sberg created this revision.
sberg added reviewers: cfe-commits, rsmith.
As discussed in [[ https://llvm.org/bugs/show_bug.cgi?id=24115 | "llvm-nm fails
to build with gcc 5.1's libstdc++," ]] both llvm-nm and LibreOffice fail to
build against GCC 5.1 libstdc++, due to Clang trying (and failing) to
instantiate constexpr function templates referenced from within a decltype.
See [[
https://groups.google.com/a/isocpp.org/forum/#!topic/std-discussion/6udLpHDvShU
| "Implicit instantiation of constexpr function template specialization called
in decltype expression?" ]] for a stripped-down test case and a discussion why
the Clang behaviour is considered wrong.
There is a comment in test/SemaTemplate/constexpr-instantiate.cpp claiming this
behaviour is in line with g++, but I cannot reproduce that neither with some
GCC 5.1.1 nor with a recent GCC trunk build. The comment also says "FIXME:
None of this is required by the C++ standard. The rules in this area are poorly
specified, so this is subject to change.", and from the above build breakage I
think it would be best to change the behaviour to not instantiate constexpr
function templates merely because they are referenced from within a decltype.
(Doing so of course breaks the test corresponding to that comment, so turn that
test around to check the changed behaviour now.)
http://reviews.llvm.org/D13386
Files:
lib/Sema/SemaExpr.cpp
test/SemaTemplate/constexpr-instantiate.cpp
Index: test/SemaTemplate/constexpr-instantiate.cpp
===================================================================
--- test/SemaTemplate/constexpr-instantiate.cpp
+++ test/SemaTemplate/constexpr-instantiate.cpp
@@ -77,37 +77,23 @@
}
namespace Unevaluated {
- // We follow g++ in treating any reference to a constexpr function template
- // specialization as requiring an instantiation, even if it occurs in an
- // unevaluated context.
- //
- // We go slightly further than g++, and also trigger the implicit definition
- // of a defaulted special member in the same circumstances. This seems scary,
- // since a lot of classes have constexpr special members in C++11, but the
- // only observable impact should be the implicit instantiation of constexpr
- // special member templates (defaulted special members should only be
- // generated if they are well-formed, and non-constexpr special members in a
- // base or member cause the class's special member to not be constexpr).
- //
- // FIXME: None of this is required by the C++ standard. The rules in this
- // area are poorly specified, so this is subject to change.
namespace NotConstexpr {
template<typename T> struct S {
S() : n(0) {}
S(const S&) : n(T::error) {}
int n;
};
struct U : S<int> {};
- decltype(U(U())) u; // ok, don't instantiate S<int>::S() because it wasn't
declared constexpr
+ decltype(U(U())) u; // ok, don't instantiate S<int>::S()
}
namespace Constexpr {
template<typename T> struct S {
constexpr S() : n(0) {}
- constexpr S(const S&) : n(T::error) {} // expected-error {{has no
members}}
+ constexpr S(const S&) : n(T::error) {}
int n;
};
- struct U : S<int> {}; // expected-note {{instantiation}}
- decltype(U(U())) u; // expected-note {{here}}
+ struct U : S<int> {};
+ decltype(U(U())) u; // ok, don't instantiate S<int>::S()
}
namespace PR11851_Comment0 {
Index: lib/Sema/SemaExpr.cpp
===================================================================
--- lib/Sema/SemaExpr.cpp
+++ lib/Sema/SemaExpr.cpp
@@ -12414,7 +12414,8 @@
// However, they cannot be referenced if they are deleted, and they are
// deleted whenever the implicit definition of the special member would
// fail.
- if (!Func->isConstexpr() || Func->getBody())
+ if (!Func->isConstexpr() || Func->getBody() ||
+ ExprEvalContexts.back().IsDecltype)
return;
CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(Func);
if (!Func->isImplicitlyInstantiable() && (!MD || MD->isUserProvided()))
Index: test/SemaTemplate/constexpr-instantiate.cpp
===================================================================
--- test/SemaTemplate/constexpr-instantiate.cpp
+++ test/SemaTemplate/constexpr-instantiate.cpp
@@ -77,37 +77,23 @@
}
namespace Unevaluated {
- // We follow g++ in treating any reference to a constexpr function template
- // specialization as requiring an instantiation, even if it occurs in an
- // unevaluated context.
- //
- // We go slightly further than g++, and also trigger the implicit definition
- // of a defaulted special member in the same circumstances. This seems scary,
- // since a lot of classes have constexpr special members in C++11, but the
- // only observable impact should be the implicit instantiation of constexpr
- // special member templates (defaulted special members should only be
- // generated if they are well-formed, and non-constexpr special members in a
- // base or member cause the class's special member to not be constexpr).
- //
- // FIXME: None of this is required by the C++ standard. The rules in this
- // area are poorly specified, so this is subject to change.
namespace NotConstexpr {
template<typename T> struct S {
S() : n(0) {}
S(const S&) : n(T::error) {}
int n;
};
struct U : S<int> {};
- decltype(U(U())) u; // ok, don't instantiate S<int>::S() because it wasn't declared constexpr
+ decltype(U(U())) u; // ok, don't instantiate S<int>::S()
}
namespace Constexpr {
template<typename T> struct S {
constexpr S() : n(0) {}
- constexpr S(const S&) : n(T::error) {} // expected-error {{has no members}}
+ constexpr S(const S&) : n(T::error) {}
int n;
};
- struct U : S<int> {}; // expected-note {{instantiation}}
- decltype(U(U())) u; // expected-note {{here}}
+ struct U : S<int> {};
+ decltype(U(U())) u; // ok, don't instantiate S<int>::S()
}
namespace PR11851_Comment0 {
Index: lib/Sema/SemaExpr.cpp
===================================================================
--- lib/Sema/SemaExpr.cpp
+++ lib/Sema/SemaExpr.cpp
@@ -12414,7 +12414,8 @@
// However, they cannot be referenced if they are deleted, and they are
// deleted whenever the implicit definition of the special member would
// fail.
- if (!Func->isConstexpr() || Func->getBody())
+ if (!Func->isConstexpr() || Func->getBody() ||
+ ExprEvalContexts.back().IsDecltype)
return;
CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(Func);
if (!Func->isImplicitlyInstantiable() && (!MD || MD->isUserProvided()))
_______________________________________________
cfe-commits mailing list
cfe-commits@lists.llvm.org
http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
