[lldb][ClangASTImporter] Don't ASTImport LambdaExpr nodes (#154962)

This patch works around an assertion that we hit in the `LambdaExpr`
constructor when we call it from `ASTNodeImporter::VisitLambdaExpr` (see
#149477). The lambda that we
imported doesn't have the `NumCaptures` field accurately set to the one
on the source decl. This is because in `MinimalImport` mode, we skip
importing of lambda definitions:
https://github.com/llvm/llvm-project/blob/e21b0dd81928a3266df0e3ede008fb7a6676ff95/clang/lib/AST/ASTImporter.cpp#L2499

In practice we have seen this assertion occur in our `import-std-module`
test-suite when libc++ headers decide to use lambdas inside inline
function bodies (the latest failure being caused by
#144602).

To avoid running into this whenever libc++ decides to use lambdas in
headers, this patch skips `ASTImport` of lambdas alltogether. Ideally
this would bubble up to the user or log as an error, but we swallow the
`ASTImportError`s currently. The only way this codepath is hit is when
lambdas are used inside functions in defined in the expression
evaluator, or when importing AST nodes from Clang modules. Both of these
are very niche use-cases (for now), so a workaround seemed appropriate.

Author: Michael137

lldb/source/Plugins/ExpressionParser/Clang/ClangASTImporter.cpp

@@ -1048,6 +1048,16 @@ ClangASTImporter::MapCompleter::~MapCompleter() = default;
 
 llvm::Expected<Decl *>
 ClangASTImporter::ASTImporterDelegate::ImportImpl(Decl *From) {
+  // FIXME: The Minimal import mode of clang::ASTImporter does not correctly
+  // import Lambda definitions. Work around this for now by not importing
+  // lambdas at all. This is most likely encountered when importing decls from
+  // the `std` module (not from debug-info), where lambdas can be defined in
+  // inline function bodies. Those will be imported by LLDB.
+  if (const auto *CXX = llvm::dyn_cast<clang::CXXRecordDecl>(From))
+    if (CXX->isLambda())
+      return llvm::make_error<ASTImportError>(
+          ASTImportError::UnsupportedConstruct);
+
   if (m_std_handler) {
     std::optional<Decl *> D = m_std_handler->Import(From);
     if (D) {

lldb/test/API/lang/cpp/lambdas/TestLambdas.py

@@ -1,4 +1,12 @@
 from lldbsuite.test import lldbinline
 from lldbsuite.test import decorators
 
-lldbinline.MakeInlineTest(__file__, globals())
+lldbinline.MakeInlineTest(
+    __file__,
+    globals(),
+    [
+        decorators.expectedFailureAll(
+            bugnumber="https://github.com/llvm/llvm-project/issues/149477"
+        )
+    ],
+)

lldb/test/Shell/Expr/TestLambdaExprImport.test

@@ -0,0 +1,26 @@
+# Test that we can successfully ASTImport clang::LambdaExpr nodes.
+# Currently this is not supported in MinimalImport mode (which LLDB
+# uses always).
+
+# RUN: split-file %s %t
+# RUN: %clang_host -g -gdwarf %t/main.cpp -o %t.out
+# RUN: %lldb -o "settings set interpreter.stop-command-source-on-error false" \
+# RUN:       -x -b -s %t/commands.input %t.out 2>&1 \
+# RUN:       | FileCheck %s
+
+#--- main.cpp
+
+int main() {
+  __builtin_debugtrap();
+}
+
+#--- commands.input
+
+run
+expression --top-level -- void method(int x) { [x=x] { ; }; }
+target dump typesystem
+
+# CHECK:      expression
+# CHECK:      target dump typesystem
+# CHECK-NOT:  FunctionDecl
+# CHECK-NOT:  LambdaExpr