[TASK] Raise PHPStan to level 4

Also allow `assertInstanceOf` checks in the tests (as we find
those useful).

Author: oliverklee

config/phpstan-baseline.neon

@@ -6,6 +6,12 @@ parameters:
 			count: 1
 			path: ../src/CSSList/AtRuleBlockList.php
 
+		-
+			message: '#^Call to function is_string\(\) with Sabberworm\\CSS\\Parsing\\ParserState will always evaluate to false\.$#'
+			identifier: function.impossibleType
+			count: 1
+			path: ../src/CSSList/CSSList.php
+
 		-
 			message: '#^Loose comparison via "\!\=" is not allowed\.$#'
 			identifier: notEqual.notAllowed
@@ -36,6 +42,18 @@ parameters:
 			count: 1
 			path: ../src/CSSList/Document.php
 
+		-
+			message: '#^Call to function in_array\(\) with arguments null, list\<string\> and true will always evaluate to false\.$#'
+			identifier: function.impossibleType
+			count: 1
+			path: ../src/Parsing/ParserState.php
+
+		-
+			message: '#^Negated boolean expression is always true\.$#'
+			identifier: booleanNot.alwaysTrue
+			count: 1
+			path: ../src/Parsing/ParserState.php
+
 		-
 			message: '#^Only booleans are allowed in an if condition, Sabberworm\\CSS\\Value\\RuleValueList\|string\|null given\.$#'
 			identifier: if.condNotBoolean
@@ -95,3 +113,15 @@ parameters:
 			identifier: typePerfect.narrowPublicClassMethodParamType
 			count: 1
 			path: ../src/Value/Size.php
+
+		-
+			message: '#^Strict comparison using \!\=\= between non\-empty\-string and null will always evaluate to true\.$#'
+			identifier: notIdentical.alwaysTrue
+			count: 1
+			path: ../src/Value/Size.php
+
+		-
+			message: '#^Call to static method PHPUnit\\Framework\\Assert\:\:assertSame\(\) with ''red'' and Sabberworm\\CSS\\Value\\Value will always evaluate to false\.$#'
+			identifier: staticMethod.impossibleType
+			count: 1
+			path: ../tests/ParserTest.php

config/phpstan.neon

@@ -8,7 +8,7 @@ parameters:
 
   phpVersion: 70200
 
-  level: 3
+  level: 4
 
   paths:
     - %currentWorkingDirectory%/bin/
@@ -21,3 +21,8 @@ parameters:
     null_over_false: true
     narrow_param: true
     narrow_return: true
+
+  ignoreErrors:
+    -
+      message: '#^Call to static method PHPUnit\\Framework\\Assert\:\:assertInstanceOf\(\) .* will always evaluate to#'
+      path: '../tests/'