[clang][ASTImporter] Fix possible crash at import of function template (llvm#124273)

During import of a function template at specific conditions an assertion
"TemplateOrSpecialization.isNull()" can be triggered. This can
happen when the new AST is already incompatible after import failures.
Problem is fixed by returning import failure at the assert condition.

Author: balazske

clang/lib/AST/ASTImporter.cpp

@@ -6759,6 +6759,14 @@ 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}}