Report types needed to form aliased type

When substituting concrete types into alias template, the resulting type
should be well-formed even if used in forward-declarable context. I. e.,
the full template argument type is needed if used e. g. in nested name
specifier or in `sizeof(T)` expression. The resulting canonical type may
still be forward-declarable.

To ensure this, the aliased type is scanned by
`InstantiatedTemplateVisitor`. When the alias is used in fwd-declarable
context, `IwyuBaseAstVisitor::VisitTemplateSpecializationType` calls
`set_in_forward_declare_context(true)` on the current node prior to that
scan, hence the resulting type should not be reported, only its
internals should be (because of the calls of
`set_in_forward_declare_context(false)` during the traversal). When
the aliased type is fully used, there may be some redundancy
in reporting.

If someone considers that such a scheme is fragile, or doesn't work
by some reason, a `set_in_forward_declare_context(true)` call may be
placed into the `if (type->isTypeAlias())` statement body inside
`TraverseTemplateSpecializationTypeHelper`.

iwyu.cc

@@ -3090,6 +3090,9 @@ class InstantiatedTemplateVisitor
     if (CanIgnoreCurrentASTNode())
       return true;
 
+    if (type->isTypeAlias())
+      return TraverseType(type->getAliasedType());
+
     // Skip the template traversal if this occurrence of the template name is
     // just a class qualifier for an out of line method, as opposed to an object
     // instantiation, where the templated code would need to be inspected.
@@ -4219,7 +4222,7 @@ class IwyuAstConsumer
 
     // If we're not in a forward-declare context, use of a template
     // specialization requires having the full type information.
-    if (!CanForwardDeclareType(current_ast_node())) {
+    if (!CanForwardDeclareType(current_ast_node()) || type->isTypeAlias()) {
       const TemplateInstantiationData data = GetTplInstData(type);
       instantiated_template_visitor_.ScanInstantiatedType(
           current_ast_node(), data.resugar_map, data.provided_types);

tests/cxx/alias_template.cc

@@ -70,6 +70,26 @@ using AliasNested2 = FullUseTemplateArgInSizeof<FullUseTemplateArgInSizeof<T>>;
 // IWYU: IndirectClass is...*indirect.h
 AliasNested2<IndirectClass> aliasNested2;
 
+template <typename T>
+using UsingArgInternals = decltype(T::a);
+
+// IWYU: IndirectClass needs a declaration
+// IWYU: IndirectClass is...*indirect.h
+UsingArgInternals<IndirectClass> aliased_int;
+// Full type is needed even in fwd-decl context.
+// IWYU: IndirectClass needs a declaration
+// IWYU: IndirectClass is...*indirect.h
+UsingArgInternals<IndirectClass>* p_int = nullptr;
+
+template <typename T>
+struct TplWithUsingArgInternals {
+  UsingArgInternals<T>* p = nullptr;
+};
+
+// IWYU: IndirectClass needs a declaration
+// IWYU: IndirectClass is...*indirect.h
+TplWithUsingArgInternals<IndirectClass> twuai;
+
 /**** IWYU_SUMMARY
 
 tests/cxx/alias_template.cc should add these lines: