Parser: skip class members in the buggy-substitute-default-template-args path

The block's own comment already says this path is only needed for free
function templates and that "something else already instantiates them for
the class member functions". When the path is exercised on a CXXMethodDecl
(in particular a CXXConstructorDecl), `Sema::SubstDecl` on the member
template's pattern null-derefs inside
`TemplateDeclInstantiator::InitMethodInstantiation`.

Captured stack from CI (clang 22.1.4) parsing
`<boost/multiprecision/cpp_int.hpp>` with
`--buggy-substitute-default-template-args`:

  #0 clang::TemplateDeclInstantiator::InitMethodInstantiation
  #1 clang::TemplateDeclInstantiator::VisitCXXMethodDecl
  #2 lambda inside Sema::SubstDecl (runWithSufficientStackSpace)
  #3 clang::StackExhaustionHandler::runWithSufficientStackSpace
  #4 clang::Sema::runWithSufficientStackSpace
  #5 clang::Sema::SubstDecl
  #6 mrbind::ClangAstVisitor_InstTypesAndCollectNewTypes::VisitFunctionDecl
       main.cpp:2815  <- this call site
  #7 clang::RecursiveASTVisitor::TraverseCXXConstructorDecl

Adding `!llvm::isa<clang::CXXMethodDecl>(decl)` to the existing
`decl->isTemplated()` check matches the comment and avoids the crash.
This is consistent with the previous commit, which skipped primary
function templates from `InstantiateDefaultArgument` /
`InstantiateFunctionDefinition` for the same class of latent libclang
bug (public Sema API null-derefs on a primary template's pattern).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Fedr

src/parser/main.cpp

@@ -2689,7 +2689,12 @@ namespace mrbind
         {
             // If this is a template, try instantiating it.
             // This is currently disabled by default because it's buggy: https://github.com/MeshInspector/mrbind/issues/19
-            if (params->buggy_substitute_default_template_args && decl->isTemplated() && !ShouldRejectFunction(*decl, *ctx, *ci, *params, printing_policies, ShouldRejectFlags::allow_uninstantiated_templates))
+            //
+            // Skip class members entirely (constructors, methods, ...). Per the comment below,
+            // they don't need this path, and `Sema::SubstDecl` on a member function template's
+            // pattern null-derefs internally on at least clang 22 — for example, a constructor
+            // template substitution crashes in `TemplateDeclInstantiator::InitMethodInstantiation`.
+            if (params->buggy_substitute_default_template_args && decl->isTemplated() && !llvm::isa<clang::CXXMethodDecl>(decl) && !ShouldRejectFunction(*decl, *ctx, *ci, *params, printing_policies, ShouldRejectFlags::allow_uninstantiated_templates))
             {
                 // Among other things, we visit the function declarations to instantiate their default arguments,
                 //   which apparently doesn't happen otherwise. This is only needed for free function templates.