) [[noreturn]]; // all-error {{'noreturn' attribute cannot be applied to types}}
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#endif
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_Noreturn void func1(void); // ok, using the function specifier
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[[noreturn]] void func2(void);
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// This is deprecated because it's only for compatibility with inclusion of the
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// <stdnoreturn.h> header where the noreturn macro expands to _Noreturn.
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[[_Noreturn]] void func3(void); // all-warning {{the '[[_Noreturn]]' attribute spelling is deprecated in C2x; use '[[noreturn]]' instead}}
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// Test the behavior of including <stdnoreturn.h>
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#include <stdnoreturn.h> // [email protected]:* {{the '<stdnoreturn.h>' header is deprecated in C2x}}
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[[noreturn]] void func6(void); // all-warning {{the '[[_Noreturn]]' attribute spelling is deprecated in C2x; use '[[noreturn]]' instead}} \
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// c2x-warning {{macro 'noreturn' has been marked as deprecated}} \
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// [email protected]:* {{macro marked 'deprecated' here}}
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void func7 [[noreturn]] (void); // all-warning {{the '[[_Noreturn]]' attribute spelling is deprecated in C2x; use '[[noreturn]]' instead}} \
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// c2x-warning {{macro 'noreturn' has been marked as deprecated}} \
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// [email protected]:* {{macro marked 'deprecated' here}}
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noreturn void func8(void); // c2x-warning {{macro 'noreturn' has been marked as deprecated}} \
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// [email protected]:* {{macro marked 'deprecated' here}}
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// Ensure the function specifier form still works
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void noreturn func9(void); // c2x-warning {{macro 'noreturn' has been marked as deprecated}} \
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// [email protected]:* {{macro marked 'deprecated' here}}
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// Test preprocessor functionality after including <stdnoreturn.h>.
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#if !__has_c_attribute(noreturn) // c2x-warning {{macro 'noreturn' has been marked as deprecated}} \
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// [email protected]:* {{macro marked 'deprecated' here}}
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#error "No noreturn attribute support?"
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#endif
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#if !__has_c_attribute(_Noreturn)
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#error "No _Noreturn attribute support?"
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#endif

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