Make kotlinx.cinterop.convert follow C rules

backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/ir/Ir.kt

@@ -149,6 +149,16 @@ internal class KonanSymbols(context: Context, val symbolTable: SymbolTable, val
 
     val allIntegerClasses = signedIntegerClasses + unsignedIntegerClasses
 
+    val unsignedToSignedOfSameBitWidth = unsignedIntegerClasses.associate {
+        it to when (it) {
+            uByte -> byte
+            uShort -> short
+            uInt -> int
+            uLong -> long
+            else -> error(it.descriptor)
+        }
+    }
+
     val integerConversions = allIntegerClasses.flatMap { fromClass ->
         allIntegerClasses.map { toClass ->
             val name = Name.identifier("to${toClass.descriptor.name.asString().capitalize()}")

backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/lower/InteropLowering.kt

@@ -906,21 +906,20 @@ private class InteropTransformer(val context: Context, val irFile: IrFile) : IrB
                 val integerClasses = symbols.allIntegerClasses
                 val typeOperand = expression.getTypeArgument(0)!!
                 val receiverType = expression.symbol.owner.extensionReceiverParameter!!.type
-                val receiverClass = receiverType.classifierOrFail as IrClassSymbol
-                assert(receiverClass in integerClasses)
+                val source = receiverType.classifierOrFail as IrClassSymbol
+                assert(source in integerClasses)
 
                 if (typeOperand is IrSimpleType && typeOperand.classifier in integerClasses && !typeOperand.hasQuestionMark) {
-                    val typeOperandClass = typeOperand.classifier as IrClassSymbol
+                    val target = typeOperand.classifier as IrClassSymbol
+                    val valueToConvert = expression.extensionReceiver!!
 
-                    val conversion = symbols.integerConversions[receiverClass to typeOperandClass]!!.owner
-
-                    builder.irCall(conversion).apply {
-                        val valueToConvert = expression.extensionReceiver!!
-                        if (conversion.dispatchReceiverParameter != null) {
-                            dispatchReceiver = valueToConvert
-                        } else {
-                            extensionReceiver = valueToConvert
-                        }
+                    if (source in symbols.signedIntegerClasses && target in symbols.unsignedIntegerClasses) {
+                        // Default Kotlin signed-to-unsigned widening integer conversions don't follow C rules.
+                        val signedTarget = symbols.unsignedToSignedOfSameBitWidth[target]!!
+                        val widened = builder.irConvertInteger(source, signedTarget, valueToConvert)
+                        builder.irConvertInteger(signedTarget, target, widened)
+                    } else {
+                        builder.irConvertInteger(source, target, valueToConvert)
                     }
                 } else {
                     context.reportCompilationError(
@@ -987,6 +986,21 @@ private class InteropTransformer(val context: Context, val irFile: IrFile) : IrB
         }
     }
 
+    private fun IrBuilderWithScope.irConvertInteger(
+            source: IrClassSymbol,
+            target: IrClassSymbol,
+            value: IrExpression
+    ): IrExpression {
+        val conversion = symbols.integerConversions[source to target]!!
+        return irCall(conversion.owner).apply {
+            if (conversion.owner.dispatchReceiverParameter != null) {
+                dispatchReceiver = value
+            } else {
+                extensionReceiver = value
+            }
+        }
+    }
+
     private fun IrType.ensureSupportedInCallbacks(isReturnType: Boolean, reportError: (String) -> Nothing) {
         this.checkCTypeNullability(reportError)