did some debugging but I cannot wrap my head arround the TypeCombinator yet.

I think I can only leave this here as is, with my initial notes for someone else to pick up.

maybe @herndlm or @arnaud-lb have an idea on how this could/should work?

I think this line is completely wrong:

TypeCombinator::intersect(new ArrayType(new IntegerType(), $keyType), ...TypeUtils::getAccessoryTypes($valueType));

It makes sense for stuff like NonEmptyArrayType but that's probably the only one.

For other cases it just mixes unrelated things together. Because array_keys produces array<int, X> from array<X, Y>. But when the $valueType (the array<X, Y>) has HasOffsetType(string), it doesn't make sense to intersect it with array<int, X>.

Please try something like this instead:

diff --git a/src/Type/Php/ArrayKeysFunctionDynamicReturnTypeExtension.php b/src/Type/Php/ArrayKeysFunctionDynamicReturnTypeExtension.php
index e0f0d451c..665b91543 100644
--- a/src/Type/Php/ArrayKeysFunctionDynamicReturnTypeExtension.php
+++ b/src/Type/Php/ArrayKeysFunctionDynamicReturnTypeExtension.php
@@ -5,6 +5,7 @@ namespace PHPStan\Type\Php;
 use PhpParser\Node\Expr\FuncCall;
 use PHPStan\Analyser\Scope;
 use PHPStan\Reflection\FunctionReflection;
+use PHPStan\Type\Accessory\NonEmptyArrayType;
 use PHPStan\Type\ArrayType;
 use PHPStan\Type\Constant\ConstantArrayType;
 use PHPStan\Type\DynamicFunctionReturnTypeExtension;
@@ -33,7 +34,11 @@ class ArrayKeysFunctionDynamicReturnTypeExtension implements DynamicFunctionRetu
 					return $valueType->getKeysArray();
 				}
 				$keyType = $valueType->getIterableKeyType();
-				return TypeCombinator::intersect(new ArrayType(new IntegerType(), $keyType), ...TypeUtils::getAccessoryTypes($valueType));
+				$array = new ArrayType(new IntegerType(), $keyType);
+				if ($valueType->isIterableAtLeastOnce()->yes()) {
+					$array = TypeCombinator::intersect($array, new NonEmptyArrayType());
+				}
+				return $array;
 			}
 		}

Thanks ondrej for the suggestion. I applied it to the source and locally it works.. lets see what CI results will look like.

I think it needs another double check in these places, which use a similar logic:

• phpstan-src/src/Type/Php/ArrayMapFunctionReturnTypeExtension.php

  Lines 82 to 85 in 1c8b236

  	$mappedArrayType = TypeCombinator::intersect(new ArrayType(
  	$arrayType->getIterableKeyType(),
  	$valueType,
  	), ...TypeUtils::getAccessoryTypes($arrayType));

• phpstan-src/src/Type/Php/ArrayMapFunctionReturnTypeExtension.php

  Lines 93 to 96 in 1c8b236

  	$mappedArrayType = TypeCombinator::intersect(new ArrayType(
  	new IntegerType(),
  	$valueType,
  	), ...TypeUtils::getAccessoryTypes($arrayType));

• phpstan-src/src/Type/Php/ArrayValuesFunctionDynamicReturnTypeExtension.php

  Lines 30 to 35 in 1c8b236

  	if ($valueType->isArray()->yes()) {
  	if ($valueType instanceof ConstantArrayType) {
  	return $valueType->getValuesArray();
  	}
  	return TypeCombinator::intersect(new ArrayType(new IntegerType(), $valueType->getIterableValueType()), ...TypeUtils::getAccessoryTypes($valueType));
  	}

what do you think?

Try to break these places and then try to fix them :)

I couldn't come up with a case for array_map which breaks, but fixed a array_values similar case

Thank you.