cgen: fix match comparing with ident of type string (#7921)

vlib/v/gen/cgen.v

@@ -3393,7 +3393,7 @@ fn (mut g Gen) match_expr_classic(node ast.MatchExpr, is_expr bool, cond_var str
 					g.write(' || ')
 				}
 				if type_sym.kind == .string {
-					if (expr as ast.StringLiteral).val == '' {
+					if expr is ast.StringLiteral && (expr as ast.StringLiteral).val == '' {
 						g.write('${cond_var}.len == 0')
 					} else {
 						g.write('string_eq(')

vlib/v/tests/match_test.v

@@ -230,3 +230,18 @@ fn test_sub_expression() {
 	c := true || match 1 {0 {false} else {false}}
 	assert c
 }
+
+const (
+	one = 'one'
+)
+
+fn test_match_constant_string() {
+	match one {
+		one {
+			assert true
+		}
+		else {
+			assert false
+		}
+	}
+}