Handle invalid variable template specialization whose type depends on itself

Author: zwuis

clang/docs/ReleaseNotes.rst

@@ -414,6 +414,8 @@ Bug Fixes to C++ Support
 - Clang now issues an error when placement new is used to modify a const-qualified variable
   in a ``constexpr`` function. (#GH131432)
 - Clang now emits a warning when class template argument deduction for alias templates is used in C++17. (#GH133806)
+- No longer crashes when instantiating invalid variable template specialization
+  whose type depends on itself. (#GH51347)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -5259,6 +5259,9 @@ def err_template_member_noparams : Error<
   "extraneous 'template<>' in declaration of member %0">;
 def err_template_tag_noparams : Error<
   "extraneous 'template<>' in declaration of %0 %1">;
+def err_var_template_spec_type_depends_on_self : Error<
+  "the type of variable template specialization %0 declared with deduced type "
+  "%1 depends on itself">;
 
 def warn_unqualified_call_to_std_cast_function : Warning<
   "unqualified call to '%0'">, InGroup<DiagGroup<"unqualified-std-cast-call">>;

clang/lib/Sema/SemaTemplate.cpp

@@ -4377,6 +4377,13 @@ Sema::CheckVarTemplateId(VarTemplateDecl *Template, SourceLocation TemplateLoc,
   if (VarTemplateSpecializationDecl *Spec =
           Template->findSpecialization(CTAI.CanonicalConverted, InsertPos)) {
     checkSpecializationReachability(TemplateNameLoc, Spec);
+    if (Spec->getType()->isUndeducedType()) {
+      // We are substituting the initializer of this variable template
+      // specialization.
+      Diag(TemplateNameLoc, diag::err_var_template_spec_type_depends_on_self)
+          << Spec << Spec->getType();
+      return true;
+    }
     // If we already have a variable template specialization, return it.
     return Spec;
   }

clang/test/SemaTemplate/instantiate-var-template.cpp

@@ -47,3 +47,21 @@ namespace InvalidInsertPos {
   template<> int v<int, 0>;
   int k = v<int, 500>;
 }
+
+namespace GH51347 {
+  template <typename T>
+  auto p = p<T>; // expected-error {{the type of variable template specialization 'p<int>'}}
+
+  auto x = p<int>; // expected-note {{in instantiation of variable template specialization 'GH51347::p'}}
+}
+
+namespace GH97881_comment {
+  template <bool B>
+  auto g = sizeof(g<!B>);
+  // expected-error@-1 {{the type of variable template specialization 'g<false>'}}
+  // expected-note@-2 {{in instantiation of variable template specialization 'GH97881_comment::g'}}
+
+  void test() {
+    (void)sizeof(g<false>); // expected-note {{in instantiation of variable template specialization 'GH97881_comment::g'}}
+  }
+}