cgen: execute defer block *after* return expression is evaluated (#9893)

doc/docs.md

@@ -1383,6 +1383,48 @@ fn read_log() {
 }
 ```
 
+If the function returns a value the `defer` block is executed *after* the return
+expression is evaluated: 
+
+```v
+import os
+
+enum State {
+	normal
+	write_log
+	return_error
+}
+
+// write log file and return number of bytes written
+fn write_log(s State) ?int {
+	mut f := os.create('log.txt') ?
+	defer {
+		f.close()
+	}
+	if s == .write_log {
+		// `f.close()` will be called after `f.write()` has been
+		// executed, but before `write_log()` finally returns the
+		// number of bytes written to `main()`
+		return f.writeln('This is a log file')
+	} else if s == .return_error {
+		// the file will be closed after the `error()` function
+		// has returned - so the error message will still report
+		// it as open
+		return error('nothing written; file open: $f.is_opened')
+	}
+	// the file will be closed here, too
+	return 0
+}
+
+fn main() {
+	n := write_log(.return_error) or {
+		println('Error: $err')
+		0
+	}
+	println('$n bytes written')
+}
+```
+
 ## Structs
 
 ```v