[Clang] Don't consider default constructors ineligible if the more constrained constructor is a template

[royjacobson]

Summary

Partially solves https://github.com/llvm/llvm-project/issues/59206:

We now mark trivial constructors as eligible even if there's a more constrained templated default constructor. Although technically non-conformant, this solves problems with pretty reasonable uses cases like

template<int n>
struct Foo {
	constexpr Foo() = default;

	template<class... Ts>
	Foo(Ts... vals) requires(sizeof...(Ts) == n) {}
};

where we currently consider the default constructor to be ineligible and therefor inheriting/containing classes have non trivial constructors. This is aligned with GCC: https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=c75ebe76ae12ac4020f20a24f34606a594a40d15

This doesn't change __is_trivial. Although we're technically standard conformant in this regard, GCC/MSVC exhibit different behaviors that seem to make more sense. An issue has been filed to CWG and we await their response.

[royjacobson]

Update so this doesn't change type triviality.

clang/lib/AST/DeclCXX.cpp

 }
 
 void CXXRecordDecl::addedSelectedDestructor(CXXDestructorDecl *DD) {
   DD->setIneligibleOrNotSelected(false);
   addedEligibleSpecialMemberFunction(DD, SMF_Destructor);
 }
 
 void CXXRecordDecl::addedEligibleSpecialMemberFunction(const CXXMethodDecl *MD,
                                                        unsigned SMKind) {
+  // FIXME: We shouldn't change DeclaredNonTrivialSpecialMembers if `MD` is
+  // a function template, but this needs CWG attention before we break ABI.
+  // See https://github.com/llvm/llvm-project/issues/59206
+
   if (const auto *DD = dyn_cast<CXXDestructorDecl>(MD)) {
     if (DD->isUserProvided())
       data().HasIrrelevantDestructor = false;
     // If the destructor is explicitly defaulted and not trivial or not public
     // or if the destructor is deleted, we clear HasIrrelevantDestructor in
     // finishedDefaultedOrDeletedMember.
 
     // C++11 [class.dtor]p5:

clang/lib/Sema/SemaDecl.cpp

 /// - they are both copy or move constructors with the same first parameter
 ///   type, or
 /// - they are both copy or move assignment operators with the same first
 ///   parameter type and the same cv-qualifiers and ref-qualifier, if any.
 static bool AreSpecialMemberFunctionsSameKind(ASTContext &Context,
                                               CXXMethodDecl *M1,
                                               CXXMethodDecl *M2,
                                               Sema::CXXSpecialMember CSM) {
+  // We don't want to compare templates to non-templates: See
+  // https://github.com/llvm/llvm-project/issues/59206
   if (CSM == Sema::CXXDefaultConstructor)
-    return true;
+    return bool(M1->getDescribedFunctionTemplate()) ==
+           bool(M2->getDescribedFunctionTemplate());
   if (!Context.hasSameType(M1->getParamDecl(0)->getType(),
                            M2->getParamDecl(0)->getType()))
     return false;
   if (!Context.hasSameType(M1->getThisType(), M2->getThisType()))
     return false;
 
   return true;
 }

clang/test/SemaCXX/constrained-special-member-functions.cpp

struct S {
   consteval S() requires (N==2) = default; // expected-note {{non-constexpr constructor 'Foo' cannot be used in a constant expression}}
 };
 
 void func() {
   S<2, 1> s1; // expected-error {{is not a constant expression}} expected-note {{in call to 'S()'}}
   S<2, 2> s2;
 }
 }
+
+namespace GH59206 {
+
+struct A {
+  A() = default; //eligible, second constructor unsatisfied
+  template<class... Args>
+  A(Args&&... args) requires (sizeof...(Args) > 0) {}
+};
+
+struct B {
+  B() = default; //ineligible, second constructor more constrained
+  template<class... Args>
+  B(Args&&... args) requires (sizeof...(Args) == 0) {}
+};
+
+struct C {
+  C() = default; //eligible, but
+  template<class... Args> //also eligible and non-trivial
+  C(Args&&... args) {}
+};
+
+struct D : B {};
+
+static_assert(__is_trivially_copyable(A), "");
+static_assert(__is_trivially_copyable(B), "");
+static_assert(__is_trivially_copyable(C), "");
+static_assert(__is_trivially_copyable(D), "");
+
+// FIXME: Update when https://github.com/llvm/llvm-project/issues/59206 is
+// resolved.
+static_assert(!__is_trivial(A), "");
+static_assert(!__is_trivial(B), "");
+static_assert(!__is_trivial(C), "");
+static_assert(__is_trivial(D), "");
+static_assert(__is_trivially_constructible(A), "");
+static_assert(__is_trivially_constructible(B), "");
+static_assert(__is_trivially_constructible(C), "");
+static_assert(__is_trivially_constructible(D), "");
+
+}