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[clang] Reject 'auto' storage class with type specifier in C++
Previously, clang allowed 'auto int x = 1;' in C++ as an extension (for C compatibility), emitting only a warning. This was confusing since 'auto' in C++11+ is a type specifier, not a storage class. This patch: - Adds a new error diagnostic 'err_auto_type_specifier' - Updates the parser to emit an error (instead of warning) when 'auto' is used as a storage class with a type specifier in C++ mode - Preserves C23 behavior where 'auto int' is valid - Adds comprehensive tests Fixes #164273 Signed-off-by: Osama Abdelkader <osama.abdelkader@gmail.com>
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‎clang/include/clang/Basic/DiagnosticParseKinds.td‎

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Original file line numberDiff line numberDiff line change
@@ -399,6 +399,8 @@ def err_requires_clause_on_declarator_not_declaring_a_function : Error<
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"trailing requires clause can only be used when declaring a function">;
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def err_requires_clause_inside_parens : Error<
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"trailing requires clause should be placed outside parentheses">;
402+
def err_auto_type_specifier : Error<
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"'auto' cannot be combined with a type specifier in C++">;
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def ext_auto_storage_class : ExtWarn<
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"'auto' storage class specifier is not permitted in C++11, and will not "
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"be supported in future releases">, InGroup<DiagGroup<"auto-storage-class">>;

‎clang/lib/Parse/ParseDecl.cpp‎

Lines changed: 256 additions & 236 deletions
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‎clang/test/CXX/dcl.dcl/dcl.spec/dcl.stc/p2.cpp‎

Lines changed: 23 additions & 13 deletions
Original file line numberDiff line numberDiff line change
@@ -5,14 +5,18 @@
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// The auto or register specifiers can be applied only to names of objects
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// declared in a block (6.3) or to function parameters (8.4).
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8-
auto int ao; // expected-error {{illegal storage class on file-scoped variable}}
8+
auto int ao;
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#if __cplusplus >= 201103L // C++11 or later
10-
// expected-warning@-2 {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
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// expected-error@-2 {{'auto' cannot be combined with a type specifier in C++}}
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#else
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// expected-error@-4 {{illegal storage class on file-scoped variable}}
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#endif
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13-
auto void af(); // expected-error {{illegal storage class on function}}
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auto void af();
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#if __cplusplus >= 201103L // C++11 or later
15-
// expected-warning@-2 {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
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// expected-error@-2 {{'auto' cannot be combined with a type specifier in C++}}
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#else
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// expected-error@-4 {{illegal storage class on function}}
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#endif
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register int ro; // expected-error {{illegal storage class on file-scoped variable}}
@@ -25,13 +29,17 @@ register int ro; // expected-error {{illegal storage class on file-scoped variab
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register void rf(); // expected-error {{illegal storage class on function}}
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struct S {
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auto int ao; // expected-error {{storage class specified for a member declaration}}
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auto int ao;
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#if __cplusplus >= 201103L // C++11 or later
30-
// expected-warning@-2 {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
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// expected-error@-2 {{'auto' cannot be combined with a type specifier in C++}}
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#else
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// expected-error@-4 {{storage class specified for a member declaration}}
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#endif
32-
auto void af(); // expected-error {{storage class specified for a member declaration}}
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auto void af();
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#if __cplusplus >= 201103L // C++11 or later
34-
// expected-warning@-2 {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
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// expected-error@-2 {{'auto' cannot be combined with a type specifier in C++}}
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#else
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// expected-error@-4 {{storage class specified for a member declaration}}
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#endif
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register int ro; // expected-error {{storage class specified for a member declaration}}
@@ -40,19 +48,21 @@ struct S {
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void foo(auto int ap, register int rp) {
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#if __cplusplus >= 201703L
43-
// expected-warning@-2 {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
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// expected-error@-2 {{'auto' cannot be combined with a type specifier in C++}}
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// expected-error@-3 {{ISO C++17 does not allow 'register' storage class specifier}}
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#elif __cplusplus >= 201103L
46-
// expected-warning@-5 {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
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// expected-error@-5 {{'auto' cannot be combined with a type specifier in C++}}
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// expected-warning@-6 {{'register' storage class specifier is deprecated}}
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#endif
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auto int abo;
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#if __cplusplus >= 201103L // C++11 or later
51-
// expected-warning@-2 {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
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// expected-error@-2 {{'auto' cannot be combined with a type specifier in C++}}
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#endif
53-
auto void abf(); // expected-error {{illegal storage class on function}}
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auto void abf();
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#if __cplusplus >= 201103L // C++11 or later
55-
// expected-warning@-2 {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
63+
// expected-error@-2 {{'auto' cannot be combined with a type specifier in C++}}
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#else
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// expected-error@-4 {{illegal storage class on function}}
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#endif
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register int rbo;

‎clang/test/CXX/dcl.dcl/dcl.spec/dcl.type/dcl.spec.auto/p3-1y.cpp‎

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -56,7 +56,7 @@ namespace p3_example {
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auto x = 5;
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const auto *v = &x, u = 6;
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static auto y = 0.0;
59-
auto int r; // expected-warning {{storage class}} expected-error {{file-scope}}
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auto int r; // expected-error {{'auto' cannot be combined with a type specifier in C++}}
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static_assert(is_same<decltype(x), int>(), "");
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static_assert(is_same<decltype(v), const int*>(), "");

