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[Clang][Sema] Warn when 'exclude_from_explicit_instantiation' attribu…
…te is used on local classes and members thereof (#88777) A local class and its members declared in a function template are instantiated alongside the definition of that template. It therefore does not make sense to apply the `exclude_from_explicit_instantiation` attribute to such declarations, and this patch adds a warning to diagnose these cases (in addition to ignoring the attribute). (The motivation for this patch is to fix a failing test in libc++ for #84050. In particular, line 199 in `include/__memory/uses_allocator_construction.h` in libc++ contains the expression `this->__value_` (reduced to https://godbolt.org/z/KqEerKWPd) which will be looked up prior to instantiation once #84050 lands (the lookup context is the current instantiation). `_LIBCPP_HIDE_FROM_ABI` includes `__attribute__((exclude_from_explicit_instantiation))`, which in the reduced example results in `Local` being instantiated with `Local::operator A` as its `DeclContext`)
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clang/test/SemaCXX/attr-exclude_from_explicit_instantiation.local-class.cpp
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// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s | ||
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// Test that the exclude_from_explicit_instantiation attribute is ignored | ||
// for local classes and members thereof. | ||
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#define EXCLUDE_FROM_EXPLICIT_INSTANTIATION __attribute__((exclude_from_explicit_instantiation)) // expected-note 0+{{expanded from macro}} | ||
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namespace N0 { | ||
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template<typename T> | ||
void f() { | ||
struct EXCLUDE_FROM_EXPLICIT_INSTANTIATION A { // expected-warning {{attribute ignored on local class}} | ||
// expected-note@-1 2{{in instantiation of}} | ||
EXCLUDE_FROM_EXPLICIT_INSTANTIATION void g(T t) { // expected-warning {{attribute ignored on local class member}} | ||
*t; // expected-error {{indirection requires pointer operand ('int' invalid)}} | ||
} | ||
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struct EXCLUDE_FROM_EXPLICIT_INSTANTIATION B { // expected-warning {{attribute ignored on local class}} | ||
void h(T t) { | ||
*t; // expected-error {{indirection requires pointer operand ('int' invalid)}} | ||
} | ||
}; | ||
}; | ||
} | ||
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template void f<int>(); // expected-note 2{{in instantiation of}} | ||
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} | ||
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// This is a reduced example from libc++ which required that 'value' | ||
// be prefixed with 'this->' because the definition of 'Local::operator A' | ||
// was not instantiated when the definition of 'g' was. | ||
namespace N1 { | ||
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struct A { }; | ||
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struct B { | ||
operator A() { | ||
return A(); | ||
} | ||
}; | ||
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template<typename T> | ||
auto f(T t) { | ||
return A(t); | ||
} | ||
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template<typename T> | ||
auto g(T t) { | ||
struct Local { | ||
T value; | ||
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EXCLUDE_FROM_EXPLICIT_INSTANTIATION // expected-warning {{attribute ignored on local class member}} | ||
operator A() { | ||
return A(value); | ||
} | ||
}; | ||
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return f(Local(t)); | ||
} | ||
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auto x = g(B()); | ||
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} |