cgen: emit interface type-table index as a real symbol under -usecache (fix undefined _IError_None___index)

vlib/v/gen/c/cgen.v

@@ -13512,7 +13512,18 @@ return ${cast_shared_struct_str};
 			}
 			iin_idx := already_generated_mwrappers[interface_index_name] - iinidx_minimum_base + 1
 			if g.pref.build_mode != .build_module {
-				sb.writeln('enum { ${interface_index_name} = ${iin_idx} };')
+				if g.pref.use_cache {
+					// With -usecache, modules like `builtin` are compiled separately
+					// in build_module mode, where the index is emitted as
+					// `extern const u32 ..._index;` and referenced. The main program
+					// must therefore provide a real, externally-linked definition
+					// (not a compile-time `enum` constant, which has no linker
+					// symbol), otherwise the reference is undefined at link time -
+					// e.g. `undefined symbol: _IError_None___index` on FreeBSD/clang.
+					sb.writeln('const u32 ${interface_index_name} = ${iin_idx};')
+				} else {
+					sb.writeln('enum { ${interface_index_name} = ${iin_idx} };')
+				}
 			} else {
 				sb.writeln('extern const u32 ${interface_index_name};')
 			}

vlib/v/tests/usecache_interface_index_symbol_test.v

@@ -0,0 +1,39 @@
+import os
+
+const vexe = @VEXE
+
+// Regression test for https://github.com/vlang/v/issues/27330
+//
+// With -usecache, modules like `builtin` are compiled separately in
+// build_module mode, where the interface type-table index is emitted as
+// `extern const u32 ..._index;` and referenced. The main program must therefore
+// provide a real, externally-linked `const u32 ..._index = N;` definition - a
+// compile-time `enum` constant has no linker symbol, so the reference would be
+// undefined at link time (e.g. `undefined symbol: _IError_None___index` on
+// FreeBSD/clang).
+fn test_usecache_interface_index_is_real_symbol() {
+	tmp_dir := os.join_path(os.vtmp_dir(), 'v_issue_27330')
+	os.mkdir_all(tmp_dir) or { panic(err) }
+	defer {
+		os.rmdir_all(tmp_dir) or {}
+	}
+	source_path := os.join_path(tmp_dir, 'issue_27330.v')
+	os.write_file(source_path, "fn main() {\n\tprintln('hello world')\n}\n") or { panic(err) }
+
+	// -o - dumps the generated C of the main program to stdout.
+	res := os.execute('${os.quoted_path(vexe)} -usecache -o - ${os.quoted_path(source_path)}')
+	if res.exit_code != 0 {
+		panic(res.output)
+	}
+	// The index must be a real (externally-linked) definition, not a bare enum.
+	assert res.output.contains('const u32 _IError_None___index =')
+	assert !res.output.contains('enum { _IError_None___index =')
+
+	// Sanity check: without -usecache the compile-time enum form is kept (it is
+	// the tcc-friendly form and needs no external symbol in a single build).
+	res2 := os.execute('${os.quoted_path(vexe)} -o - ${os.quoted_path(source_path)}')
+	if res2.exit_code != 0 {
+		panic(res2.output)
+	}
+	assert res2.output.contains('enum { _IError_None___index =')
+}