Revert ConstantArrayTypeBuilder changes, decompose union offsets in callers instead

Move the union offset decomposition entirely into flipArray() and
fillKeysArray() — each individual scalar type is passed to the builder
separately with optional=true, avoiding both the value-replacement
and partial-match degradation issues without modifying the builder.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phpstan-bot

src/Type/Constant/ConstantArrayType.php

@@ -969,9 +969,19 @@ public function fillKeysArray(Type $valueType): Type
 					return $stringKeyType;
 				}
 
-				$builder->setOffsetValueType($stringKeyType, $valueType, $this->isOptionalKey($i) || count($stringKeyType->getConstantScalarTypes()) > 1);
+				$offsetType = $stringKeyType;
 			} else {
-				$builder->setOffsetValueType($keyType, $valueType, $this->isOptionalKey($i) || count($keyType->getConstantScalarTypes()) > 1);
+				$offsetType = $keyType;
+			}
+
+			$isOptional = $this->isOptionalKey($i);
+			$scalarTypes = $offsetType->getConstantScalarTypes();
+			if (count($scalarTypes) > 1) {
+				foreach ($scalarTypes as $scalarType) {
+					$builder->setOffsetValueType($scalarType, $valueType, true);
+				}
+			} else {
+				$builder->setOffsetValueType($offsetType, $valueType, $isOptional);
 			}
 		}
 
@@ -984,12 +994,25 @@ public function flipArray(): Type
 
 		foreach ($this->keyTypes as $i => $keyType) {
 			$valueType = $this->valueTypes[$i];
-			$offsetType = $valueType->toArrayKey();
-			$builder->setOffsetValueType(
-				$offsetType,
-				$keyType,
-				$this->isOptionalKey($i) || count($offsetType->getConstantScalarTypes()) > 1,
-			);
+			$isOptional = $this->isOptionalKey($i);
+			$arrayKeyType = $valueType->toArrayKey();
+
+			$scalarTypes = $arrayKeyType->getConstantScalarTypes();
+			if (count($scalarTypes) > 1) {
+				foreach ($scalarTypes as $scalarType) {
+					$builder->setOffsetValueType(
+						$scalarType,
+						$keyType,
+						true,
+					);
+				}
+			} else {
+				$builder->setOffsetValueType(
+					$arrayKeyType,
+					$keyType,
+					$isOptional,
+				);
+			}
 		}
 
 		return $builder->getArray();

src/Type/Constant/ConstantArrayTypeBuilder.php

@@ -270,7 +270,6 @@ public function setOffsetValueType(?Type $offsetType, Type $valueType, bool $opt
 			if (count($scalarTypes) > 0 && count($scalarTypes) < self::ARRAY_COUNT_LIMIT) {
 				$match = true;
 				$hasMatch = false;
-				$unmatchedScalarTypes = [];
 				$valueTypes = $this->valueTypes;
 				foreach ($scalarTypes as $scalarType) {
 					$offsetMatch = false;
@@ -294,7 +293,6 @@ public function setOffsetValueType(?Type $offsetType, Type $valueType, bool $opt
 						continue;
 					}
 
-					$unmatchedScalarTypes[] = $scalarType;
 					$match = false;
 				}
 
@@ -303,10 +301,8 @@ public function setOffsetValueType(?Type $offsetType, Type $valueType, bool $opt
 					return;
 				}
 
-				if (!$hasMatch || $optional) {
-					$this->valueTypes = $valueTypes;
-
-					foreach ($unmatchedScalarTypes as $scalarType) {
+				if (!$hasMatch) {
+					foreach ($scalarTypes as $scalarType) {
 						$this->keyTypes[] = $scalarType;
 						$this->valueTypes[] = $valueType;
 						$this->optionalKeys[] = count($this->keyTypes) - 1;