checker: allow i32 in enum MyEnum as i32 { (#18172)

vlib/v/checker/checker.v

@@ -1627,10 +1627,14 @@ fn (mut c Checker) enum_decl(mut node ast.EnumDecl) {
 			signed, enum_umin, enum_umax = false, 0, 0xFFFF_FFFF_FFFF_FFFF
 		}
 		else {
-			c.error('`${senum_type}` is not one of `i8`,`i16`,`int`,`i64`,`u8`,`u16`,`u32`,`u64`',
+			if senum_type == 'i32' {
+				signed, enum_imin, enum_imax = true, -2_147_483_648, 0x7FFF_FFFF
+			} else {
+				c.error('`${senum_type}` is not one of `i8`,`i16`,`i32`,`int`,`i64`,`u8`,`u16`,`u32`,`u64`',
 					node.typ_pos)
 			}
 		}
+	}
 	if enum_imin > 0 {
 		// ensure that the minimum value is negative, even with msvc, which has a bug that makes -2147483648 positive ...
 		enum_imin *= -1

vlib/v/checker/tests/enum_field_value_overflow.out

@@ -5,7 +5,7 @@ vlib/v/checker/tests/enum_field_value_overflow.vv:3:10: error: enum value `21474
       |             ~~~~~~~~~~
     4 |     blue
     5 | }
-vlib/v/checker/tests/enum_field_value_overflow.vv:7:21: error: `string` is not one of `i8`,`i16`,`int`,`i64`,`u8`,`u16`,`u32`,`u64`
+vlib/v/checker/tests/enum_field_value_overflow.vv:7:21: error: `string` is not one of `i8`,`i16`,`i32`,`int`,`i64`,`u8`,`u16`,`u32`,`u64`
     5 | }
     6 |
     7 | enum ColorString as string {

vlib/v/tests/enum_test.v

@@ -88,6 +88,20 @@ fn test_nums() {
 	assert d == unsafe { Foo(-10) }
 }
 
+enum Number as i32 {
+	a = 100
+	b = 200
+	c = 300
+	d = 400
+}
+
+fn test_enum_as_i32() {
+	assert int(Number.a) == 100
+	assert int(Number.b) == 200
+	assert int(Number.c) == 300
+	assert int(Number.d) == 400
+}
+
 /*
 enum Expr {
 	BoolExpr(bool)