dlj created this revision.
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dlj added subscribers: cfe-commits, EricWF.
Per https://wg21.link/CWG1677, the C++11 standard did not clarify that constant
initialization of an object allowed constexpr brace-or-equal initialization of
subobjects:
struct foo_t { union { int i; volatile int j; } u; };
__attribute__((__require_constant_initialization__))
static const foo_t x = {{0}};
Because foo_t::u has a volatile member, the initializer for x fails. However,
there is really no good reason, because this:
union foo_u { int i; volatile int j; };
__attribute__((__require_constant_initialization__))
static const foo_u x = {0};
does have a constant initializer.
(This was triggered by musl's pthread_mutex_t type when building under C++11.)
https://reviews.llvm.org/D28427
Files:
lib/AST/ExprConstant.cpp
Index: lib/AST/ExprConstant.cpp
===================================================================
--- lib/AST/ExprConstant.cpp
+++ lib/AST/ExprConstant.cpp
@@ -1627,8 +1627,17 @@
// C++1y: A constant initializer for an object o [...] may also invoke
// constexpr constructors for o and its subobjects even if those objects
// are of non-literal class types.
- if (Info.getLangOpts().CPlusPlus14 && This &&
- Info.EvaluatingDecl == This->getLValueBase())
+ //
+ // C++11 missed this detail for aggregates, so classes like this:
+ // struct foo_t { union { int i; volatile int j; } u; };
+ // are not (obviously) initializable like so:
+ // __attribute__((__require_constant_initialization__))
+ // static const foo_t x = {{0}};
+ // because "i" is a subobject with non-literal initialization (due to the
+ // volatile member of the union). See:
+ // http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1677
+ // Therefore, we use the C++1y behavior.
+ if (This && Info.EvaluatingDecl == This->getLValueBase())
return true;
// Prvalue constant expressions must be of literal types.
Index: lib/AST/ExprConstant.cpp
===================================================================
--- lib/AST/ExprConstant.cpp
+++ lib/AST/ExprConstant.cpp
@@ -1627,8 +1627,17 @@
// C++1y: A constant initializer for an object o [...] may also invoke
// constexpr constructors for o and its subobjects even if those objects
// are of non-literal class types.
- if (Info.getLangOpts().CPlusPlus14 && This &&
- Info.EvaluatingDecl == This->getLValueBase())
+ //
+ // C++11 missed this detail for aggregates, so classes like this:
+ // struct foo_t { union { int i; volatile int j; } u; };
+ // are not (obviously) initializable like so:
+ // __attribute__((__require_constant_initialization__))
+ // static const foo_t x = {{0}};
+ // because "i" is a subobject with non-literal initialization (due to the
+ // volatile member of the union). See:
+ // http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1677
+ // Therefore, we use the C++1y behavior.
+ if (This && Info.EvaluatingDecl == This->getLValueBase())
return true;
// Prvalue constant expressions must be of literal types.
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