https://github.com/HamzaHassanain created https://github.com/llvm/llvm-project/pull/174281
### **The Problem** Clang crashes when encountering a `static` data member declared with an alias template but lacking template arguments (e.g., `static E u;`). Because of C++ CTAD rules, Clang's parser assigns this a placeholder `AutoType`. If this member is later referenced, the backend (`CodeGen`) attempts to emit the variable. Since the type remains undeduced and no initializer exists, the compiler hits an `llvm_unreachable` in `CodeGenTypes.cpp`. ### **The Fix** The fix is implemented in `Sema::ActOnUninitializedDecl`. We now explicitly check if a `static` data member still possesses an undeduced placeholder type after all deduction attempts have failed. * **Logic:** If the type contains an undeduced type, and the variable is a static member without an initializer, we emit `err_auto_var_requires_init`. ### **Changes** * **SemaDecl.cpp**: Added validation logic to `ActOnUninitializedDecl` to catch undeduced static members. * **alias-template-static-member.cpp**: Added a regression test to ensure this scenario results in a diagnostic rather than a crash. ### **Notes** on formatting When I `clang-format` the `SemaDecl.cpp` after applying my changes, I found the whole file got changed, so I undid the formatting. >From aafd57e112ea07a66e2864c9034ac8bbc328c0b2 Mon Sep 17 00:00:00 2001 From: Hamza Hassanain <[email protected]> Date: Sat, 3 Jan 2026 17:07:05 +0200 Subject: [PATCH 1/2] [Clang][Sema] reject undeduced static members without init --- clang/lib/Sema/SemaDecl.cpp | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp index 11323803e1910..50a6a4176d1ec 100644 --- a/clang/lib/Sema/SemaDecl.cpp +++ b/clang/lib/Sema/SemaDecl.cpp @@ -14337,8 +14337,18 @@ void Sema::ActOnUninitializedDecl(Decl *RealDecl) { DeduceVariableDeclarationType(Var, false, nullptr)) return; + Type = Var->getType(); this->CheckAttributesOnDeducedType(RealDecl); + if (auto *Deduced = Type->getContainedDeducedType()) { + if (Var->isStaticDataMember() && Deduced->getDeducedType().isNull()) { + Diag(Var->getLocation(), diag::err_auto_var_requires_init) + << Var->getDeclName() << Type; + Var->setInvalidDecl(); + return; + } + } + // C++11 [class.static.data]p3: A static data member can be declared with // the constexpr specifier; if so, its declaration shall specify // a brace-or-equal-initializer. >From 74473fd4f581b5538e01b8a3c1e9e068f9c88831 Mon Sep 17 00:00:00 2001 From: Hamza Hassanain <[email protected]> Date: Sat, 3 Jan 2026 17:31:34 +0200 Subject: [PATCH 2/2] [Clang][Test][SemaCXX] added alias tempalte static member hanlding test --- clang/test/SemaCXX/alias-template-static-member.cpp | 11 +++++++++++ 1 file changed, 11 insertions(+) create mode 100644 clang/test/SemaCXX/alias-template-static-member.cpp diff --git a/clang/test/SemaCXX/alias-template-static-member.cpp b/clang/test/SemaCXX/alias-template-static-member.cpp new file mode 100644 index 0000000000000..0089aea33cc80 --- /dev/null +++ b/clang/test/SemaCXX/alias-template-static-member.cpp @@ -0,0 +1,11 @@ +// RUN: %clang_cc1 -fsyntax-only -verify -std=c++17 %s + +template <class T> +struct A { + template <class U> + using E = U; + + static E u; // expected-error {{declaration of variable 'u' with deduced type 'E' requires an initializer}} +}; + +decltype(A<int>::u) a; \ No newline at end of file _______________________________________________ cfe-commits mailing list [email protected] https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
