Found while verifying #654.
See this example:

Hit Ctrl+Space at that cursor position. The following exception is thrown:
eclipse.buildId=4.7.3.M20180330-0640
java.version=1.8.0_181
java.vendor=Oracle Corporation
BootLoader constants: OS=win32, ARCH=x86_64, WS=win32, NL=it_IT
Command-line arguments: -os win32 -ws win32 -arch x86_64 -data file:/D:/ws/47/DCS-FATTURAPA-trunk/
org.eclipse.jdt.ui
Warning
Wed Aug 29 09:26:40 CEST 2018
The 'Groovy Code Completions' proposal computer from the 'org.codehaus.groovy.eclipse.codeassist.completion' plug-in did not complete normally. The extension has thrown a runtime exception.
java.lang.IllegalArgumentException
at org.eclipse.jdt.internal.codeassist.InternalCompletionProposal.setReplaceRange(InternalCompletionProposal.java:730)
at org.codehaus.groovy.eclipse.codeassist.processors.GroovyProposalTypeSearchRequestor.proposeType(GroovyProposalTypeSearchRequestor.java:467)
at org.codehaus.groovy.eclipse.codeassist.processors.GroovyProposalTypeSearchRequestor.processAcceptedTypes(GroovyProposalTypeSearchRequestor.java:435)
at org.codehaus.groovy.eclipse.codeassist.processors.TypeCompletionProcessor.generateProposals(TypeCompletionProcessor.java:133)
at org.codehaus.groovy.eclipse.codeassist.requestor.GroovyCompletionProposalComputer.computeCompletionProposals(GroovyCompletionProposalComputer.java:215)
at org.eclipse.jdt.internal.ui.text.java.CompletionProposalComputerDescriptor.computeCompletionProposals(CompletionProposalComputerDescriptor.java:333)
at org.eclipse.jdt.internal.ui.text.java.CompletionProposalCategory.computeCompletionProposals(CompletionProposalCategory.java:337)
at org.eclipse.jdt.internal.ui.text.java.ContentAssistProcessor.collectProposals(ContentAssistProcessor.java:331)
at org.eclipse.jdt.internal.ui.text.java.ContentAssistProcessor.computeCompletionProposals(ContentAssistProcessor.java:288)
at org.eclipse.jface.text.contentassist.ContentAssistant$3.run(ContentAssistant.java:2007)
at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
at org.eclipse.jface.text.contentassist.ContentAssistant.computeCompletionProposals(ContentAssistant.java:2004)
at org.eclipse.jface.text.contentassist.CompletionProposalPopup.computeProposals(CompletionProposalPopup.java:560)
at org.eclipse.jface.text.contentassist.CompletionProposalPopup.access$7(CompletionProposalPopup.java:555)
at org.eclipse.jface.text.contentassist.CompletionProposalPopup$2.run(CompletionProposalPopup.java:489)
at org.eclipse.swt.custom.BusyIndicator.showWhile(BusyIndicator.java:70)
at org.eclipse.jface.text.contentassist.CompletionProposalPopup.showProposals(CompletionProposalPopup.java:482)
at org.eclipse.jface.text.contentassist.ContentAssistant.showPossibleCompletions(ContentAssistant.java:1823)
at org.eclipse.jdt.internal.ui.javaeditor.CompilationUnitEditor$AdaptedSourceViewer.doOperation(CompilationUnitEditor.java:184)
at org.eclipse.ui.texteditor.ContentAssistAction$1.run(ContentAssistAction.java:84)
at org.eclipse.swt.custom.BusyIndicator.showWhile(BusyIndicator.java:70)
at org.eclipse.ui.texteditor.ContentAssistAction.run(ContentAssistAction.java:81)
at org.eclipse.jface.action.Action.runWithEvent(Action.java:473)
at org.eclipse.jface.commands.ActionHandler.execute(ActionHandler.java:118)
at org.eclipse.ui.internal.handlers.E4HandlerProxy.execute(E4HandlerProxy.java:92)
at sun.reflect.GeneratedMethodAccessor63.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.eclipse.e4.core.internal.di.MethodRequestor.execute(MethodRequestor.java:55)
at org.eclipse.e4.core.internal.di.InjectorImpl.invokeUsingClass(InjectorImpl.java:305)
at org.eclipse.e4.core.internal.di.InjectorImpl.invoke(InjectorImpl.java:239)
at org.eclipse.e4.core.contexts.ContextInjectionFactory.invoke(ContextInjectionFactory.java:132)
at org.eclipse.e4.core.commands.internal.HandlerServiceHandler.execute(HandlerServiceHandler.java:152)
at org.eclipse.core.commands.Command.executeWithChecks(Command.java:494)
at org.eclipse.core.commands.ParameterizedCommand.executeWithChecks(ParameterizedCommand.java:487)
at org.eclipse.e4.core.commands.internal.HandlerServiceImpl.executeHandler(HandlerServiceImpl.java:210)
at org.eclipse.e4.ui.bindings.keys.KeyBindingDispatcher.executeCommand(KeyBindingDispatcher.java:287)
at org.eclipse.e4.ui.bindings.keys.KeyBindingDispatcher.press(KeyBindingDispatcher.java:527)
at org.eclipse.e4.ui.bindings.keys.KeyBindingDispatcher.processKeyEvent(KeyBindingDispatcher.java:577)
at org.eclipse.e4.ui.bindings.keys.KeyBindingDispatcher.filterKeySequenceBindings(KeyBindingDispatcher.java:385)
at org.eclipse.e4.ui.bindings.keys.KeyBindingDispatcher.access$0(KeyBindingDispatcher.java:331)
at org.eclipse.e4.ui.bindings.keys.KeyBindingDispatcher$KeyDownFilter.handleEvent(KeyBindingDispatcher.java:88)
at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:86)
at org.eclipse.swt.widgets.Display.filterEvent(Display.java:1271)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1078)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1103)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1088)
at org.eclipse.swt.widgets.Widget.sendKeyEvent(Widget.java:1130)
at org.eclipse.swt.widgets.Widget.sendKeyEvent(Widget.java:1126)
at org.eclipse.swt.widgets.Widget.wmChar(Widget.java:1547)
at org.eclipse.swt.widgets.Control.WM_CHAR(Control.java:4962)
at org.eclipse.swt.widgets.Canvas.WM_CHAR(Canvas.java:364)
at org.eclipse.swt.widgets.Control.windowProc(Control.java:4843)
at org.eclipse.swt.widgets.Canvas.windowProc(Canvas.java:359)
at org.eclipse.swt.widgets.Display.windowProc(Display.java:5178)
at org.eclipse.swt.internal.win32.OS.DispatchMessageW(Native Method)
at org.eclipse.swt.internal.win32.OS.DispatchMessage(OS.java:2560)
at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3815)
at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine$5.run(PartRenderingEngine.java:1150)
at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:336)
at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine.run(PartRenderingEngine.java:1039)
at org.eclipse.e4.ui.internal.workbench.E4Workbench.createAndRunUI(E4Workbench.java:153)
at org.eclipse.ui.internal.Workbench.lambda$3(Workbench.java:680)
at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:336)
at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:594)
at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:148)
at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:151)
at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:196)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:134)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:104)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:388)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:243)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:653)
at org.eclipse.equinox.launcher.Main.basicRun(Main.java:590)
at org.eclipse.equinox.launcher.Main.run(Main.java:1499)
Found while verifying #654.
See this example:
Hit Ctrl+Space at that cursor position. The following exception is thrown: