[Clang] prevent assertion failure by avoiding required literal type checking in C context (#101426)

Fixes #101304

Author: a-tarasyuk

clang/docs/ReleaseNotes.rst

@@ -156,6 +156,7 @@ Bug Fixes in This Version
 
 - Fixed the definition of ``ATOMIC_FLAG_INIT`` in ``<stdatomic.h>`` so it can
   be used in C++.
+- Fixed a failed assertion when checking required literal types in C context. (#GH101304).
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaDecl.cpp

@@ -8756,7 +8756,8 @@ void Sema::CheckVariableDeclarationType(VarDecl *NewVD) {
     return;
   }
 
-  if (NewVD->isConstexpr() && !T->isDependentType() &&
+  if (getLangOpts().CPlusPlus && NewVD->isConstexpr() &&
+      !T->isDependentType() &&
       RequireLiteralType(NewVD->getLocation(), T,
                          diag::err_constexpr_var_non_literal)) {
     NewVD->setInvalidDecl();

clang/test/Sema/constexpr.c

@@ -357,3 +357,6 @@ void infsNaNs() {
   constexpr double db5 = LD_SNAN; // expected-error {{constexpr initializer evaluates to nan which is not exactly representable in type 'const double'}}
   constexpr double db6 = INF;
 }
+
+constexpr struct S9 s9 = {  }; // expected-error {{variable has incomplete type 'const struct S9'}} \
+                               // expected-note {{forward declaration of 'struct S9'}}