Harvest provided types from alias template args

iwyu.cc

@@ -4332,14 +4332,20 @@ class IwyuAstConsumer
 
   set<const Type*> GetProvidedByTplArg(const Type* type) {
     set<const Type*> res;
-    if (const auto* elaborated = dyn_cast_or_null<ElaboratedType>(type)) {
+    if (!type)
+      return res;
+    if (const auto* elaborated = dyn_cast<ElaboratedType>(type)) {
       const NestedNameSpecifier* nns = elaborated->getQualifier();
       while (nns) {
         if (const Type* host = nns->getAsType())
           InsertAllInto(GetTplInstData(host).provided_types, &res);
         nns = nns->getPrefix();
       }
     }
+    if (const auto* tpl_spec = type->getAs<TemplateSpecializationType>()) {
+      if (tpl_spec->isTypeAlias())
+        InsertAllInto(GetTplInstData(tpl_spec).provided_types, &res);
+    }
     return res;
   }
 

tests/cxx/alias_template.cc

@@ -24,6 +24,11 @@ constexpr auto s = sizeof(ic);
 // IWYU: IndirectClass needs a declaration
 Identity<IndirectClass>* pic = nullptr;
 
+// IWYU: IndirectClass is...*indirect.h
+using Providing = IndirectClass;
+
+Identity<Providing> type_is_provided_by_arg;
+
 template<class T> struct FullUseTemplateArgInSizeof {
   char argument[sizeof(T)];
 };

tests/cxx/iwyu_stricter_than_cpp.cc

@@ -274,6 +274,11 @@ void TestAliasTemplates() {
   // IWYU: IndirectStruct2 is...*iwyu_stricter_than_cpp-i2.h
   // IWYU: TplIndirectStruct3 is...*iwyu_stricter_than_cpp-i5.h
   TemplateNotProvidedArgumentNotUsed<IndirectStruct3> tnpanu;
+
+  TemplateProvidedArgumentUsed<DoesNotForwardDeclareAndIncludes>
+      argument_provided;
+  // IWYU: IndirectStruct2 is...*iwyu_stricter_than_cpp-i2.h
+  TemplateProvidedArgumentUsed<DoesEverythingRight> argument_not_provided;
 }
 
 void TestAutocast() {