checker: fix comptime "ident is type" (#18747)

vlib/v/checker/if.v

@@ -161,6 +161,13 @@ fn (mut c Checker) if_expr(mut node ast.IfExpr) ast.Type {
 							is_comptime_type_is_expr = true
 							left_type := c.unwrap_generic(left.typ)
 							skip_state = c.check_compatible_types(left_type, right as ast.TypeNode)
+						} else if left is ast.Ident {
+							is_comptime_type_is_expr = true
+							mut checked_type := ast.void_type
+							if var := left.scope.find_var(left.name) {
+								checked_type = c.unwrap_generic(var.typ)
+							}
+							skip_state = c.check_compatible_types(checked_type, right as ast.TypeNode)
 						}
 					}
 				} else if branch.cond.op in [.eq, .ne] {

vlib/v/checker/tests/run/comptime_ident_is_type.run.out

@@ -0,0 +1,6 @@
+u64: true
+u64 array: 0
+other array: 1
+other array: 0
+other int: false
+unknown type

vlib/v/checker/tests/run/comptime_ident_is_type.vv

@@ -0,0 +1,22 @@
+fn test[T](val T) {
+	$if val is u64 {
+		println('u64: ${val == u64(0)}')
+	} $else $if val is []u64 {
+		println('u64 array: ${val.len}')
+	} $else $if val is $int {
+		println('other int: ${val == 0}')
+	} $else $if val is $array {
+		println('other array: ${val.len}')
+	} $else {
+		println('unknown type')
+	}
+}
+
+fn main() {
+	test(u64(0))
+	test([]u64{})
+	test(['string'])
+	test([]u32{})
+	test(int(12))
+	test('string')
+}