Only treat implode() separator as guaranteeing a non-empty result when the array has at least two elements

- The separator passed to `implode()`/`join()` only appears *between* elements, so a single-element array (e.g. `implode(',', [''])`) drops it entirely and can still produce an empty/falsy string.
- `ImplodeFunctionReturnTypeExtension` now gates the separator's `non-empty-string`/`non-falsy-string` contribution on the array being provably of size >= 2 (via `getArraySize()` and `IntegerRangeType::createAllGreaterThanOrEqualTo(2)`). The value-type contribution is unchanged since it holds for any non-empty array.
- Updated existing `non-empty-string`/`non-falsy-string` assertions that encoded the previous, unsound behavior to the now-correct `string`/`non-empty-string` results.
- Added regression tests in `implode.php` covering `non-empty-array`/`non-empty-list` of `string` and `non-empty-string`, plus `array{string, string}` (provably >= 2 elements stays `non-falsy-string`).
- Taught PHPStan's own analysis that `PhpParser\Node\Name::getParts()` returns `non-empty-list<non-empty-string>` (dev-only stub) so building a class name with `implode('\\', ...)` stays `non-empty-string`.
- Probed sibling string builders `str_repeat()` and `str_pad()`: both already gate the relevant accessory on the multiplier/length, so no change needed.

Author: phpstan-bot

build/stubs/PhpParserName.stub

@@ -22,4 +22,8 @@ class Name extends NodeAbstract
     /** @return non-empty-string */
     public function toCodeString() : string {
     }
+
+    /** @return non-empty-list<non-empty-string> */
+    public function getParts() : array {
+    }
 }

src/Type/Php/ImplodeFunctionReturnTypeExtension.php

@@ -15,6 +15,7 @@
 use PHPStan\Type\Constant\ConstantArrayType;
 use PHPStan\Type\Constant\ConstantStringType;
 use PHPStan\Type\DynamicFunctionReturnTypeExtension;
+use PHPStan\Type\IntegerRangeType;
 use PHPStan\Type\IntersectionType;
 use PHPStan\Type\StringType;
 use PHPStan\Type\Type;
@@ -85,9 +86,14 @@ private function implode(Type $arrayType, Type $separatorType): Type
 		$accessoryTypes = [];
 		$valueTypeAsString = $arrayType->getIterableValueType()->toString();
 		if ($arrayType->isIterableAtLeastOnce()->yes()) {
-			if ($valueTypeAsString->isNonFalsyString()->yes() || $separatorType->isNonFalsyString()->yes()) {
+			// The separator only appears between elements, so it can only
+			// guarantee a non-empty/non-falsy result when the array has at
+			// least two elements. A single-element array drops the separator
+			// entirely (e.g. implode(',', ['']) === '').
+			$separatorAppears = IntegerRangeType::createAllGreaterThanOrEqualTo(2)->isSuperTypeOf($arrayType->getArraySize())->yes();
+			if ($valueTypeAsString->isNonFalsyString()->yes() || ($separatorAppears && $separatorType->isNonFalsyString()->yes())) {
 				$accessoryTypes[] = new AccessoryNonFalsyStringType();
-			} elseif ($valueTypeAsString->isNonEmptyString()->yes() || $separatorType->isNonEmptyString()->yes()) {
+			} elseif ($valueTypeAsString->isNonEmptyString()->yes() || ($separatorAppears && $separatorType->isNonEmptyString()->yes())) {
 				$accessoryTypes[] = new AccessoryNonEmptyStringType();
 			}
 		}

tests/PHPStan/Analyser/nsrt/implode.php

@@ -78,6 +78,38 @@ public function unsealedConstArr($unsealed) {
 		assertType('non-falsy-string', implode(',', $unsealed));
 	}
 
