ASTParser.createAST looses ability to resolve some bindings (#1942)

Having root unit set in the lookupEnvironment (when focusing on a single
unit) can be necessary for further binding resolution.

Fixes #1915

org.eclipse.jdt.core.tests.model/src/org/eclipse/jdt/core/tests/dom/ASTConverter15Test.java

@@ -27,6 +27,7 @@
 
 import org.eclipse.core.resources.IFolder;
 import org.eclipse.core.resources.IResource;
+import org.eclipse.core.resources.ResourcesPlugin;
 import org.eclipse.core.runtime.CoreException;
 import org.eclipse.jdt.core.BindingKey;
 import org.eclipse.jdt.core.IAnnotation;
@@ -7891,6 +7892,29 @@ public void test0238_2() throws JavaModelException {
 		assertEquals("wrong size", 2, methodBindings.length);
 	}
 
+	/*
+	 * https://github.com/eclipse-jdt/eclipse.jdt.core/issues/1915
+	 */
+	public void test0238_ASTParser() throws JavaModelException, CoreException {
+		this.workingCopy = getWorkingCopy("/Converter15/src/test0238/X.java",
+			"""
+			package test0238;
+			public class X extends A {
+			}
+			""", true /*resolve*/);
+		ASTParser parser = ASTParser.newParser(AST.getJLSLatest());
+		parser.setBindingsRecovery(true);
+		parser.setResolveBindings(true);
+		parser.setStatementsRecovery(true);
+		parser.setSource(this.workingCopy);
+		parser.setProject(JavaCore.create(ResourcesPlugin.getWorkspace().getRoot().getProject("Converter15")));
+		CompilationUnit unit = (CompilationUnit) parser.createAST(null);
+		TypeDeclaration typeDeclaration = (TypeDeclaration) unit.types().get(0);
+		ITypeBinding superTypeBinding = typeDeclaration.resolveBinding().getSuperclass();
+		IMethodBinding[] methodBindings = superTypeBinding.getDeclaredMethods();
+		assertEquals("wrong size", 2, methodBindings.length);
+	}
+
 	/*
 	 * https://bugs.eclipse.org/bugs/show_bug.cgi?id=173338
 	 */

org.eclipse.jdt.core/dom/org/eclipse/jdt/core/dom/CompilationUnitResolver.java

@@ -816,6 +816,10 @@ public static CompilationUnitDeclaration resolve(
 				CancelableNameEnvironment cancelableNameEnvironment = (CancelableNameEnvironment) environment;
 				cancelableNameEnvironment.printTimeSpent();
 			}
+			if (unit != null && unit.scope != null && unit.scope.environment != null
+				&& unit.scope.environment.unitBeingCompleted == null) {
+				unit.scope.environment.unitBeingCompleted = unit;
+			}
 			return unit;
 		} finally {
 			if (environment != null) {