[C++] Fix a crash with __thread and dependent types (#140542)

We were checking whether the initializer is a valid constant expression
even if the variable was dependent. Now we delay that checking until
after the template has been instantiated.

Fixes #140509

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -558,6 +558,9 @@ Improvements to Clang's diagnostics
   between different Unicode character types (``char8_t``, ``char16_t``, ``char32_t``).
   This warning only triggers in C++ as these types are aliases in C. (#GH138526)
 
+- Fixed a crash when checking a ``__thread``-specified variable declaration
+  with a dependent type in C++. (#GH140509)
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/lib/Sema/SemaDecl.cpp

@@ -14608,6 +14608,10 @@ void Sema::CheckCompleteVariableDeclaration(VarDecl *var) {
   std::optional<bool> CacheHasConstInit;
   const Expr *CacheCulprit = nullptr;
   auto checkConstInit = [&]() mutable {
+    const Expr *Init = var->getInit();
+    if (Init->isInstantiationDependent())
+      return true;
+
     if (!CacheHasConstInit)
       CacheHasConstInit = var->getInit()->isConstantInitializer(
             Context, var->getType()->isReferenceType(), &CacheCulprit);

clang/test/SemaCXX/thread-specifier.cpp

@@ -0,0 +1,30 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++23 -verify %s
+
+namespace GH140509 {
+template <typename T>
+void not_instantiated() {
+  static __thread T my_wrapper;
+}
+
+template <typename T>
+void instantiated() {
+  static __thread T my_wrapper = T{}; // expected-error {{initializer for thread-local variable must be a constant expression}} \
+                                         expected-note {{use 'thread_local' to allow this}}
+}
+
+template <typename T>
+void nondependent_var() {
+  // Verify that the dependence of the initializer is what really matters.
+  static __thread int my_wrapper = T{};
+}
+
+struct S {
+  S() {}
+};
+
+void f() {
+  instantiated<int>();
+  instantiated<S>(); // expected-note {{in instantiation of function template specialization 'GH140509::instantiated<GH140509::S>' requested here}}
+  nondependent_var<int>();
+}
+} // namespace GH140509