[clang] Emit an error if variable ends up with incomplete array type

This adds an error if variable with incomplete type has initializer with
incomplete type, so it is not possible to deduce array size from
initializer.

Fixes #37257

Reviewed By: aaron.ballman, shafik

Differential Revision: https://reviews.llvm.org/D158615

Author: Fznamznon

clang/docs/ReleaseNotes.rst

@@ -189,6 +189,9 @@ Bug Fixes in This Version
   (`#64876 <https://github.com/llvm/llvm-project/issues/64876>`_)
 - Fixed an assertion if a function has cleanups and fatal erors.
   (`#48974 <https://github.com/llvm/llvm-project/issues/48974>`_)
+- Clang now emits an error if it is not possible to deduce array size for a
+  variable with incomplete array type.
+  (`#37257 <https://github.com/llvm/llvm-project/issues/37257>`_)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaDecl.cpp

@@ -13445,6 +13445,18 @@ void Sema::AddInitializerToDecl(Decl *RealDecl, Expr *Init, bool DirectInit) {
     IsParenListInit = !InitSeq.steps().empty() &&
                       InitSeq.step_begin()->Kind ==
                           InitializationSequence::SK_ParenthesizedListInit;
+    QualType VDeclType = VDecl->getType();
+    if (Init && !Init->getType().isNull() &&
+        !Init->getType()->isDependentType() && !VDeclType->isDependentType() &&
+        Context.getAsIncompleteArrayType(VDeclType) &&
+        Context.getAsIncompleteArrayType(Init->getType())) {
+      // Bail out if it is not possible to deduce array size from the
+      // initializer.
+      Diag(VDecl->getLocation(), diag::err_typecheck_decl_incomplete_type)
+          << VDeclType;
+      VDecl->setInvalidDecl();
+      return;
+    }
   }
 
   // Check for self-references within variable initializers.

clang/test/SemaCXX/gh37257.cpp

@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+template<class T>
+T&& create();
+
+template<class T, class... Args>
+void test() {
+  T t(create<Args>()...); // expected-error{{variable has incomplete type 'int[]'}}
+  (void) t;
+}
+
+struct A;
+
+int main() {
+  test<int[]>(); // expected-note {{in instantiation of function template specialization 'test<int[]>' requested here}}
+}