Fix #925 - set correct flag when coercing class-like string to specific

src/Psalm/Checker/TypeChecker.php

@@ -638,6 +638,7 @@ class TypeChecker
 
             if ($input_type_part instanceof TClassString) {
                 $type_coerced = true;
+                $type_coerced_from_scalar = true;
 
                 return false;
             }

tests/ConstantTest.php

@@ -221,6 +221,23 @@ class ConstantTest extends TestCase
                     fwrite(STDOUT, "asd");
                     fwrite(STDERR, "zcx");'
             ],
+            'classStringArrayOffset' => [
+                '<?php
+                    class A {}
+                    class B {}
+
+                    const C = [
+                        A::class => 1,
+                        B::class => 2,
+                    ];
+
+                    /**
+                     * @param class-string $s
+                     */
+                    function foo(string $s) : void {
+                        if (isset(C[$s])) {}
+                    }',
+            ],
         ];
     }