Fix completion that relies on pseudobinding in script blocks (PowerShell#25122)

Author: MartinGC94

src/System.Management.Automation/engine/CommandCompletion/PseudoParameterBinder.cs

@@ -1406,7 +1406,6 @@ private CommandProcessorBase PrepareFromAst(ExecutionContext context, out string
             }
 
             ast.Visit(exportVisitor);
-
             CommandProcessorBase commandProcessor = null;
 
             resolvedCommandName = _commandAst.GetCommandName();

src/System.Management.Automation/engine/Modules/ScriptAnalysis.cs

@@ -277,10 +277,22 @@ public override AstVisitAction VisitAssignmentStatement(AssignmentStatementAst a
         //  - Exporting module members
         public override AstVisitAction VisitCommand(CommandAst commandAst)
         {
-            string commandName =
-                commandAst.GetCommandName() ??
-                GetSafeValueVisitor.GetSafeValue(commandAst.CommandElements[0], null, GetSafeValueVisitor.SafeValueContext.ModuleAnalysis) as string;
+            string commandName = commandAst.GetCommandName();
+            if (commandName is null)
+            {
+                // GetCommandName only works if the name is a string constant. GetSafeValueVistor can evaluate some safe dynamic expressions
+                try
+                {
+                    commandName = GetSafeValueVisitor.GetSafeValue(commandAst.CommandElements[0], null, GetSafeValueVisitor.SafeValueContext.ModuleAnalysis) as string;
+                }
+                catch (ParseException)
+                {
+                    // The script is invalid so we can't use GetSafeValue to get the name either.
+                }
+            }
 
+            // We couldn't get the name of the command. Either it's an anonymous scriptblock: & {"Some script"}
+            // Or it's a dynamic expression we couldn't safely resolve.
             if (commandName == null)
                 return AstVisitAction.SkipChildren;
 

test/powershell/Host/TabCompletion/TabCompletion.Tests.ps1

@@ -837,6 +837,11 @@ using `
         $res.CompletionMatches.CompletionText | Should -Contain "GetType"
     }
 
+    It 'Should complete variable member inferred from command inside scriptblock' {
+        $res = TabExpansion2 -inputScript '& {(New-Guid).'
+        $res.CompletionMatches.Count | Should -BeGreaterThan 0
+    }
+
     It 'Should not complete void instance members' {
         $res = TabExpansion2 -inputScript '([void]("")).'
         $res.CompletionMatches | Should -BeNullOrEmpty