Infer literal types for word date format characters

.typos.toml

@@ -16,3 +16,5 @@ is_writeable = "is_writeable"
 [default.extend-words]
 # override false-positives
 Excluder = "Excluder"
+# English ordinal suffix ("2nd"), from the "S" date format
+nd = "nd"

src/Type/Php/DateFunctionReturnTypeHelper.php

@@ -37,12 +37,6 @@ public function getTypeFromFormatType(Type $formatType, bool $useMicrosec): Type
 			$type = TypeCombinator::union(...$types);
 		}
 
-		if ($type->isNumericString()->no() && $formatType->isNonEmptyString()->yes()) {
-			$type = TypeCombinator::union($type, new IntersectionType([
-				new StringType(), new AccessoryNonEmptyStringType(),
-			]));
-		}
-
 		return $type;
 	}
 
@@ -84,6 +78,20 @@ private function buildReturnTypeFromFormat(string $formatString, bool $useMicros
 				return $useMicrosec
 					? new IntersectionType([new StringType(), new AccessoryNonFalsyStringType(), new AccessoryNumericStringType()])
 					: new ConstantStringType('000');
+			case 'a':
+				return $this->buildConstantStringsType(['am', 'pm']);
+			case 'A':
+				return $this->buildConstantStringsType(['AM', 'PM']);
+			case 'S':
+				return $this->buildConstantStringsType(['st', 'nd', 'rd', 'th']);
+			case 'D':
+				return $this->buildConstantStringsType(['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']);
+			case 'l':
+				return $this->buildConstantStringsType(['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday']);
+			case 'M':
+				return $this->buildConstantStringsType(['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec']);
+			case 'F':
+				return $this->buildConstantStringsType(['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December']);
 		}
 
 		$date = date($formatString);
@@ -117,4 +125,18 @@ private function buildNumericRangeType(int $min, int $max, bool $zeroPad): Type
 		return new UnionType($types);
 	}
 
+	/**
+	 * @param list<string> $values
+	 */
+	private function buildConstantStringsType(array $values): Type
+	{
+		$types = [];
+
+		foreach ($values as $value) {
+			$types[] = new ConstantStringType($value);
+		}
+
+		return new UnionType($types);
+	}
+
 }

tests/PHPStan/Analyser/nsrt/bug-15537.php

@@ -0,0 +1,29 @@
+<?php declare(strict_types = 1);
+
+namespace Bug15537;
+
+use DateTimeImmutable;
+use function PHPStan\Testing\assertType;
+
+/**
+ * @param 'am'|'pm'|'AM'|'PM' $meridiem
+ */
+function translate_meridiem( string $meridiem ): string {
+	return $meridiem;
+}
+
+function test( DateTimeImmutable $datetime, int $timestamp ): void {
+	echo translate_meridiem( $datetime->format( 'a' ) );
+	echo translate_meridiem( date( 'A', $timestamp ) );
+
+	assertType( "'am'|'pm'", $datetime->format( 'a' ) );
+	assertType( "'AM'|'PM'", $datetime->format( 'A' ) );
+	assertType( "'nd'|'rd'|'st'|'th'", $datetime->format( 'S' ) );
+	assertType( "'Fri'|'Mon'|'Sat'|'Sun'|'Thu'|'Tue'|'Wed'", $datetime->format( 'D' ) );
+	assertType( "'Friday'|'Monday'|'Saturday'|'Sunday'|'Thursday'|'Tuesday'|'Wednesday'", $datetime->format( 'l' ) );
+	assertType( "'Apr'|'Aug'|'Dec'|'Feb'|'Jan'|'Jul'|'Jun'|'Mar'|'May'|'Nov'|'Oct'|'Sep'", $datetime->format( 'M' ) );
+	assertType( "'April'|'August'|'December'|'February'|'January'|'July'|'June'|'March'|'May'|'November'|'October'|'September'", $datetime->format( 'F' ) );
+
+	assertType( "'am'|'pm'", date( 'a', $timestamp ) );
+	assertType( "'Friday'|'Monday'|'Saturday'|'Sunday'|'Thursday'|'Tuesday'|'Wednesday'", date_format( $datetime, 'l' ) );
+}

tests/PHPStan/Analyser/nsrt/date-format.php

@@ -7,39 +7,39 @@
 function (string $s): void {
 	assertType('\'\'', date(''));
 	assertType('string', date($s));
-	assertType('non-falsy-string', date('D'));
+	assertType("'Fri'|'Mon'|'Sat'|'Sun'|'Thu'|'Tue'|'Wed'", date('D'));
 	assertType('numeric-string', date('Y'));
 	assertType('numeric-string', date('Ghi'));
 };
 
 function (\DateTime $dt, string $s): void {
 	assertType('\'\'', date_format($dt, ''));
 	assertType('string', date_format($dt, $s));
-	assertType('non-falsy-string', date_format($dt, 'D'));
+	assertType("'Fri'|'Mon'|'Sat'|'Sun'|'Thu'|'Tue'|'Wed'", date_format($dt, 'D'));
 	assertType('numeric-string', date_format($dt, 'Y'));
 	assertType('numeric-string', date_format($dt, 'Ghi'));
 };
 
 function (\DateTimeInterface $dt, string $s): void {
 	assertType('\'\'', $dt->format(''));
 	assertType('string', $dt->format($s));
-	assertType('non-falsy-string', $dt->format('D'));
+	assertType("'Fri'|'Mon'|'Sat'|'Sun'|'Thu'|'Tue'|'Wed'", $dt->format('D'));
 	assertType('numeric-string', $dt->format('Y'));
 	assertType('numeric-string', $dt->format('Ghi'));
 };
 
 function (\DateTime $dt, string $s): void {
 	assertType('\'\'', $dt->format(''));
 	assertType('string', $dt->format($s));
-	assertType('non-falsy-string', $dt->format('D'));
+	assertType("'Fri'|'Mon'|'Sat'|'Sun'|'Thu'|'Tue'|'Wed'", $dt->format('D'));
 	assertType('numeric-string', $dt->format('Y'));
 	assertType('numeric-string', $dt->format('Ghi'));
 };
 
 function (\DateTimeImmutable $dt, string $s): void {
 	assertType('\'\'', $dt->format(''));
 	assertType('string', $dt->format($s));
-	assertType('non-falsy-string', $dt->format('D'));
+	assertType("'Fri'|'Mon'|'Sat'|'Sun'|'Thu'|'Tue'|'Wed'", $dt->format('D'));
 	assertType('numeric-string', $dt->format('Y'));
 	assertType('numeric-string', $dt->format('Ghi'));
 };