checker: fix comptime evaluation is/!is operator with typenode (#18773)

vlib/v/checker/comptime.v

@@ -666,7 +666,6 @@ fn (mut c Checker) comptime_if_branch(cond ast.Expr, pos token.Pos) ComptimeBran
 						sym := c.table.sym(cond.right.typ)
 						if sym.kind != .interface_ {
 							c.expr(cond.left)
-							// c.error('`$sym.name` is not an interface', cond.right.pos())
 						}
 						return .unknown
 					} else if cond.left is ast.TypeNode && cond.right is ast.ComptimeType {

vlib/v/checker/if.v

@@ -100,7 +100,7 @@ fn (mut c Checker) if_expr(mut node ast.IfExpr) ast.Type {
 					is_comptime_type_is_expr = true
 				}
 			} else if mut branch.cond is ast.InfixExpr {
-				if branch.cond.op == .key_is {
+				if branch.cond.op in [.not_is, .key_is] {
 					left := branch.cond.left
 					right := branch.cond.right
 					if right !in [ast.TypeNode, ast.ComptimeType] {
@@ -170,7 +170,10 @@ fn (mut c Checker) if_expr(mut node ast.IfExpr) ast.Type {
 							skip_state = c.check_compatible_types(checked_type, right as ast.TypeNode)
 						}
 					}
-				} else if branch.cond.op in [.eq, .ne] {
+					if branch.cond.op == .not_is && skip_state != .unknown {
+						skip_state = if skip_state == .eval { .skip } else { .eval }
+					}
+				} else if branch.cond.op in [.eq, .ne, .gt, .lt, .ge, .le] {
 					left := branch.cond.left
 					right := branch.cond.right
 					if left is ast.SelectorExpr && right is ast.IntegerLiteral {

vlib/v/tests/comptime_is_check_test.v

@@ -0,0 +1,15 @@
+fn test[T](val T) string {
+	$if T is u32 {
+		$compile_error('u32')
+		return ''
+	} $else $if T !is string {
+		$compile_error('not string')
+		return ''
+	} $else {
+		return val
+	}
+}
+
+fn test_main() {
+	assert test('str') == 'str'
+}