RFC: Allow disabling coercion between JSON primitives

formats/json/commonMain/src/kotlinx/serialization/json/Json.kt

@@ -658,6 +658,8 @@ public class JsonBuilder internal constructor(json: Json) {
      */
     public var useArrayPolymorphism: Boolean = json.configuration.useArrayPolymorphism
 
+    public var allowPrimitiveCoercion: Boolean = json.configuration.allowPrimitiveCoercion
+
     /**
      * Module with contextual and polymorphic serializers to be used in the resulting [Json] instance.
      *
@@ -695,7 +697,8 @@ public class JsonBuilder internal constructor(json: Json) {
             allowStructuredMapKeys, prettyPrint, explicitNulls, prettyPrintIndent,
             coerceInputValues, useArrayPolymorphism,
             classDiscriminator, allowSpecialFloatingPointValues, useAlternativeNames,
-            namingStrategy, decodeEnumsCaseInsensitive, allowTrailingComma, allowComments, classDiscriminatorMode
+            namingStrategy, decodeEnumsCaseInsensitive, allowTrailingComma, allowComments, classDiscriminatorMode,
+            allowPrimitiveCoercion
         )
     }
 }

formats/json/commonMain/src/kotlinx/serialization/json/JsonConfiguration.kt

@@ -45,6 +45,8 @@ public class JsonConfiguration @OptIn(ExperimentalSerializationApi::class) inter
         level = DeprecationLevel.ERROR
     )
     public var classDiscriminatorMode: ClassDiscriminatorMode = ClassDiscriminatorMode.POLYMORPHIC,
+    @ExperimentalSerializationApi
+    public val allowPrimitiveCoercion: Boolean = true,
 ) {
 
     /** @suppress Dokka **/

formats/json/commonMain/src/kotlinx/serialization/json/internal/StreamingJsonDecoder.kt

@@ -25,6 +25,7 @@ internal open class StreamingJsonDecoder(
     descriptor: SerialDescriptor,
     discriminatorHolder: DiscriminatorHolder?
 ) : JsonDecoder, ChunkedDecoder, AbstractDecoder() {
+    private val coercePrimitives = json.configuration.allowPrimitiveCoercion
 
     // A mutable reference to the discriminator that have to be skipped when in optimistic phase
     // of polymorphic serialization, see `decodeSerializableValue`
@@ -273,47 +274,51 @@ internal open class StreamingJsonDecoder(
     }
 
     /*
-     * The primitives are allowed to be quoted and unquoted
-     * to simplify map key parsing and integrations with third-party API.
-     */
+         * The primitives are allowed to be quoted and unquoted
+         * to simplify map key parsing and integrations with third-party API.
+         */
     override fun decodeBoolean(): Boolean {
-        return lexer.consumeBooleanLenient()
+        return if (coercePrimitives) {
+            lexer.consumeBooleanLenient()
+        } else {
+            lexer.consumeBoolean()
+        }
     }
 
     override fun decodeByte(): Byte {
-        val value = lexer.consumeNumericLiteral()
+        val value = lexer.consumeNumericLiteral(coercePrimitives)
         // Check for overflow
         if (value != value.toByte().toLong()) lexer.fail("Failed to parse byte for input '$value'")
         return value.toByte()
     }
 
     override fun decodeShort(): Short {
-        val value = lexer.consumeNumericLiteral()
+        val value = lexer.consumeNumericLiteral(coercePrimitives)
         // Check for overflow
         if (value != value.toShort().toLong()) lexer.fail("Failed to parse short for input '$value'")
         return value.toShort()
     }
 
     override fun decodeInt(): Int {
-        val value = lexer.consumeNumericLiteral()
+        val value = lexer.consumeNumericLiteral(coercePrimitives)
         // Check for overflow
         if (value != value.toInt().toLong()) lexer.fail("Failed to parse int for input '$value'")
         return value.toInt()
     }
 
     override fun decodeLong(): Long {
-        return lexer.consumeNumericLiteral()
+        return lexer.consumeNumericLiteral(coercePrimitives)
     }
 
     override fun decodeFloat(): Float {
-        val result = lexer.parseString("float") { toFloat() }
+        val result = lexer.parseString("float", coercePrimitives) { toFloat() }
         val specialFp = json.configuration.allowSpecialFloatingPointValues
         if (specialFp || result.isFinite()) return result
         lexer.throwInvalidFloatingPointDecoded(result)
     }
 
     override fun decodeDouble(): Double {
-        val result = lexer.parseString("double") { toDouble() }
+        val result = lexer.parseString("double", coercePrimitives) { toDouble() }
         val specialFp = json.configuration.allowSpecialFloatingPointValues
         if (specialFp || result.isFinite()) return result
         lexer.throwInvalidFloatingPointDecoded(result)
@@ -374,15 +379,16 @@ internal class JsonDecoderForUnsignedTypes(
 ) : AbstractDecoder() {
     override val serializersModule: SerializersModule = json.serializersModule
     override fun decodeElementIndex(descriptor: SerialDescriptor): Int = error("unsupported")
+    private val coercePrimitves = json.configuration.allowPrimitiveCoercion
 
