cgen: fix nested or in assign decl (fix #25864) (#25865)

Author: kbkpbot

vlib/v/gen/c/assign.v

@@ -37,16 +37,24 @@ fn (mut g Gen) expr_with_opt_or_block(expr ast.Expr, expr_typ ast.Type, var_expr
 			or_expr := (expr as ast.Ident).or_expr
 			stmts := or_expr.stmts
 			scope := or_expr.scope
+			last_stmt := stmts.last()
 			// handles stmt block which returns something
 			// e.g. { return none }
-			if stmts.len > 0 && stmts.last() is ast.ExprStmt && stmts.last().typ != ast.void_type {
-				g.gen_or_block_stmts(c_name(var_expr.str()), '', stmts, ret_typ, false,
-					scope, expr.pos())
+			if stmts.len > 0 && last_stmt is ast.ExprStmt && last_stmt.typ != ast.void_type {
+				var_expr_name := c_name(var_expr.str())
+				if last_stmt.expr is ast.Ident && last_stmt.expr.or_expr.kind != .absent {
+					g.write('${var_expr_name} = ')
+					g.expr_with_opt_or_block(last_stmt.expr, last_stmt.typ, var_expr,
+						ret_typ, in_heap)
+				} else {
+					g.gen_or_block_stmts(var_expr_name, '', stmts, ret_typ, false, scope,
+						expr.pos())
+				}
 			} else {
 				// handles stmt block which doesn't returns value
 				// e.g. { return }
 				g.stmts(stmts)
-				if stmts.len > 0 && stmts.last() is ast.ExprStmt {
+				if stmts.len > 0 && last_stmt is ast.ExprStmt {
 					g.writeln(';')
 				}
 				g.write_defer_stmts(scope, false, expr.pos())

vlib/v/tests/nested_or_in_assign_decl_test.v

@@ -0,0 +1,23 @@
+module main
+
+fn fx1() ?int {
+	return none
+}
+
+fn fx2() ?int {
+	return none
+}
+
+fn fx3() ?int {
+	return none
+}
+
+fn test_nested_or_in_assign_decl() {
+	x1 := fx1()
+	x2 := fx2()
+	x3 := fx3()
+	def := 123
+
+	y := x1 or { x2 or { x3 or { def } } }
+	assert y == 123
+}