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Index: test/SemaCXX/libstdcxx_common_type_hack.cpp
===================================================================
--- test/SemaCXX/libstdcxx_common_type_hack.cpp (revision 0)
+++ test/SemaCXX/libstdcxx_common_type_hack.cpp (revision 166455)
@@ -0,0 +1,33 @@
+// RUN: %clang_cc1 -fsyntax-only %s -std=c++11 -verify
+
+// This is a test for an egregious hack in Clang that works around
+// an issue with GCC's <type_traits> implementation. std::common_type
+// relies on pre-standard rules for decltype(), in which it doesn't
+// produce reference types so frequently.
+
+#ifdef BE_THE_HEADER
+
+#pragma GCC system_header
+namespace std {
+ template<typename T> T &&declval();
+
+ template<typename...Ts> struct common_type {};
+ template<typename A, typename B> struct common_type<A, B> {
+ // Under the rules in the standard, this always produces a
+ // reference type.
+ typedef decltype(true ? declval<A>() : declval<B>()) type;
+ };
+}
+
+#else
+
+#define BE_THE_HEADER
+#include "libstdcxx_common_type_hack.cpp"
+
+using T = int;
+using T = std::common_type<int, int>::type;
+
+using U = int; // expected-note {{here}}
+using U = decltype(true ? std::declval<int>() : std::declval<int>()); // expected-error {{different types}}
+
+#endif
Index: lib/Sema/SemaTemplateInstantiateDecl.cpp
===================================================================
--- lib/Sema/SemaTemplateInstantiateDecl.cpp (revision 166454)
+++ lib/Sema/SemaTemplateInstantiateDecl.cpp (revision 166455)
@@ -158,6 +158,22 @@
SemaRef.MarkDeclarationsReferencedInType(D->getLocation(), DI->getType());
}
+ // HACK: g++ has a bug where it gets the value kind of ?: wrong.
+ // libstdc++ relies upon this bug in its implementation of common_type.
+ // If we happen to be processing that implementation, fake up the g++ ?:
+ // semantics. See LWG issue 2141 for more information on the bug.
+ const DecltypeType *DT = DI->getType()->getAs<DecltypeType>();
+ CXXRecordDecl *RD = dyn_cast<CXXRecordDecl>(D->getDeclContext());
+ if (DT && RD && isa<ConditionalOperator>(DT->getUnderlyingExpr()) &&
+ DT->isReferenceType() &&
+ RD->getEnclosingNamespaceContext() == SemaRef.getStdNamespace() &&
+ RD->getIdentifier() && RD->getIdentifier()->isStr("common_type") &&
+ D->getIdentifier() && D->getIdentifier()->isStr("type") &&
+ SemaRef.getSourceManager().isInSystemHeader(D->getLocStart()))
+ // Fold it to the (non-reference) type which g++ would have produced.
+ DI = SemaRef.Context.getTrivialTypeSourceInfo(
+ DI->getType().getNonReferenceType());
+
// Create the new typedef
TypedefNameDecl *Typedef;
if (IsTypeAlias)
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