84a3aadf0f
Reapplication of 7339c0f782d5c70e0928f8991b0c05338a90c84c with a fix for a crash involving arrays without a size expression. Clang supports VLAs in C++ as an extension, but we currently only warn on their use when you pass -Wvla, -Wvla-extension, or -pedantic. However, VLAs as they're expressed in C have been considered by WG21 and rejected, are easy to use accidentally to the surprise of users (e.g., https://ddanilov.me/default-non-standard-features/), and they have potential security implications beyond constant-size arrays (https://wiki.sei.cmu.edu/confluence/display/c/ARR32-C.+Ensure+size+arguments+for+variable+length+arrays+are+in+a+valid+range). C++ users should strongly consider using other functionality such as std::vector instead. This seems like sufficiently compelling evidence to warn users about VLA use by default in C++ modes. This patch enables the -Wvla-extension diagnostic group in C++ language modes by default, and adds the warning group to -Wall in GNU++ language modes. The warning is still opt-in in C language modes, where support for VLAs is somewhat less surprising to users. RFC: https://discourse.llvm.org/t/rfc-diagnosing-use-of-vlas-in-c/73109 Fixes https://github.com/llvm/llvm-project/issues/62836 Differential Revision: https://reviews.llvm.org/D156565
29 lines
1.5 KiB
C++
29 lines
1.5 KiB
C++
// RUN: %clang_cc1 %s -Wno-uninitialized -std=c++1z -fsyntax-only -verify
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const extern int arr[];
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constexpr auto p = arr; // ok
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constexpr int f(int i) {return p[i];} // expected-note {{read of dereferenced one-past-the-end pointer}}
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constexpr int arr[] {1, 2, 3};
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constexpr auto p2 = arr + 2; // ok
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constexpr int x = f(2); // ok
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constexpr int y = f(3); // expected-error {{constant expression}}
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// expected-note-re@-1 {{in call to 'f({{.*}})'}}
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// FIXME: consider permitting this case
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struct A {int m[];} a;
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constexpr auto p3 = a.m; // expected-error {{constant expression}} expected-note {{without known bound}}
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constexpr auto p4 = a.m + 1; // expected-error {{constant expression}} expected-note {{without known bound}}
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void g(int i) { // expected-note 2{{declared here}}
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int arr[i]; // expected-warning {{variable length arrays in C++ are a Clang extension}} \
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expected-note {{function parameter 'i' with unknown value cannot be used in a constant expression}}
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constexpr auto *p = arr + 2; // expected-error {{constant expression}} expected-note {{without known bound}}
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// FIXME: Give a better diagnostic here. The issue is that computing
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// sizeof(*arr2) within the array indexing fails due to the VLA.
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int arr2[2][i]; // expected-warning 2{{variable length arrays in C++ are a Clang extension}} \
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expected-note {{function parameter 'i' with unknown value cannot be used in a constant expression}}
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constexpr int m = ((void)arr2[2], 0); // expected-error {{constant expression}}
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}
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