Add helper to ensure we're not including PG files

.gitignore

@@ -1,3 +1,4 @@
+include/generated/
 .clangd
 .depend
 .deps

Makefile

@@ -22,6 +22,8 @@ DUCKDB_CMAKE_VARS = -DBUILD_SHELL=0 -DBUILD_PYTHON=0 -DBUILD_UNITTESTS=0
 # DuckDB agains the release build of MotherDuck.
 DUCKDB_DISABLE_ASSERTIONS ?= 0
 
+CPP_ONLY_FILE = include/generated/cpp_only_file.hpp
+
 DUCKDB_BUILD_CXX_FLAGS=
 DUCKDB_BUILD_TYPE=
 ifeq ($(DUCKDB_BUILD), Debug)
@@ -47,7 +49,7 @@ include Makefile.global
 # includes those header files. This does mean that we rebuild our .o files
 # whenever we change the DuckDB version, but that seems like a fairly
 # reasonable thing to do anyway, even if not always strictly necessary always.
-$(OBJS): .git/modules/third_party/duckdb/HEAD
+$(OBJS): .git/modules/third_party/duckdb/HEAD $(CPP_ONLY_FILE)
 
 COMPILE.cc.bc += $(PG_CPPFLAGS)
 COMPILE.cxx.bc += $(PG_CXXFLAGS)
@@ -97,6 +99,16 @@ clean-duckdb:
 install: install-duckdb
 
 clean-all: clean clean-regression clean-duckdb
+	rm -f $(CPP_ONLY_FILE)
+
+$(CPP_ONLY_FILE):
+	@rm -f $(CPP_ONLY_FILE)
+	@echo "// Auto-generated file, run 'make update_cpp_guard' to update" > $(CPP_ONLY_FILE)
+	@echo "#if \\" >> $(CPP_ONLY_FILE)
+	for m in $$(grep -R '_H$$' "$(INCLUDEDIR)" | grep define | cut -d \: -f 2 | cut -d \  -f 2 | sort); do echo "defined($$m) || \\"; done >> $(CPP_ONLY_FILE)
+	@echo "false" >> $(CPP_ONLY_FILE)
+	@echo "static_assert(false, \"No Postgres header should be included in this file.\");" >> $(CPP_ONLY_FILE)
+	@echo "#endif" >> $(CPP_ONLY_FILE)
 
 lintcheck:
 	clang-tidy $(SRCS) -- -I$(INCLUDEDIR) -I$(INCLUDEDIR_SERVER) -Iinclude $(CPPFLAGS) -std=c++17

src/catalog/pgduckdb_catalog.cpp

@@ -5,6 +5,8 @@
 #include "pgduckdb/catalog/pgduckdb_storage.hpp"
 #include "pgduckdb/catalog/pgduckdb_transaction.hpp"
 
+#include "generated/cpp_only_file.hpp" // Must be last include.
+
 namespace pgduckdb {
 
 PostgresCatalog::PostgresCatalog(duckdb::AttachedDatabase &db, const duckdb::string &connection_string,

src/catalog/pgduckdb_schema.cpp

@@ -3,6 +3,8 @@
 #include "pgduckdb/catalog/pgduckdb_transaction.hpp"
 #include "duckdb/parser/parsed_data/create_table_info.hpp"
 
+#include "generated/cpp_only_file.hpp" // Must be last include.
+
 namespace pgduckdb {
 
 PostgresSchema::PostgresSchema(duckdb::Catalog &catalog, duckdb::CreateSchemaInfo &info, Snapshot snapshot)

src/catalog/pgduckdb_storage.cpp

@@ -2,6 +2,8 @@
 #include "pgduckdb/catalog/pgduckdb_catalog.hpp"
 #include "pgduckdb/catalog/pgduckdb_transaction_manager.hpp"
 
+#include "generated/cpp_only_file.hpp" // Must be last include.
+
 namespace pgduckdb {
 
 static duckdb::unique_ptr<duckdb::TransactionManager>