[IR] Change the return type of getFunctionDeclaration to be nullable

#KT-64807

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/lower/inline/SaveInlineFunctionsBeforeInlining.kt

@@ -37,8 +37,8 @@ internal class JsInlineFunctionResolver(context: JsIrBackendContext) : InlineFun
         return symbol == enumEntriesIntrinsic || super.shouldExcludeFunctionFromInlining(symbol)
     }
 
-    override fun getFunctionDeclaration(symbol: IrFunctionSymbol): IrFunction {
+    override fun getFunctionDeclaration(symbol: IrFunctionSymbol): IrFunction? {
-        val function = super.getFunctionDeclaration(symbol)
+        val function = super.getFunctionDeclaration(symbol) ?: return null
         return inlineFunctionsBeforeInlining[function] ?: return function
     }
 }

compiler/ir/ir.inline/src/org/jetbrains/kotlin/ir/inline/FunctionInlining.kt

@@ -39,7 +39,10 @@ import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.util.OperatorNameConventions
 
 abstract class InlineFunctionResolver {
-    open fun getFunctionDeclaration(symbol: IrFunctionSymbol): IrFunction = symbol.owner
+    open fun getFunctionDeclaration(symbol: IrFunctionSymbol): IrFunction? {
+        return symbol.owner
+    }
+
     open fun shouldExcludeFunctionFromInlining(symbol: IrFunctionSymbol): Boolean {
         return Symbols.isLateinitIsInitializedPropertyGetter(symbol) || Symbols.isTypeOfIntrinsic(symbol)
     }
@@ -56,7 +59,7 @@ fun IrFunction.isBuiltInSuspendCoroutineUninterceptedOrReturn(): Boolean =
     )
 
 open class InlineFunctionResolverReplacingCoroutineIntrinsics(open val context: CommonBackendContext) : InlineFunctionResolver() {
-    override fun getFunctionDeclaration(symbol: IrFunctionSymbol): IrFunction {
+    override fun getFunctionDeclaration(symbol: IrFunctionSymbol): IrFunction? {
         val function = symbol.owner
         // TODO: Remove these hacks when coroutine intrinsics are fixed.
         return when {
@@ -106,7 +109,7 @@ class FunctionInlining(
 
         val target = (callee as? IrSimpleFunction)?.resolveFakeOverride() ?: callee
         val actualCallee = inlineFunctionResolver.getFunctionDeclaration(target.symbol)
-        if (actualCallee.body == null) {
+        if (actualCallee?.body == null) {
             return expression
         }
 
@@ -382,7 +385,7 @@ class FunctionInlining(
                 return inlineFunctionReference(
                     irCall, irFunctionReference,
                     if (inlinedFunction.needsInlining)
-                        inlineFunctionResolver.getFunctionDeclaration(inlinedFunction.symbol)
+                        inlineFunctionResolver.getFunctionDeclaration(inlinedFunction.symbol) ?: inlinedFunction
                     else inlinedFunction
                 )
             }

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

@@ -32,8 +32,8 @@ internal class InlineFunctionsSupport(mapping: NativeMapping) {
 
 // TODO: This is a bit hacky. Think about adopting persistent IR ideas.
 internal class NativeInlineFunctionResolver(override val context: Context, val generationState: NativeGenerationState) : InlineFunctionResolverReplacingCoroutineIntrinsics(context) {
-    override fun getFunctionDeclaration(symbol: IrFunctionSymbol): IrFunction {
+    override fun getFunctionDeclaration(symbol: IrFunctionSymbol): IrFunction? {
-        val function = super.getFunctionDeclaration(symbol)
+        val function = super.getFunctionDeclaration(symbol) ?: return null
 
         generationState.inlineFunctionOrigins[function]?.let { return it.irFunction }