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

[Fznamznon]

Summary

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 https://github.com/llvm/llvm-project/issues/37257

[Fznamznon]

clang/lib/Sema/SemaDecl.cpp

13456

Not sure about the error message though, maybe it makes sense to add a message similar to err_new_array_size_unknown_from_init .

[shafik]

I Think this change makes sense.

[aaron.ballman]

LGTM with a small nit

clang/lib/Sema/SemaDecl.cpp

13449–13462

We can cram everything into one predicate; but this is largely intended to rearrange the logic so we're checking the cheap conditions before trying to call getAsIncompleteArrayType twice.

[Fznamznon]

Apply suggestion, rebase to maybe pass precommit

[Fznamznon]

The failures in CI are unrelated and caused by another patch - https://reviews.llvm.org/D112921.

clang/docs/ReleaseNotes.rst

 - Clang now prints unnamed members in diagnostic messages instead of giving an
   empty ''. Fixes
   (`#63759 <https://github.com/llvm/llvm-project/issues/63759>`_)
 - Fix crash in __builtin_strncmp and related builtins when the size value
   exceeded the maximum value representable by int64_t. Fixes
   (`#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
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Bug Fixes to Attribute Support
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Bug Fixes to C++ Support

clang/lib/Sema/SemaDecl.cpp

if (Result.isInvalid()) {
         VDecl->setInit(RecoveryExpr.get());
       return;
     }
 
     Init = Result.getAs<Expr>();
     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.
   // Variables declared within a function/method body (except for references)
   // are handled by a dataflow analysis.
   // This is undefined behavior in C++, but valid in C.
   if (getLangOpts().CPlusPlus)
     if (!VDecl->hasLocalStorage() || VDecl->getType()->isRecordType() ||
         VDecl->getType()->isReferenceType())
       CheckSelfReference(*this, RealDecl, Init, DirectInit);
 

clang/test/SemaCXX/gh37257.cpp

+// 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}}
+}