Index: clang/include/clang/AST/Decl.h
===================================================================
--- clang/include/clang/AST/Decl.h
+++ clang/include/clang/AST/Decl.h
@@ -614,7 +614,12 @@
     if (!isInline())
       return false;
     auto X = lookup(Name);
-    auto Y = getParent()->lookup(Name);
+    // We should not perform a lookup within a transparent context, so walk
+    // up the context chain until we find a non-transparent context.
+    const DeclContext *Parent = getParent();
+    while (Parent->isTransparentContext())
+      Parent = Parent->getParent();
+    auto Y = Parent->lookup(Name);
     return std::distance(X.begin(), X.end()) ==
       std::distance(Y.begin(), Y.end());
   }
Index: clang/test/Misc/diag-inline-namespace.cpp
===================================================================
--- clang/test/Misc/diag-inline-namespace.cpp
+++ clang/test/Misc/diag-inline-namespace.cpp
@@ -48,3 +48,14 @@
   T<struct A::B::C::i> t4; // expected-error {{implicit instantiation of undefined template 'N::T<N::A::i>'}}
   T<struct A::B::C::j> t5; // expected-error {{implicit instantiation of undefined template 'N::T<N::B::C::j>'}}
 }
+
+namespace dont_crash {
+// A malformed lookup involving inline namespaces in a linkage specification
+// would previous cause an assertion due to the way diagnostics are emitted.
+extern "C++" inline namespace {
+namespace a {
+  a : b // expected-error {{unexpected ':' in nested name specifier; did you mean '::'?}} \
+        // expected-error {{no type named 'b' in namespace 'dont_crash::a'}}
+} // expected-error {{expected unqualified-id}}
+} // inline namespace
+} // dont_crash