Infer types of variables with dynamic name

Author: ondrejmirtes

src/Analyser/MutatingScope.php

@@ -2000,12 +2000,19 @@ static function (Node $node, Scope $scope) use ($arrowScope, &$arrowFunctionImpu
 			);
 		}
 
-		if ($node instanceof Variable && is_string($node->name)) {
-			if ($this->hasVariableType($node->name)->no()) {
-				return new ErrorType();
+		if ($node instanceof Variable) {
+			if (is_string($node->name)) {
+				if ($this->hasVariableType($node->name)->no()) {
+					return new ErrorType();
+				}
+
+				return $this->getVariableType($node->name);
 			}
 
-			return $this->getVariableType($node->name);
+			$nameType = $this->getType($node->name);
+			if (count($nameType->getConstantStrings()) > 0) {
+				return TypeCombinator::union(...array_map(fn ($constantString) => $this->getVariableType($constantString->getValue()), $nameType->getConstantStrings()));
+			}
 		}
 
 		if ($node instanceof Expr\ArrayDimFetch && $node->dim !== null) {

tests/PHPStan/Analyser/nsrt/bug-12398.php

@@ -0,0 +1,16 @@
+<?php
+
+namespace Bug12398;
+
+use function PHPStan\Testing\assertType;
+
+class Foo
+{
+
+	public function doFoo(string $foo): void
+	{
+		$bar = 'foo';
+		assertType('string', $$bar);
+	}
+
+}