+	/**
+	 * @param non-empty-array<string> $arr
+	 * @param non-empty-list<string> $list
+	 */
+	public function nonEmptyArrayOfStrings(array $arr, array $list) {
+		// A non-empty array can have a single element, which drops the
+		// separator entirely (implode(',', ['']) === ''), so the non-falsy
+		// separator must not make the whole result non-falsy.
+		assertType('string', implode(',', $arr));
+		assertType('string', implode('', $arr));
+		assertType('string', implode(',', $list));
+	}
+
+	/**
+	 * @param non-empty-array<non-empty-string> $arr
+	 */
+	public function nonEmptyArrayOfNonEmptyStrings(array $arr) {
+		assertType('non-empty-string', implode(',', $arr));
+		assertType('non-empty-string', implode('', $arr));
+	}
+
+	/**
+	 * @param array{string, string} $arr
+	 * @param array{string, string, string} $arr3
+	 */
+	public function atLeastTwoElements(array $arr, array $arr3) {
+		// At least two elements guarantees the separator appears.
+		assertType('non-falsy-string', implode(',', $arr));
+		assertType('non-falsy-string', implode(',', $arr3));
+		assertType('string', implode('', $arr));
+	}
+
 	/** @param array{'a', 'b', ...<int, string>} $unsealed */
 	public function unsealedConstArrEmptySeparator($unsealed) {
 		// Empty separator + a possibly-empty unsealed value type leaves no

tests/PHPStan/Analyser/nsrt/non-empty-string.php

@@ -182,7 +182,7 @@ public function doFoo2(string $s, array $nonEmptyArrayWithStrings): void
 	{
 		assertType('string', implode($s, $nonEmptyArrayWithStrings));
 		assertType('string', implode('', $nonEmptyArrayWithStrings));
-		assertType('non-falsy-string', implode(' ', $nonEmptyArrayWithStrings));
+		assertType('string', implode(' ', $nonEmptyArrayWithStrings));
 		assertType('string', implode($nonEmptyArrayWithStrings));
 	}
 
@@ -204,7 +204,7 @@ public function doFoo4(string $s, array $nonEmptyArrayWithNonEmptyStrings): void
 	{
 		assertType('non-empty-string', implode($s, $nonEmptyArrayWithNonEmptyStrings));
 		assertType('non-empty-string', implode('', $nonEmptyArrayWithNonEmptyStrings));
-		assertType('non-falsy-string', implode(' ', $nonEmptyArrayWithNonEmptyStrings));
+		assertType('non-empty-string', implode(' ', $nonEmptyArrayWithNonEmptyStrings));
 		assertType('non-empty-string', implode($nonEmptyArrayWithNonEmptyStrings));
 	}
 
@@ -227,7 +227,9 @@ public function nonE($glue, array $a)
 			return "xyz";
 		}
 
-		assertType('non-empty-string', implode($glue, $a));
+		// A single-element array drops the glue entirely, so the result can
+		// still be an empty/falsy string even with a non-empty glue.
+		assertType('string', implode($glue, $a));
 	}
 
 	public function sayHello2(int $i): void
@@ -248,7 +250,7 @@ public function nonE2($glue, array $a)
 			return "xyz";
 		}
 
-		assertType('non-empty-string', join($glue, $a));
+		assertType('string', join($glue, $a));
 	}
 
 }

tests/PHPStan/Analyser/nsrt/non-falsy-string.php

@@ -78,7 +78,7 @@ function concat(string $s, string $nonFalsey, $numericS, $nonEmpty, $literalStri
 	function stringFunctions(string $s, $nonFalsey, $arrayOfNonFalsey, $nonEmptyArray, array $arr)
 	{
 		assertType('string', implode($nonFalsey, []));
-		assertType('non-falsy-string', implode($nonFalsey, $nonEmptyArray));
+		assertType('string', implode($nonFalsey, $nonEmptyArray));
 		assertType('non-falsy-string', implode($nonFalsey, $arrayOfNonFalsey));
 		assertType('non-falsy-string', implode($s, $arrayOfNonFalsey));