Pass non-null arguments to avoid nullability warning

src/Compilers/CSharp/Portable/Binder/Imports.cs

@@ -179,7 +179,7 @@ internal string GetDebuggerDisplay()
 
                             // construct the alias sym with the binder for which we are building imports. That
                             // way the alias target can make use of extern alias definitions.
-                            usingAliases.Add(identifierValueText, new AliasAndUsingDirective(new AliasSymbol(usingsBinder, usingDirective), usingDirective));
+                            usingAliases.Add(identifierValueText, new AliasAndUsingDirective(new AliasSymbol(usingsBinder, usingDirective.Name, usingDirective.Alias), usingDirective));
                         }
                     }
                     else

src/Compilers/CSharp/Portable/Compilation/SyntaxTreeSemanticModel.cs

@@ -1858,7 +1858,7 @@ public override ILabelSymbol GetDeclaredSymbol(SwitchLabelSyntax declarationSynt
             else
             {
                 // Case: multiple aliases, not the first (see DevDiv #9368)
-                return new AliasSymbol(binder, declarationSyntax).GetPublicSymbol();
+                return new AliasSymbol(binder, declarationSyntax.Name, declarationSyntax.Alias).GetPublicSymbol();
             }
         }
 

src/Compilers/CSharp/Portable/Symbols/AliasSymbol.cs

@@ -75,12 +75,13 @@ private AliasSymbol(Binder binder, SyntaxToken aliasName)
             _binder = binder;
         }
 
-        internal AliasSymbol(Binder binder, UsingDirectiveSyntax syntax)
-#nullable disable // Can 'syntax.Alias' be null here? https://github.com/dotnet/roslyn/issues/39166
-            : this(binder, syntax.Alias.Name.Identifier)
-#nullable enable
+        internal AliasSymbol(Binder binder, NameSyntax name, NameEqualsSyntax alias)
+            : this(binder, alias.Name.Identifier)
         {
-            _aliasTargetName = syntax.Name;
+            Debug.Assert(name.Parent.IsKind(SyntaxKind.UsingDirective));
+            Debug.Assert(name.Parent == alias.Parent);
+
+            _aliasTargetName = name;
         }
 
         internal AliasSymbol(Binder binder, ExternAliasDirectiveSyntax syntax)