infer non-empty-string on substr() comparison with constant string

src/Analyser/TypeSpecifier.php

@@ -242,6 +242,27 @@ public function specifyTypesInCondition(
 						}
 					}
 				}
+
+				if (
+					$exprNode instanceof FuncCall
+					&& $exprNode->name instanceof Name
+					&& strtolower($exprNode->name->toString()) === 'substr'
+					&& isset($exprNode->getArgs()[0])
+					&& $constantType instanceof ConstantStringType
+					&& $constantType->getValue() !== ''
+				) {
+					$argType = $scope->getType($exprNode->getArgs()[0]->value);
+
+					if ($argType->isString()->yes()) {
+						return $this->create(
+							$exprNode->getArgs()[0]->value,
+							TypeCombinator::intersect($argType, new AccessoryNonEmptyStringType()),
+							$context,
+							false,
+							$scope,
+						);
+					}
+				}
 			}
 
 			$rightType = $scope->getType($expr->right);

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -911,6 +911,7 @@ public function dataFileAsserts(): iterable
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/constant-array-type-identical.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/non-empty-string-str-containing-fns.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-6609.php');
+		yield from $this->gatherAssertTypes(__DIR__ . '/data/non-empty-string-substr-specifying.php');
 	}
 
 	/**

tests/PHPStan/Analyser/data/non-empty-string-substr-specifying.php

@@ -0,0 +1,63 @@
+<?php
+
+namespace NonEmptyStringSubstr;
+
+use function PHPStan\Testing\assertType;
+
+class Foo {
+	public function nonEmptySubstr(string $s, int $offset, int $length): void
+	{
+		if (substr($s, 10) === 'hallo') {
+			assertType('non-empty-string', $s);
+		}
+		assertType('string', $s);
+
+		if (substr($s, -10) === 'hallo') {
+			assertType('non-empty-string', $s);
+		}
+		assertType('string', $s);
+
+		if (substr($s, 10, 5) === 'hallo') {
+			assertType('non-empty-string', $s);
+		}
+		assertType('string', $s);
+
+		if (substr($s, 10, -5) === 'hallo') {
+			assertType('non-empty-string', $s);
+		}
+		assertType('string', $s);
+
+		if (substr($s, $offset) === 'hallo') {
+			assertType('non-empty-string', $s);
+		}
+		assertType('string', $s);
+
+		if (substr($s, $offset, $length) === 'hallo') {
+			assertType('non-empty-string', $s);
+		}
+		assertType('string', $s);
+
+		if (substr($s, $offset, $length) !== 'hallo') {
+			assertType('string', $s);
+		}
+		assertType('string', $s);
+
+		if (substr($s, $offset, $length) === '') {
+			assertType('string', $s);
+		}
+		assertType('string', $s);
+
+		if (substr($s, $offset, $length) == '') {
+			assertType('string', $s);
+		}
+		assertType('string', $s);
+
+		$x = (substr($s, 10) === 'hallo');
+		assertType('string', $s);
+		var_dump($x);
+
+		$x = (substr($s, 10) !== 'hallo');
+		assertType('string', $s);
+		var_dump($x);
+	}
+}