[clang] Fix wrong warning about missing init for flexible array members (#66341)

9108897 shouldn't have removed an
additional check that field has incomplete array type.

Fixes #66300

Co-authored-by: Aaron Ballman <[email protected]>

Author: Fznamznon

clang/lib/Sema/SemaInit.cpp

@@ -2395,7 +2395,8 @@ void InitListChecker::CheckStructUnionTypes(
       if (HasDesignatedInit && InitializedFields.count(*it))
         continue;
 
-      if (!it->isUnnamedBitfield() && !it->hasInClassInitializer()) {
+      if (!it->isUnnamedBitfield() && !it->hasInClassInitializer() &&
+          !it->getType()->isIncompleteArrayType()) {
         SemaRef.Diag(IList->getSourceRange().getEnd(),
                      diag::warn_missing_field_initializers)
             << *it;

clang/test/Sema/missing-field-initializers.c

@@ -50,3 +50,14 @@ struct { int:5; int a; int:5; int b; int:5; } noNamedImplicit[] = {
   { 1, 2 },
   { 1 } // expected-warning {{missing field 'b' initializer}}
 };
+
+// GH66300
+struct S {
+  int f0;
+  int f1[];
+};
+
+// We previously would accidentally diagnose missing a field initializer for
+// f1, now we no longer issue that warning (note, this code is still unsafe
+// because of the buffer overrun).
+struct S s = {1, {1, 2}};