Implement Any?.String using java.lang.String.valueOf

This is how kotlinc does it, and doesn't involve an unchecked null deref like the existing use of Object.toString.

java/kotlin-extractor/src/main/kotlin/KotlinFileExtractor.kt

@@ -1102,6 +1102,25 @@ open class KotlinFileExtractor(
         return result
     }
 
+    val javaLangString by lazy {
+        val result = pluginContext.referenceClass(FqName("java.lang.String"))?.owner
+        result?.let { extractExternalClassLater(it) }
+        result
+    }
+
+    val stringValueOfObjectMethod by lazy {
+        val result = javaLangString?.declarations?.find {
+            it is IrFunction &&
+            it.name.asString() == "valueOf" &&
+            it.valueParameters.size == 1 &&
+            it.valueParameters[0].type == pluginContext.irBuiltIns.anyNType
+        } as IrFunction?
+        if (result == null) {
+            logger.error("Couldn't find declaration java.lang.String.valueOf(Object)")
+        }
+        result
+    }
+
     fun extractCall(c: IrCall, callable: Label<out DbCallable>, parent: Label<out DbExprparent>, idx: Int, enclosingStmt: Label<out DbStmt>) {
         with("call", c) {
             fun isFunction(pkgName: String, className: String, fName: String, hasQuestionMark: Boolean? = false): Boolean {
@@ -1394,13 +1413,9 @@ open class KotlinFileExtractor(
                     logger.errorElement("Unhandled builtin", c)
                 }
                 isFunction("kotlin", "Any", "toString", true) -> {
-                    // TODO: this is not correct. `a.toString()` becomes `(a?:"\"null\"").toString()`
+                    stringValueOfObjectMethod?.let {
-                    val func = pluginContext.irBuiltIns.anyType.classOrNull?.owner?.declarations?.find { it is IrFunction && it.name.asString() == "toString" }
+                        extractRawMethodAccess(it, c, callable, parent, idx, enclosingStmt, listOf(c.extensionReceiver), null, null)
-                    if (func == null) {
-                        logger.errorElement("Couldn't find toString function", c)
-                        return
                     }
-                    extractMethodAccess(func as IrFunction)
                 }
                 isBuiltinCallKotlin(c, "enumValues") -> {
                     extractSpecialEnumFunction("values")

java/ql/test/kotlin/library-tests/exprs/PrintAst.expected

@@ -976,7 +976,8 @@ exprs.kt:
 #  200|       5: [BlockStmt] { ... }
 #  202|         0: [LocalVariableDeclStmt] var ...;
 #  202|           1: [LocalVariableDeclExpr] a
-#  202|             0: [MethodAccess] toString(...)
+#  202|             0: [MethodAccess] valueOf(...)
+#  202|               -1: [TypeAccess] String
 #  202|               0: [VarAccess] aa
 #  203|         1: [LocalVariableDeclStmt] var ...;
 #  203|           1: [LocalVariableDeclExpr] b0

java/ql/test/kotlin/library-tests/exprs/exprs.expected

@@ -579,7 +579,8 @@
 | exprs.kt:199:1:212:1 | <obinit>(...) | exprs.kt:199:1:212:1 | Class2 | MethodAccess |
 | exprs.kt:202:9:202:29 | a | exprs.kt:200:5:211:5 | x | LocalVariableDeclExpr |
 | exprs.kt:202:17:202:18 | aa | exprs.kt:200:5:211:5 | x | VarAccess |
-| exprs.kt:202:20:202:29 | toString(...) | exprs.kt:200:5:211:5 | x | MethodAccess |
+| exprs.kt:202:20:202:29 | String | exprs.kt:200:5:211:5 | x | TypeAccess |
+| exprs.kt:202:20:202:29 | valueOf(...) | exprs.kt:200:5:211:5 | x | MethodAccess |
 | exprs.kt:203:9:203:27 | b0 | exprs.kt:200:5:211:5 | x | LocalVariableDeclExpr |
 | exprs.kt:203:19:203:19 | s | exprs.kt:200:5:211:5 | x | VarAccess |
 | exprs.kt:203:21:203:27 | Intrinsics | exprs.kt:200:5:211:5 | x | TypeAccess |