[clang] diagnose block pointer types as invalid for constant template parameters (#190464)

Fixes a crash by making it ill-formed to have a constant template
parameter with a block pointer type.

Fixes #189247

clang/docs/ReleaseNotes.rst

@@ -412,6 +412,7 @@ Bug Fixes to Attribute Support
 
 Bug Fixes to C++ Support
 ^^^^^^^^^^^^^^^^^^^^^^^^
+- Clang now rejects constant template parameters with block pointer types, since these are not implemented anyway and would lead to crashes. (#GH189247)
 - Fixed a crash on error recovery when dealing with invalid templates. (#GH183075)
 - Fixed a crash when instantiating ``requires`` expressions involving substitution failures in C++ concepts. (#GH176402)
 - Fixed an incorrect template argument deduction when matching packs of template

clang/lib/Sema/SemaTemplate.cpp

@@ -1459,6 +1459,11 @@ QualType Sema::CheckNonTypeTemplateParameterType(QualType T,
     return QualType();
   }
 
+  if (T->isBlockPointerType()) {
+    Diag(Loc, diag::err_template_nontype_parm_bad_type) << T;
+    return QualType();
+  }
+
   // C++ [temp.param]p4:
   //
   // A non-type template-parameter shall have one of the following

clang/test/SemaCXX/blocks.cpp

@@ -163,3 +163,7 @@ void static_data_member() {
     };
   };
 }
+
+namespace gh189247 {
+  template<void (^)()> struct A; // expected-error {{a non-type template parameter cannot have type 'void (^)()'}}
+}