‎clang/test/CXX/dcl.dcl/dcl.spec/dcl.type/dcl.spec.auto/p3.cpp‎

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@@ -42,7 +42,12 @@ void p3example() {
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static auto y = 0.0;
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// In C++98: 'auto' storage class specifier is redundant and incompatible with C++0x
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// In C++0x: 'auto' storage class specifier is not permitted in C++0x, and will not be supported in future releases
45-
auto int r; // expected-warning {{'auto' storage class specifier}}
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auto int r;
46+
#if __cplusplus >= 201103L
47+
// expected-error@-2 {{'auto' cannot be combined with a type specifier in C++}}
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#else
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// expected-warning@-4 {{'auto' storage class specifier}}
50+
#endif
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4752
same<__typeof(x), int> xHasTypeInt;
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same<__typeof(v), const int*> vHasTypeConstIntPtr;

‎clang/test/CXX/drs/cwg3xx.cpp‎

Lines changed: 8 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -1732,11 +1732,16 @@ namespace cwg395 { // cwg395: 3.0
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namespace cwg396 { // cwg396: 3.0
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void f() {
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auto int a();
1735-
// since-cxx11-error@-1 {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
1736-
// expected-error@-2 {{illegal storage class on function}}
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// since-cxx11-error@-1 {{'auto' cannot be combined with a type specifier in C++}}
1736+
#if __cplusplus < 201103L
1737+
// expected-error@-3 {{illegal storage class on function}}
1738+
#else
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// expected-warning@-5 {{empty parentheses interpreted as a function declaration}}
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// expected-note@-6 {{replace parentheses with an initializer to declare a variable}}
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#endif
17371742
int (i); // #cwg396-i
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auto int (i);
1739-
// since-cxx11-error@-1 {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
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// since-cxx11-error@-1 {{'auto' cannot be combined with a type specifier in C++}}
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// expected-error@-2 {{redefinition of 'i'}}
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// expected-note@#cwg396-i {{previous definition is here}}
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}

‎clang/test/Parser/cxx-auto-type-specifier.cpp‎

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@@ -0,0 +1,22 @@
1+
// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 %s
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// RUN: %clang_cc1 -fsyntax-only -verify -std=c++14 %s
3+
// RUN: %clang_cc1 -fsyntax-only -verify -std=c++17 %s
4+
// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
5+
// RUN: %clang_cc1 -fsyntax-only -verify -std=c++23 %s
6+
7+
// Test that 'auto' cannot be combined with a type specifier in C++.
8+
void f() {
9+
auto int x = 1; // expected-error {{'auto' cannot be combined with a type specifier in C++}}
10+
auto char c = 'a'; // expected-error {{'auto' cannot be combined with a type specifier in C++}}
11+
auto float f = 1.0f; // expected-error {{'auto' cannot be combined with a type specifier in C++}}
12+
auto double d = 1.0; // expected-error {{'auto' cannot be combined with a type specifier in C++}}
13+
auto long l = 1L; // expected-error {{'auto' cannot be combined with a type specifier in C++}}
14+
}
15+
16+
// Test that regular 'auto' (type deduction) still works in C++.
17+
void h() {
18+
auto x = 1;
19+
auto y = 2.0;
20+
auto z = 'c';
21+
}
22+

‎clang/test/SemaCXX/auto-cxx0x.cpp‎

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1,7 +1,7 @@
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// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++11
22
// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++1y
33
void f() {
4-
auto int a; // expected-warning {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
4+
auto int a; // expected-error {{'auto' cannot be combined with a type specifier in C++}}
55
int auto b; // expected-error{{cannot combine with previous 'int' declaration specifier}}
66
}
77

‎clang/test/SemaCXX/class.cpp‎

Lines changed: 4 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -2,11 +2,12 @@
22
// RUN: %clang_cc1 -fsyntax-only -verify=expected,cxx98 -Wc++11-compat %s -std=c++98
33
class C {
44
public:
5-
auto int errx; // expected-error {{storage class specified for a member declaration}}
5+
auto int errx;
66
#if __cplusplus <= 199711L
7-
// expected-warning@-2 {{'auto' storage class specifier is redundant}}
7+
// expected-error@-2 {{storage class specified for a member declaration}}
8+
// expected-warning@-3 {{'auto' storage class specifier is redundant and incompatible with C++11}}
89
#else
9-
// expected-warning@-4 {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
10+
// expected-error@-5 {{'auto' cannot be combined with a type specifier in C++}}
1011
#endif
1112
register int erry; // expected-error {{storage class specified for a member declaration}}
1213
extern int errz; // expected-error {{storage class specified for a member declaration}}

‎clang/test/SemaCXX/static-data-member.cpp‎

Lines changed: 6 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -13,7 +13,12 @@ double ABC::a = 1.0;
1313
extern double ABC::b = 1.0; // expected-error {{static data member definition cannot specify a storage class}}
1414
static double ABC::c = 1.0; // expected-error {{'static' can only be specified inside the class definition}}
1515
__private_extern__ double ABC::d = 1.0; // expected-error {{static data member definition cannot specify a storage class}}
16-
auto double ABC::e = 1.0; // expected-error {{static data member definition cannot specify a storage class}}
16+
auto double ABC::e = 1.0;
17+
#if __cplusplus >= 201103L
18+
// expected-error@-2 {{'auto' cannot be combined with a type specifier in C++}}
19+
#else
20+
// expected-error@-4 {{static data member definition cannot specify a storage class}}
21+
#endif
1722
#if __cplusplus < 201703L
1823
register double ABC::f = 1.0; // expected-error {{static data member definition cannot specify a storage class}}
1924
#endif

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