[lldb][ClangExpressionParser] Implement ExternalSemaSource::ReadUndefinedButUsed (#104817)

While parsing an expression, Clang tries to diagnose usage of decls
(with possibly non-external linkage) for which it hasn't been provided
with a definition. This is the case, e.g., for functions with parameters
that live in an anonymous namespace (those will have `UniqueExternal`
linkage, this is computed [here in
computeTypeLinkageInfo](https://github.com/llvm/llvm-project/blob/ea8bb4d633683f5cbfd82491620be3056f347a02/clang/lib/AST/Type.cpp#L4647-L4653)).
Before diagnosing such situations, Clang calls
`ExternalSemaSource::ReadUndefinedButUsed`. The intended use of this API
is to extend the set of "used but not defined" decls with additional
ones that the external source knows about. However, in LLDB's case, we
never provide `FunctionDecl`s with a definition, and instead rely on the
expression parser to resolve those symbols by linkage name. Thus, to
avoid the Clang parser from erroring out in these situations, this patch
implements `ReadUndefinedButUsed` which just removes the "undefined"
non-external `FunctionDecl`s that Clang found.

We also had to add an `ExternalSemaSource` to the `clang::Sema` instance
LLDB creates. We previously didn't have any source on `Sema`. Because we
add the `ExternalASTSourceWrapper` here, that means we'd also
technically be adding the `ClangExpressionDeclMap` as an
`ExternalASTSource` to `Sema`, which is fine because `Sema` will only be
calling into the `ExternalSemaSource` APIs (though nothing currently
strictly enforces this, which is a bit worrying).

Note, the decision for whether to put a function into `UndefinedButUsed`
is done in
[Sema::MarkFunctionReferenced](https://github.com/llvm/llvm-project/blob/ea8bb4d633683f5cbfd82491620be3056f347a02/clang/lib/Sema/SemaExpr.cpp#L18083-L18087).
The `UniqueExternal` linkage computation is done in
[getLVForNamespaceScopeDecl](https://github.com/llvm/llvm-project/blob/ea8bb4d633683f5cbfd82491620be3056f347a02/clang/lib/AST/Decl.cpp#L821-L833).

Fixes https://github.com/llvm/llvm-project/issues/104712

2 files changed, 22 insertions, 0 deletions

diff --git a/lldb/source/Plugins/ExpressionParser/Clang/ASTUtils.h b/lldb/source/Plugins/ExpressionParser/Clang/ASTUtils.h
index 44e52e8..5669c31 100644
--- a/lldb/source/Plugins/ExpressionParser/Clang/ASTUtils.h
+++ b/lldb/source/Plugins/ExpressionParser/Clang/ASTUtils.h
@@ -15,6 +15,7 @@
 #include "clang/Sema/Sema.h"
 #include "clang/Sema/SemaConsumer.h"
 #include "llvm/ADT/IntrusiveRefCntPtr.h"
+#include "llvm/Support/Casting.h"
 #include <optional>
 
 namespace clang {
@@ -138,6 +139,24 @@ public:
     return m_Source->layoutRecordType(Record, Size, Alignment, FieldOffsets,
                                       BaseOffsets, VirtualBaseOffsets);
   }
+
+  /// This gets called when Sema is reconciling undefined but used decls.
+  /// For LLDB's use-case, we never provide Clang with function definitions,
+  /// instead we rely on linkage names and symbol resolution to call the
+  /// correct funcitons during JITting. So this implementation clears
+  /// any "undefined" FunctionDecls that Clang found while parsing.
+  ///
+  /// \param[in,out] Undefined A set of used decls for which Clang has not
+  ///                          been provided a definition with.
+  ///
+  void ReadUndefinedButUsed(
+      llvm::MapVector<clang::NamedDecl *, clang::SourceLocation> &Undefined)
+      override {
+    Undefined.remove_if([](auto const &decl_loc_pair) {
+      const clang::NamedDecl *ND = decl_loc_pair.first;
+      return llvm::isa_and_present<clang::FunctionDecl>(ND);
+    });
+  }
 };
 
 /// Wraps an ASTConsumer into an SemaConsumer. Doesn't take ownership of the
diff --git a/lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp b/lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp
index d37787e..38fe9fa 100644
--- a/lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp
+++ b/lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp
@@ -1223,6 +1223,8 @@ ClangExpressionParser::ParseInternal(DiagnosticManager &diagnostic_manager,
 
     clang::ExternalASTSource *ast_source = decl_map->CreateProxy();
 
+    auto *ast_source_wrapper = new ExternalASTSourceWrapper(ast_source);
+
     if (ast_context.getExternalSource()) {
       auto *module_wrapper =
           new ExternalASTSourceWrapper(ast_context.getExternalSource());
@@ -1236,6 +1238,7 @@ ClangExpressionParser::ParseInternal(DiagnosticManager &diagnostic_manager,
     } else {
       ast_context.setExternalSource(ast_source);
     }
+    m_compiler->getSema().addExternalSource(ast_source_wrapper);
     decl_map->InstallASTContext(*m_ast_context);
   }