[Sema] PR26077 Fix assert when partial specialization is missing required parameters

[hintonda]

Summary

Bail-out and return true as soon as a partial template specialization that does not use any of its template parameters is detected.

This avoids an asset later on since the ctor doesn't depend on any parameters and Sema::SetCtorInitializers doesn't return early.

[hintonda]

Here's a snippet of code that will crash:

class DB {};

template <typename T> class RemovePolicy : public T {};

template <typename T, typename Policy = RemovePolicy<T>>

class Crash : T, Policy {};

template <typename Policy>
class Crash<DB/*, Policy*/> : public DB, public Policy {
public:

Crash(){}

};

[hintonda]

This patch fixes bug 26077.

https://llvm.org/bugs/show_bug.cgi?id=26077

[hintonda]

• Added test for PR26077

[hintonda]

I don't have commit access, so if this patch is accepted, could someone please commit it for me?

[hintonda]

ping...

[hintonda]

Rebase, fix test, and add FIXME note.

[hintonda]

ping...

[hintonda]

ping...

lib/Sema/SemaTemplate.cpp

if (IsPartialSpecialization) {
     // also do them during instantiation.
     bool InstantiationDependent;
     if (!Name.isDependent() &&
         !TemplateSpecializationType::anyDependentTemplateArguments(
             TemplateArgs.arguments(),
             InstantiationDependent)) {
       Diag(TemplateNameLoc, diag::err_partial_spec_fully_specialized)
           << VarTemplate->getDeclName();
-      IsPartialSpecialization = false;
+      // FIXME: Can this ever get triggered?  If so, we need test.  If not,
+      // should this if be removed?
+      return true;
     }
 
     if (isSameAsPrimaryTemplate(VarTemplate->getTemplateParameters(),
                                 Converted)) {
       // C++ [temp.class.spec]p9b3:
       //
       //   -- The argument list of the specialization shall not be identical
       //      to the implicit argument list of the primary template.
if (isPartialSpecialization) {
     // FIXME: Move this to CheckTemplatePartialSpecializationArgs so we
     // also do it during instantiation.
     bool InstantiationDependent;
     if (!Name.isDependent() &&
         !TemplateSpecializationType::anyDependentTemplateArguments(
             TemplateArgs.arguments(), InstantiationDependent)) {
       Diag(TemplateNameLoc, diag::err_partial_spec_fully_specialized)
         << ClassTemplate->getDeclName();
-      isPartialSpecialization = false;
+      return true;
     }
   }
 
   void *InsertPos = nullptr;
   ClassTemplateSpecializationDecl *PrevDecl = nullptr;
 
   if (isPartialSpecialization)
     // FIXME: Template parameter list matters, too

test/SemaTemplate/partial-spec-fully-specified.cpp

+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++98 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 %s
+
+template <class T>
+struct Base{};
+
+template <class A, class B>
+class Test {};
+
+template <class T>
+class Test<void, void> : Base<T> { // expected-error{{partial specialization of 'Test' does not use any of its template parameters}}
+  Test() {}
+};