checker: fix defer ident handling & fix defer optional error message (#10975)

vlib/v/checker/checker.v

@@ -3208,8 +3208,13 @@ pub fn (mut c Checker) check_expr_opt_call(expr ast.Expr, ret_type ast.Type) ast
 	if expr is ast.CallExpr {
 		if expr.return_type.has_flag(.optional) {
 			if expr.or_block.kind == .absent {
+				if c.inside_defer {
+					c.error('${expr.name}() returns an option, so it should have an `or {}` block at the end',
+						expr.pos)
+				} else {
 					c.error('${expr.name}() returns an option, so it should have either an `or {}` block, or `?` at the end',
 						expr.pos)
+				}
 			} else {
 				c.check_or_expr(expr.or_block, ret_type, expr.return_type.clear_flag(.optional))
 			}

vlib/v/checker/tests/defer_optional.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/defer_optional.vv:5:3: error: opt() returns an option, so it should have an `or {}` block at the end
+    3 | fn thing() ?string {
+    4 |     defer {
+    5 |         opt()
+      |         ~~~~~
+    6 |     }
+    7 |     return 'ok'

vlib/v/checker/tests/defer_optional.vv

@@ -0,0 +1,8 @@
+fn opt() ? {}
+
+fn thing() ?string {
+	defer {
+		opt()
+	}
+	return 'ok'
+}

vlib/v/gen/c/cgen.v

@@ -4902,7 +4902,9 @@ fn (mut g Gen) return_stmt(node ast.Return) {
 			// `tmp := foo(a, b, c); free(a); free(b); free(c); return tmp;`
 			// Save return value in a temp var so that all args (a,b,c) can be freed
 			// Don't use a tmp var if a variable is simply returned: `return x`
-			if node.exprs[0] !is ast.Ident {
+			// Just in case of defer statements exists, that the return values cannot
+			// be modified.
+			if node.exprs[0] !is ast.Ident || use_tmp_var {
 				g.write('$ret_typ $tmpvar = ')
 			} else {
 				use_tmp_var = false

vlib/v/tests/defer_test.v

@@ -142,3 +142,15 @@ fn test_defer_str_interpol() {
 		t << '${t[0]}'
 	}
 }
+
+fn test_defer_not_change_return_values() {
+	assert num() == 10
+}
+
+fn num() int {
+	mut ret := 10
+	defer {
+		ret = 20
+	}
+	return ret
+}