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[Clang][Sema] Improve support for explicit specializations of constra…
…ined member functions & member function templates (#88963) Consider the following snippet from the discussion of CWG2847 on the core reflector: ``` template<typename T> concept C = sizeof(T) <= sizeof(long); template<typename T> struct A { template<typename U> void f(U) requires C<U>; // #1, declares a function template void g() requires C<T>; // #2, declares a function template<> void f(char); // #3, an explicit specialization of a function template that declares a function }; template<> template<typename U> void A<short>::f(U) requires C<U>; // #4, an explicit specialization of a function template that declares a function template template<> template<> void A<int>::f(int); // #5, an explicit specialization of a function template that declares a function template<> void A<long>::g(); // #6, an explicit specialization of a function that declares a function ``` A number of problems exist: - Clang rejects `#4` because the trailing _requires-clause_ has `U` substituted with the wrong template parameter depth when `Sema::AreConstraintExpressionsEqual` is called to determine whether it matches the trailing _requires-clause_ of the implicitly instantiated function template. - Clang rejects `#5` because the function template specialization instantiated from `A<int>::f` has a trailing _requires-clause_, but `#5` does not (nor can it have one as it isn't a templated function). - Clang rejects `#6` for the same reasons it rejects `#5`. This patch resolves these issues by making the following changes: - To fix `#4`, `Sema::AreConstraintExpressionsEqual` is passed `FunctionTemplateDecl`s when comparing the trailing _requires-clauses_ of `#4` and the function template instantiated from `#1`. - To fix `#5` and `#6`, the trailing _requires-clauses_ are not compared for explicit specializations that declare functions. In addition to these changes, `CheckMemberSpecialization` now considers constraint satisfaction/constraint partial ordering when determining which member function is specialized by an explicit specialization of a member function for an implicit instantiation of a class template (we previously would select the first function that has the same type as the explicit specialization). With constraints taken under consideration, we match EDG's behavior for these declarations.
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Original file line number | Diff line number | Diff line change |
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// RUN: %clang_cc1 -std=c++20 -verify %s | ||
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template<int I> | ||
concept C = I >= 4; | ||
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template<int I> | ||
concept D = I < 8; | ||
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template<int I> | ||
struct A { | ||
constexpr static int f() { return 0; } | ||
constexpr static int f() requires C<I> && D<I> { return 1; } | ||
constexpr static int f() requires C<I> { return 2; } | ||
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constexpr static int g() requires C<I> { return 0; } // #candidate-0 | ||
constexpr static int g() requires D<I> { return 1; } // #candidate-1 | ||
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constexpr static int h() requires C<I> { return 0; } // expected-note {{member declaration nearly matches}} | ||
}; | ||
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template<> | ||
constexpr int A<2>::f() { return 3; } | ||
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template<> | ||
constexpr int A<4>::f() { return 4; } | ||
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template<> | ||
constexpr int A<8>::f() { return 5; } | ||
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static_assert(A<3>::f() == 0); | ||
static_assert(A<5>::f() == 1); | ||
static_assert(A<9>::f() == 2); | ||
static_assert(A<2>::f() == 3); | ||
static_assert(A<4>::f() == 4); | ||
static_assert(A<8>::f() == 5); | ||
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template<> | ||
constexpr int A<0>::g() { return 2; } | ||
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template<> | ||
constexpr int A<8>::g() { return 3; } | ||
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template<> | ||
constexpr int A<6>::g() { return 4; } // expected-error {{ambiguous member function specialization 'A<6>::g' of 'A::g'}} | ||
// expected-note@#candidate-0 {{member function specialization matches 'g'}} | ||
// expected-note@#candidate-1 {{member function specialization matches 'g'}} | ||
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static_assert(A<9>::g() == 0); | ||
static_assert(A<1>::g() == 1); | ||
static_assert(A<0>::g() == 2); | ||
static_assert(A<8>::g() == 3); | ||
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template<> | ||
constexpr int A<4>::h() { return 1; } | ||
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template<> | ||
constexpr int A<0>::h() { return 2; } // expected-error {{out-of-line definition of 'h' does not match any declaration in 'A<0>'}} | ||
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static_assert(A<5>::h() == 0); | ||
static_assert(A<4>::h() == 1); |
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