Handle sugared template specs in template args

Non-sugared template specialization type template arguments are already
analyzed due to 'TraverseType' call inside
'InstantiatedTemplateVisitor::TraverseSubstTemplateTypeParmTypeHelper'
and subsequent 'TraverseTemplateSpecializationTypeHelper' call.
Components of sugared template specialization types are added into
resugar_map, otherwise their template arguments would not be reported.
Test case has been extended.

iwyu.cc

@@ -2682,8 +2682,7 @@ class IwyuBaseAstVisitor : public BaseAstVisitor<Derived> {
   // void fn(const S<Class>& s) { // Forward declarations are sufficient here.
   //   (void)s.t; // Full 'Class' type is needed due to template instantiation.
   // }
-  void ReportTplSpecComponentTypes(const TemplateSpecializationType*) {
-  }
+  void ReportTplSpecComponentTypes(const TemplateSpecializationType*) = delete;
 
   // Do not add any variables here!  If you do, they will not be shared
   // between the normal iwyu ast visitor and the
@@ -3236,6 +3235,12 @@ class InstantiatedTemplateVisitor
     return Base::VisitCXXConstructExpr(expr);
   }
 
+  // --- Handler declared in IwyuBaseASTVisitor.
+
+  void ReportTplSpecComponentTypes(const TemplateSpecializationType* type) {
+    TraverseDataAndTypeMembersOfClassHelper(type);
+  }
+
  private:
   // Clears the state of the visitor.
   void Clear() {

iwyu_ast_util.cc

@@ -547,12 +547,35 @@ class TypeEnumerator : public RecursiveASTVisitor<TypeEnumerator> {
   }
 
   // --- Methods on RecursiveASTVisitor
+  bool TraverseType(QualType type) {
+    TraverseArgumentsOfSugaredTemplates(type);
+    return Base::TraverseType(type);
+  }
+
+  bool TraverseTypeLoc(TypeLoc type_loc) {
+    TraverseArgumentsOfSugaredTemplates(type_loc.getType());
+    return Base::TraverseTypeLoc(type_loc);
+  }
+
   bool VisitType(Type* type) {
     seen_types_.insert(type);
     return true;
   }
 
  private:
+  // Clang doesn't traverse underlying type for most of the sugar types.
+  // If a TemplateSpecializationType occurs in a sugaring chain, traverse its
+  // arguments explicitly, otherwise they aren't put into resugar_map.
+  void TraverseArgumentsOfSugaredTemplates(QualType type) {
+    if (type.isNull())
+      return;
+
+    if (const auto* template_spec = type->getAs<TemplateSpecializationType>()) {
+      for (const TemplateArgument& arg : template_spec->template_arguments())
+        TraverseTemplateArgument(arg);
+    }
+  }
+
   set<const Type*> seen_types_;
 };
 

tests/cxx/template_args.cc

@@ -80,6 +80,10 @@ void PointerClassArguments() {
 
 template<typename T> struct Outer { T t; };
 template<typename T> struct Inner { T t; };
+struct StaticTemplateFieldStruct {
+  // IWYU: IndirectClass needs a declaration
+  static Inner<IndirectClass> tpl;
+};
 
 void NestedTemplateArguments() {
   // IWYU: IndirectClass needs a declaration
@@ -94,6 +98,23 @@ void NestedTemplateArguments() {
   // IWYU: IndirectClass needs a declaration
   Outer<Inner<IndirectClass> >* opi;
   (void)opi;
+
+  // Test that use of template specialization type template argument is not
+  // hidden by any sugar in the AST.
+
+  // IWYU: IndirectClass is...*indirect.h
+  Outer<decltype(StaticTemplateFieldStruct::tpl)> osi;
+  // Member referencing also requires template instantiation and nested member
+  // full-type-use reporting.
+  // IWYU: IndirectClass is...*indirect.h
+  (void)osi.t;
+
+  Outer<decltype(StaticTemplateFieldStruct::tpl)*> osip;
+  (void)osip.t;
+
+  Outer<decltype(StaticTemplateFieldStruct::tpl)>* opsi;
+  // IWYU: IndirectClass is...*indirect.h
+  (void)opsi->t;
 }
 
 // ---------------------------------------------------------------