checker: fix x.$(field.name) not working outside of $if (#12802)

vlib/v/checker/checker.v

@@ -101,6 +101,7 @@ mut:
 	prevent_sum_type_unwrapping_once bool       // needed for assign new values to sum type, stopping unwrapping then
 	loop_label                       string     // set when inside a labelled for loop
 	timers                           &util.Timers = util.get_timers()
+	comptime_fields_default_type     ast.Type
 	comptime_fields_type             map[string]ast.Type
 	fn_scope                         &ast.Scope = voidptr(0)
 	main_fn_decl_node                ast.FnDecl
@@ -2596,6 +2597,7 @@ fn (mut c Checker) comptime_for(node ast.ComptimeFor) {
 	}
 	if node.kind == .fields {
 		c.comptime_fields_type[node.val_var] = node.typ
+		c.comptime_fields_default_type = node.typ
 	}
 	c.stmts(node.stmts)
 }

vlib/v/checker/if.v

@@ -124,7 +124,7 @@ pub fn (mut c Checker) if_expr(mut node ast.IfExpr) ast.Type {
 				node.branches[i].stmts = []
 			}
 			if comptime_field_name.len > 0 {
-				c.comptime_fields_type.delete(comptime_field_name)
+				c.comptime_fields_type[comptime_field_name] = c.comptime_fields_default_type
 			}
 			c.skip_flags = cur_skip_flags
 			if c.fn_level == 0 && c.pref.output_cross_c && c.ct_cond_stack.len > 0 {

vlib/v/tests/for_t_fields_with_comptime_if_test.v

@@ -0,0 +1,54 @@
+struct Abc {
+	a byte
+	b byte
+	c int
+}
+
+fn decode<T>() T {
+	mut x := T{}
+	$for field in T.fields {
+		$if field.typ is byte {
+			x.$(field.name) = 1
+		} $else {
+			x.$(field.name) = 3
+		}
+		if x.$(field.name) == 1 {
+			x.$(field.name) = 5
+		}
+	}
+	return x
+}
+
+fn test_decode() {
+	abc := decode<Abc>()
+	assert abc.a == 5
+	assert abc.b == 5
+	assert abc.c == 3
+}
+
+struct Abc2 {
+	an_int   int
+	a_byte   byte
+	a_string string
+}
+
+fn decode2<T>() T {
+	mut x := T{}
+	$for field in T.fields {
+		$if field.typ is byte {
+			x.$(field.name) = byte(-1)
+		} $else $if field.typ is int {
+			x.$(field.name) = int(-1)
+		} $else {
+			x.$(field.name) = 'hi'
+		}
+	}
+	return x
+}
+
+fn test_decode2() {
+	abc := decode2<Abc2>()
+	assert abc.an_int == -1
+	assert abc.a_byte == 0xff
+	assert abc.a_string == 'hi'
+}