Parser: skip primary function templates in Sema instantiation calls

[Fedr]

Summary

The visitor unconditionally called Sema::InstantiateDefaultArgument and Sema::InstantiateFunctionDefinition on every FunctionDecl it traversed, including primary function templates that pass through because of allow_uninstantiated_templates. On a primary template both APIs null-deref inside libclang and crash with STATUS_ACCESS_VIOLATION. Skipping the call when decl.isTemplated() is true fixes both crashes — the specific instantiations are visited separately and processed correctly.

Stack traces (clang 22.1.4, MSYS2 mingw-w64-clang-x86_64)

InstantiateDefaultArgument at main.cpp:2838:

#0  clang::FunctionDecl::getNumParams() const                      (this = invalid)
#1  clang::Sema::addInstantiatedParametersToScope
#2  clang::Sema::SubstDefaultArgument                              (ForCallExpr = true)
#3  clang::Sema::InstantiateDefaultArgument
#4  mrbind::ClangAstVisitor_InstTypesAndCollectNewTypes::VisitFunctionDecl  (main.cpp:2838)

InstantiateFunctionDefinition at main.cpp:1072:

#0  clang::FunctionDecl::isDefined(FunctionDecl const*&, bool)     (this = invalid)
#1  clang::Sema::InstantiateFunctionDefinition
#2  mrbind::InstantiateReturnTypeIfNeeded                          (main.cpp:1072)
#3  mrbind::ClangAstVisitor_InstTypesAndCollectNewTypes::VisitFunctionDecl  (main.cpp:2859)

Both inputs the same: PatternFD returned by FunctionDecl::getTemplateInstantiationPattern(/*ForDefinition*/ false) on a primary template is null, and Sema dereferences it without a guard.

Tested clang versions

clang	repro without fix	with fix
18.1.8	crashes	clean
19.1.7	crashes	clean
21.1.8	crashes	clean
22.1.4	crashes	clean

Source-diff of Sema::SubstDefaultArgument, Sema::addInstantiatedParametersToScope, Sema::InstantiateDefaultArgument, FunctionDecl::getTemplateInstantiationPattern across llvmorg-18.1.8 ... llvmorg-22.1.4 is logically identical (only type/RAII renames), so the bug is at least as old as v18.

Standalone clang-tooling reproducer (no mrbind dependency):

// repro_input.cpp
template <typename T> struct Wrap {
    template <typename U = T> void method(int x = sizeof(U)) {}
};
inline void use() { Wrap<int> w; w.method(); }

A RecursiveASTVisitor whose VisitFunctionDecl calls Sema::InstantiateDefaultArgument on every ParmVarDecl with hasUninstantiatedDefaultArg() (no isTemplated() guard) crashes identically inside libclang. Will be filed upstream against LLVM as API hardening (Sema::InstantiateDefaultArgument's documented precondition is only assert(Param->hasUninstantiatedDefaultArg()) — it should also assert FD->isTemplateInstantiation() or fail gracefully on a null pattern).

Verification on real input

Local mrbind build with this fix, parsing #include <boost/multiprecision/cpp_int.hpp> on clang 22.1.4:

• Without fix: silent crash after 0 lines of output (the original CI signature).
• With fix: parser progresses past Sema instantiation, reaches a separate cppdecl error on _Complex _Float16 — that's a different issue in a different library, not addressed here.

Test plan

• CI on this branch is green (mrbind has its own tests in test/).
• Picked up as a submodule bump in MeshLib PR #6021 (clang 22 toolchain) and observed to clear the STATUS_ACCESS_VIOLATION segfault during Generate C bindings.