Use the definition to check if decl is provided by template

TypeToDeclAsWritten would not return the written definition in all cases.

For templates, for example, the selected declaration could be the one
where it was explicitly instantiatiated. That, in turn, affects the source
location that is returned and ultimately the outcome of the function.

Make sure to always get to the definition before using it to determine
whether the template being instantiated provides the decl.

Fixes #558

iwyu.cc

@@ -3207,7 +3207,7 @@ class InstantiatedTemplateVisitor
          ast_node != caller_ast_node_; ast_node = ast_node->parent()) {
       if (preprocessor_info().PublicHeaderIntendsToProvide(
               GetFileEntry(ast_node->GetLocation()),
-              GetFileEntry(decl)))
+              GetFileEntry(decl->getLocation())))
         return ast_node->GetLocation();
     }
     return SourceLocation();   // an invalid source-loc
@@ -3219,8 +3219,10 @@ class InstantiatedTemplateVisitor
   bool IsProvidedByTemplate(const Type* type) const {
     type = RemoveSubstTemplateTypeParm(type);
     type = RemovePointersAndReferences(type);  // get down to the decl
-    if (const NamedDecl* decl = TypeToDeclAsWritten(type))
+    if (const NamedDecl* decl = TypeToDeclAsWritten(type)) {
+      decl = GetDefinitionAsWritten(decl);
       return GetLocOfTemplateThatProvides(decl).isValid();
+    }
     return true;   // we always provide non-decl types like int, etc.
   }
 

tests/cxx/explicit_instantiation2-template_helpers.h

@@ -37,4 +37,12 @@ class TemplateTemplateArgShortFwd {
   T<short>* t;
 };
 
+template <class T>
+class ProvidedTemplate {};
+
+template <class U = short, class T = ProvidedTemplate<U>>
+class TemplateAsDefaultFullProvided {
+  T t;
+};
+
 #endif  // INCLUDE_WHAT_YOU_USE_TESTS_CXX_EXPLICIT_INSTANTIATION2_TEMPLATE_HELPERS_H_

tests/cxx/explicit_instantiation2-template_short.h

@@ -12,4 +12,6 @@
 
 extern template class Template<short>;
 
+extern template class ProvidedTemplate<short>;
+
 #endif  // INCLUDE_WHAT_YOU_USE_TESTS_CXX_EXPLICIT_INSTANTIATION2_TEMPLATE_SHORT_H_

tests/cxx/explicit_instantiation2.cc

@@ -29,6 +29,8 @@
 // 6: Fwd-decl use in a template, provided as a default parameter.
 // 9: Implicit instantiation of Template<int>
 // 10: Specialization of Template<T>
+// 11: Explicit instantiation with a dependent instantiation declaration, but
+//     provided by the template helper itself.
 
 
 // IWYU: Template is...*explicit_instantiation-template.h
@@ -81,6 +83,8 @@ template <> class Template<char> {};
 
 TemplateAsDefaultFull<char> t10; // 10
 
+TemplateAsDefaultFullProvided<> t11; // 11
+
 /**** IWYU_SUMMARY
 
 tests/cxx/explicit_instantiation2.cc should add these lines:
@@ -93,7 +97,7 @@ tests/cxx/explicit_instantiation2.cc should remove these lines:
 
 The full include-list for tests/cxx/explicit_instantiation2.cc:
 #include "explicit_instantiation-template.h"  // for Template
-#include "explicit_instantiation2-template_helpers.h"  // for FullUseArg, FwdDeclUseArg, TemplateAsDefaultFull, TemplateAsDefaultFwd, TemplateTemplateArgShortFull, TemplateTemplateArgShortFwd
+#include "explicit_instantiation2-template_helpers.h"  // for FullUseArg, FwdDeclUseArg, TemplateAsDefaultFull, TemplateAsDefaultFullProvided, TemplateAsDefaultFwd, TemplateTemplateArgShortFull, TemplateTemplateArgShortFwd
 #include "explicit_instantiation2-template_short.h"  // for Template
 
 ***** IWYU_SUMMARY */