[clang][ASTImporter] Fix possible crash at import of function template

clang/lib/AST/ASTImporter.cpp

@@ -6755,6 +6755,14 @@ ASTNodeImporter::VisitFunctionTemplateDecl(FunctionTemplateDecl *D) {
                               Params, TemplatedFD))
     return ToFunc;
 
+  // Fail if TemplatedFD is already part of a template.
+  // The template should have been found by structural equivalence check before,
+  // or ToFunc should be already imported.
+  // If not, there is AST incompatibility that can be caused by previous import
+  // errors. (NameConflict is not exact here.)
+  if (TemplatedFD->getDescribedTemplate())
+    return make_error<ASTImportError>(ASTImportError::NameConflict);
+
   TemplatedFD->setDescribedFunctionTemplate(ToFunc);
 
   ToFunc->setAccess(D->getAccess());

clang/test/Analysis/Inputs/ctu-test-import-failure-import.cpp

@@ -0,0 +1,52 @@
+namespace std {
+inline namespace __cxx11 {
+template <typename _CharT, typename = int, typename = _CharT>
+class basic_string;
+}
+template <typename, typename> class basic_istream;
+template <typename> struct __get_first_arg;
+struct allocator_traits {
+  using type = __get_first_arg<int>;
+};
+} // namespace std
+namespace std {
+inline namespace __cxx11 {
+template <typename, typename, typename> class basic_string {
+  allocator_traits _M_allocated_capacity;
+  void _M_assign();
+};
+} // namespace __cxx11
+} // namespace std
+namespace std {
+template <typename _CharT, typename _Alloc> void operator!=(_Alloc, _CharT);
+template <typename _CharT, typename _Traits, typename _Alloc>
+basic_istream<_CharT, _Traits> &getline(basic_istream<_CharT, _Traits> &,
+                                        basic_string<_CharT, _Traits, _Alloc> &,
+                                        _CharT);
+} // namespace std
+namespace std {
+template <typename _CharT, typename _Traits, typename _Alloc>
+void basic_string<_CharT, _Traits, _Alloc>::_M_assign() {
+  this != 0;
+}
+template <typename _CharT, typename _Traits, typename _Alloc>
+basic_istream<_CharT, _Traits> &getline(basic_istream<_CharT, _Traits> &,
+                                        basic_string<_CharT, _Traits, _Alloc> &,
+                                        _CharT) {}
+} // namespace std
+struct CommandLineOptionDefinition {
+  void *OutAddress;
+};
+struct CommandLineCommand {
+  CommandLineOptionDefinition Options;
+};
+namespace CommandLine {
+extern const CommandLineCommand RootCommands[];
+extern const int RootExamples[];
+} // namespace CommandLine
+using utf8 = char;
+using u8string = std::basic_string<utf8>;
+u8string _rct2DataPath;
+CommandLineOptionDefinition StandardOptions{&_rct2DataPath};
+const CommandLineCommand CommandLine::RootCommands[]{StandardOptions};
+const int CommandLine::RootExamples[]{};

clang/test/Analysis/Inputs/ctu-test-import-failure-import.cpp.externalDefMap.ast-dump.txt

@@ -0,0 +1,5 @@
+47:c:@N@std@S@allocator_traits@F@allocator_traits# ctu-test-import-failure-import.cpp.ast
+29:c:@N@CommandLine@RootCommands ctu-test-import-failure-import.cpp.ast
+55:c:@N@std@N@__cxx11@ST>3#T#T#T@basic_string@F@_M_assign# ctu-test-import-failure-import.cpp.ast
+97:c:@S@CommandLineOptionDefinition@F@CommandLineOptionDefinition#&1$@S@CommandLineOptionDefinition# ctu-test-import-failure-import.cpp.ast
+29:c:@N@CommandLine@RootExamples ctu-test-import-failure-import.cpp.ast

clang/test/Analysis/ctu-test-import-failure.cpp

@@ -0,0 +1,34 @@
+// RUN: rm -rf %t && mkdir %t
+// RUN: mkdir -p %t/ctudir
+// RUN: %clang_cc1 -triple x86_64-pc-linux-gnu -std=c++17 \
+// RUN:   -emit-pch -o %t/ctudir/ctu-test-import-failure-import.cpp.ast %S/Inputs/ctu-test-import-failure-import.cpp
+// RUN: cp %S/Inputs/ctu-test-import-failure-import.cpp.externalDefMap.ast-dump.txt %t/ctudir/externalDefMap.txt
+// RUN: %clang_cc1 -triple x86_64-pc-linux-gnu -std=c++17 -analyze \
+// RUN:   -analyzer-checker=core \
+// RUN:   -analyzer-config experimental-enable-naive-ctu-analysis=true \
+// RUN:   -analyzer-config ctu-dir=%t/ctudir \
+// RUN:   -verify %s
+
+// Check that importing this code does not cause crash.
+// Import intentionally fails because mismatch of '__get_first_arg'.
+
+namespace std {
+inline namespace __cxx11 {}
+template <typename _CharT, typename> class basic_istream;
+struct __get_first_arg;
+inline namespace __cxx11 {
+template <typename, typename, typename> class basic_string;
+}
+template <typename _CharT, typename _Traits, typename _Alloc>
+basic_istream<_CharT, _Traits> &getline(basic_istream<_CharT, _Traits> &,
+                                        basic_string<_CharT, _Traits, _Alloc> &,
+                                        _CharT) {}
+} // namespace std
+namespace CommandLine {
+extern const int RootExamples[];
+}
+
+// expected-warning@Inputs/ctu-test-import-failure-import.cpp:14{{incompatible definitions}}
+// expected-warning@Inputs/ctu-test-import-failure-import.cpp:14{{incompatible definitions}}
+// expected-note@Inputs/ctu-test-import-failure-import.cpp:14{{no corresponding field here}}
+// expected-note@Inputs/ctu-test-import-failure-import.cpp:14{{no corresponding field here}}