fix build failure surfacing as

  _ cannot be resolved. It is indirectly referenced _

+ avoid singleton SplitPackageBinding
+ don't loop over array of non-unique modules
+ assert (if -ea is set) that plain package has no split child

org.eclipse.jdt.core.compiler.batch/src/org/eclipse/jdt/internal/compiler/lookup/LookupEnvironment.java

@@ -795,7 +795,7 @@ private PackageBinding computePackageFrom(char[][] constantPoolName, boolean isM
 			if (this.useModuleSystem) {
 				if (this.module.isUnnamed()) {
 					char[][] currentCompoundName = CharOperation.arrayConcat(parent.compoundName, constantPoolName[i]);
-					char[][] declaringModules = ((IModuleAwareNameEnvironment) this.nameEnvironment).getModulesDeclaringPackage(
+					char[][] declaringModules = ((IModuleAwareNameEnvironment) this.nameEnvironment).getUniqueModulesDeclaringPackage(
 							currentCompoundName, ModuleBinding.ANY);
 					List<PackageBinding> bindings = new ArrayList<>();
 					if (declaringModules != null) {
@@ -2403,7 +2403,7 @@ public Binding getInaccessibleBinding(char[][] compoundName, ModuleBinding clien
 		int length = compoundName.length;
 		for (int j=length; j>0; j--) {
 			char[][] candidateName = CharOperation.subarray(compoundName, 0, j);
-			char[][] moduleNames = moduleEnv.getModulesDeclaringPackage(candidateName, ModuleBinding.ANY);
+			char[][] moduleNames = moduleEnv.getUniqueModulesDeclaringPackage(candidateName, ModuleBinding.ANY);
 			if (moduleNames != null) {
 				// in some module a package named candidateName exists, verify observability & inaccessibility:
 				PackageBinding inaccessiblePackage = null;

org.eclipse.jdt.core.compiler.batch/src/org/eclipse/jdt/internal/compiler/lookup/PlainPackageBinding.java

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2019 GK Software SE, and others.
+ * Copyright (c) 2019, 2024 GK Software SE, and others.
  *
  * This program and the accompanying materials
  * are made available under the terms of the Eclipse Public License 2.0
@@ -44,4 +44,10 @@ public PlainPackageBinding getIncarnation(ModuleBinding moduleBinding) {
 			return this;
 		return null;
 	}
+
+	@Override
+	PackageBinding addPackage(PackageBinding element, ModuleBinding module) {
+		assert element instanceof PlainPackageBinding : "PlainPackageBinding cannot be parent of split " + element; //$NON-NLS-1$
+		return super.addPackage(element, module);
+	}
 }

org.eclipse.jdt.core.compiler.batch/src/org/eclipse/jdt/internal/compiler/lookup/SplitPackageBinding.java

@@ -81,6 +81,8 @@ public static PackageBinding combineAll(List<PackageBinding> bindings, ModuleBin
 							split.add(packageBinding);
 					}
 				}
+				if (split.incarnations.size() == 1) // we don't want singleton SplitPackageBinding
+					return split.incarnations.iterator().next(); // simply peel the only incarnation
 				return split;
 			}
 		}