Migrate weak-external.cpp from llvm/test/FrontendC++.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@137763 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGenCXX/weak-external.cpp

+// RUN: %clang -fexceptions %s -S -emit-llvm -o - | FileCheck %s
+// PR4262
+
+// CHECK-NOT: _ZNSs12_S_constructIPKcEEPcT_S3_RKSaIcESt20forward_iterator_tag
+
+// The "basic_string" extern template instantiation declaration is supposed to
+// suppress the implicit instantiation of non-inline member functions. Make sure
+// that we suppress the implicit instantiation of non-inline member functions
+// defined out-of-line. That we aren't instantiating the basic_string
+// constructor when we shouldn't be. Such an instantiation forces the implicit
+// instantiation of _S_construct<const char*>. Since _S_construct is a member
+// template, it's instantiation is *not* suppressed (despite being in
+// basic_string<char>), so we would emit it as a weak definition.
+
+#include <stdexcept>
+
+void dummysymbol() {
+  throw(std::runtime_error("string"));
+}