-    override fun decodeInt(): Int = lexer.parseString("UInt") { toUInt().toInt() }
-    override fun decodeLong(): Long = lexer.parseString("ULong") { toULong().toLong() }
-    override fun decodeByte(): Byte = lexer.parseString("UByte") { toUByte().toByte() }
-    override fun decodeShort(): Short = lexer.parseString("UShort") { toUShort().toShort() }
+    override fun decodeInt(): Int = lexer.parseString("UInt", coercePrimitves) { toUInt().toInt() }
+    override fun decodeLong(): Long = lexer.parseString("ULong", coercePrimitves) { toULong().toLong() }
+    override fun decodeByte(): Byte = lexer.parseString("UByte", coercePrimitves) { toUByte().toByte() }
+    override fun decodeShort(): Short = lexer.parseString("UShort", coercePrimitves) { toUShort().toShort() }
 }
 
-private inline fun <T> AbstractJsonLexer.parseString(expectedType: String, block: String.() -> T): T {
-    val input = consumeStringLenient()
+private inline fun <T> AbstractJsonLexer.parseString(expectedType: String, coercePrimitives: Boolean, block: String.() -> T): T {
+    val input = consumeOther(allowQuoted = coercePrimitives)
     try {
         return input.block()
     } catch (e: IllegalArgumentException) {

formats/json/commonMain/src/kotlinx/serialization/json/internal/TreeJsonDecoder.kt

@@ -85,6 +85,9 @@ private sealed class AbstractJsonTreeDecoder(
 
     private inline fun <T : Any> getPrimitiveValue(tag: String, primitiveName: String, convert: JsonPrimitive.() -> T?): T {
         val literal = cast<JsonPrimitive>(currentElement(tag), primitiveName, tag)
+        if (!json.configuration.allowPrimitiveCoercion && literal.isString) {
+            unparsedPrimitive(literal, primitiveName, tag)
+        }
         try {
             return literal.convert() ?: unparsedPrimitive(literal, primitiveName, tag)
         } catch (e: IllegalArgumentException) {

formats/json/commonMain/src/kotlinx/serialization/json/internal/lexer/AbstractJsonLexer.kt

@@ -438,14 +438,18 @@ internal abstract class AbstractJsonLexer {
     }
 
     // Allows consuming unquoted string
-    fun consumeStringLenient(): String {
+    fun consumeStringLenient(): String =
+        consumeOther(allowQuoted = true)
+
+    fun consumeOther(allowQuoted: Boolean): String {
+        // TODO this string peeking stuff might break things...
         if (peekedString != null) {
             return takePeeked()
         }
         var current = skipWhitespaces()
         if (current >= source.length || current == -1) fail("EOF", current)
         val token = charToTokenClass(source[current])
-        if (token == TC_STRING) {
+        if (allowQuoted && token == TC_STRING) {
             return consumeString()
         }
 
@@ -587,15 +591,18 @@ internal abstract class AbstractJsonLexer {
         throw JsonDecodingException(position, message + " at path: " + path.getPath() + hintMessage, source)
     }
 
-    fun consumeNumericLiteral(): Long {
+    fun consumeNumericLiteral(): Long =
+        consumeNumericLiteral(coercePrimitives = true)
+
+    fun consumeNumericLiteral(coercePrimitives: Boolean): Long {
         /*
          * This is an optimized (~40% for numbers) version of consumeString().toLong()
          * that doesn't allocate and also doesn't support any radix but 10
          */
         var current = skipWhitespaces()
         current = prefetchOrEof(current)
         if (current >= source.length || current == -1) fail("EOF")
-        val hasQuotation = if (source[current] == STRING) {
+        val hasQuotation = if (coercePrimitives && source[current] == STRING) {
             // Check it again
             // not sure if should call ensureHaveChars() because threshold is far greater than chars count in MAX_LONG
             if (++current == source.length) fail("EOF")