I checked why the the sources fail to transform for PHP <8 and it seems to be because InternalScopeFactory is transformed first (the union type for $function is removed) and then LazyInternalScopeFactory fails to transform because at that point the class is not a valid implementation of the interface.

It seems that one issue is that strict mixed type is not a straightforward upgrade from normal mixed type:

• It's not subtractable.
• describe methods only handle the normal mixed type (IDK if this is a feature or a bug).
• ...

I'm sorry, you're going to have to explain it to me better :) This https://phpstan.org/r/4ef0bc61-b8b3-4db0-8b03-d862c15cd37d just shows a cosmetic issue in an error message, but your PR description tells me about "hiding issues" - meaning there are errors that should be reported but aren't. Can you provide a better code sample where this is obvious? Thanks :)

Ok, sorry. How about this: https://phpstan.org/r/5e3fed61-e2c2-4c48-ac7f-e2c6d4e972dc ? IMO every line inside those two functions should have an error on level 9 (i.e. "the only allowed operation you can do with it is to pass it to another mixed").

This is the result from my branch (I used a bare-bones config file with just level 9 set and nothing else):

  9      Cannot call method foo() on T of mixed.                                    
  10     Cannot call static method bar() on T of mixed.                             
  11     Cannot access offset 5 on T of mixed.                                      
  12     Parameter #1 $string of function strlen expects string, T of mixed given.  
  13     Cannot use ++ on T of mixed.                                               
  18     Cannot call method foo() on mixed.                                         
  19     Cannot call static method bar() on mixed.                                  
  20     Cannot access offset 5 on mixed.                                           
  21     Parameter #1 $string of function strlen expects string, mixed given.       
  22     Cannot use ++ on mixed. 

And is the reason it doesn't get reported because TemplateMixedType does not get converted into StrictMixedType?

I think so (to TemplateStrictMixedType to be exact). I checked $a::foo() (it's caught for regular mixed) and $a++ (it's not caught for either) in the debugger:

For $a::foo() there is this code in StaticMethodCallCheck:

$classTypeResult = $this->ruleLevelHelper->findTypeToCheck(
	$scope,
	NullsafeOperatorHelper::getNullsafeShortcircuitedExprRespectingScope($scope, $class),
	sprintf('Call to static method %s() on an unknown class %%s.', SprintfHelper::escapeFormatString($methodName)),
	static fn (Type $type): bool => $type->canCallMethods()->yes() && $type->hasMethod($methodName)->yes(),
);
$classType = $classTypeResult->getType();
if ($classType instanceof ErrorType) {
	return [$classTypeResult->getUnknownClassErrors(), null];
}

• The generic version returns ErrorType from findTypeToCheck (getUnknownClassErrors is empty array).
• The specific version works because findTypeToCheck returns StrictMixedType.

For $a++ there is this code in InvalidIncDecOperationRule:

$varType = $scope->getType($node->var);
if (!$varType->toString() instanceof ErrorType) {
	return [];
}
if (!$varType->toNumber() instanceof ErrorType) {
	return [];
}

Both TemplateMixedType and MixedType always allow toString, so no error is reported.

In general it seems that various rules have ways of dealing with mixed (i.e. using RuleLevelHelper in some ways), but it's not consistent. That's why I thought it might be a good idea to try converting the mixed types to strict versions directly in the scope (it will probably need to happen in more places - e.g. values returned by functions, not just the parameters), so that the rules don't have to think about it individually.

It's all supposed to happen in RuleLevelHelper... I'll have to look into this when I get back in front of my computer 😊

@ondrejmirtes Thank you for the feedback (and sorry for delayed reply). You are right. I created #2885 as a replacement.