[clang] Add -Wmissing-designated-field-initializers (#81364)

#56628 changed the behavior of `-Wmissing-field-initializers`, which
introduces many new warnings in C++ code that uses partial designated
initializers. If such code is being built with `-Wextra -Werror`, this
change will break the build.

This PR adds a new flag that allows to disable these new warnings and
keep the old ones, as was suggested by @AaronBallman in the original
issue:
#56628 (comment)

Fixes  #68933

clang/docs/ReleaseNotes.rst

@@ -150,6 +150,10 @@ Non-comprehensive list of changes in this release
 New Compiler Flags
 ------------------
 
+- ``-Wmissing-designated-field-initializers``, grouped under ``-Wmissing-field-initializers``.
+  This diagnostic can be disabled to make ``-Wmissing-field-initializers`` behave
+  like it did before Clang 18.x. Fixes (`#56628 <https://github.com/llvm/llvm-project/issues/68933>`_)
+
 Deprecated Compiler Flags
 -------------------------
 

clang/include/clang/Basic/DiagnosticGroups.td

@@ -517,7 +517,15 @@ def MethodSignatures : DiagGroup<"method-signatures">;
 def MismatchedParameterTypes : DiagGroup<"mismatched-parameter-types">;
 def MismatchedReturnTypes : DiagGroup<"mismatched-return-types">;
 def MismatchedTags : DiagGroup<"mismatched-tags">;
-def MissingFieldInitializers : DiagGroup<"missing-field-initializers">;
+def MissingDesignatedFieldInitializers : DiagGroup<"missing-designated-field-initializers">{
+  code Documentation = [{
+Warn about designated initializers with some fields missing (only in C++).
+  }];
+}
+// Default -Wmissing-field-initializers matches gcc behavior,
+// but missing-designated-field-initializers can be turned off to match old clang behavior.
+def MissingFieldInitializers : DiagGroup<"missing-field-initializers",
+                                         [MissingDesignatedFieldInitializers]>;
 def ModuleLock : DiagGroup<"module-lock">;
 def ModuleBuild : DiagGroup<"module-build">;
 def ModuleImport : DiagGroup<"module-import">;

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -6170,6 +6170,10 @@ def ext_initializer_string_for_char_array_too_long : ExtWarn<
 def warn_missing_field_initializers : Warning<
   "missing field %0 initializer">,
   InGroup<MissingFieldInitializers>, DefaultIgnore;
+// The same warning, but another group is needed to disable it separately.
+def warn_missing_designated_field_initializers : Warning<
+  warn_missing_field_initializers.Summary>,
+  InGroup<MissingDesignatedFieldInitializers>, DefaultIgnore;
 def warn_braces_around_init : Warning<
   "braces around %select{scalar |}0initializer">,
   InGroup<DiagGroup<"braced-scalar-init">>;

clang/lib/Sema/SemaInit.cpp

@@ -2227,8 +2227,6 @@ void InitListChecker::CheckStructUnionTypes(
   size_t NumRecordDecls = llvm::count_if(RD->decls(), [&](const Decl *D) {
     return isa<FieldDecl>(D) || isa<RecordDecl>(D);
   });
-  bool CheckForMissingFields =
-    !IList->isIdiomaticZeroInitializer(SemaRef.getLangOpts());
   bool HasDesignatedInit = false;
 
   llvm::SmallPtrSet<FieldDecl *, 4> InitializedFields;
@@ -2269,11 +2267,6 @@ void InitListChecker::CheckStructUnionTypes(
       }
 
       InitializedSomething = true;
-
-      // Disable check for missing fields when designators are used.
-      // This matches gcc behaviour.
-      if (!SemaRef.getLangOpts().CPlusPlus)
-        CheckForMissingFields = false;
       continue;
     }
 
@@ -2285,7 +2278,7 @@ void InitListChecker::CheckStructUnionTypes(
     // These are okay for randomized structures. [C99 6.7.8p19]
     //
     // Also, if there is only one element in the structure, we allow something
-    // like this, because it's really not randomized in the tranditional sense.
+    // like this, because it's really not randomized in the traditional sense.
     //
     //   struct foo h = {bar};
     auto IsZeroInitializer = [&](const Expr *I) {
@@ -2363,8 +2356,13 @@ void InitListChecker::CheckStructUnionTypes(
   }
 
   // Emit warnings for missing struct field initializers.
-  if (!VerifyOnly && InitializedSomething && CheckForMissingFields &&
-      !RD->isUnion()) {
+  // This check is disabled for designated initializers in C.
+  // This matches gcc behaviour.
+  bool IsCDesignatedInitializer =
+      HasDesignatedInit && !SemaRef.getLangOpts().CPlusPlus;
+  if (!VerifyOnly && InitializedSomething && !RD->isUnion() &&
+      !IList->isIdiomaticZeroInitializer(SemaRef.getLangOpts()) &&
+      !IsCDesignatedInitializer) {
     // It is possible we have one or more unnamed bitfields remaining.
     // Find first (if any) named field and emit warning.
     for (RecordDecl::field_iterator it = HasDesignatedInit ? RD->field_begin()
@@ -2376,9 +2374,10 @@ void InitListChecker::CheckStructUnionTypes(
 
       if (!it->isUnnamedBitfield() && !it->hasInClassInitializer() &&
           !it->getType()->isIncompleteArrayType()) {
-        SemaRef.Diag(IList->getSourceRange().getEnd(),
-                     diag::warn_missing_field_initializers)
-            << *it;
+        auto Diag = HasDesignatedInit
+                        ? diag::warn_missing_designated_field_initializers
+                        : diag::warn_missing_field_initializers;
+        SemaRef.Diag(IList->getSourceRange().getEnd(), Diag) << *it;
         break;
       }
     }

clang/test/SemaCXX/cxx2a-initializer-aggregates.cpp

@@ -4,7 +4,8 @@
 // RUN: %clang_cc1 -std=c++20 %s -verify=cxx20,expected,reorder -Wno-c99-designator -Werror=reorder-init-list -Wno-initializer-overrides
 // RUN: %clang_cc1 -std=c++20 %s -verify=cxx20,expected,override -Wno-c99-designator -Wno-reorder-init-list -Werror=initializer-overrides
 // RUN: %clang_cc1 -std=c++20 %s -verify=cxx20,expected -Wno-c99-designator -Wno-reorder-init-list -Wno-initializer-overrides
-// RUN: %clang_cc1 -std=c++20 %s -verify=cxx20,expected,wmissing -Wmissing-field-initializers -Wno-c99-designator -Wno-reorder-init-list -Wno-initializer-overrides
+// RUN: %clang_cc1 -std=c++20 %s -verify=cxx20,expected,wmissing,wmissing-designated -Wmissing-field-initializers -Wno-c99-designator -Wno-reorder-init-list -Wno-initializer-overrides
+// RUN: %clang_cc1 -std=c++20 %s -verify=cxx20,expected,wmissing -Wmissing-field-initializers -Wno-missing-designated-field-initializers -Wno-c99-designator -Wno-reorder-init-list -Wno-initializer-overrides
 
 
 namespace class_with_ctor {
@@ -50,11 +51,11 @@ A a3 = {
 A a4 = {
   .x = 1, // override-note {{previous}}
   .x = 1 // override-error {{overrides prior initialization}}
-}; // wmissing-warning {{missing field 'y' initializer}}
+}; // wmissing-designated-warning {{missing field 'y' initializer}}
 A a5 = {
   .y = 1, // override-note {{previous}}
   .y = 1 // override-error {{overrides prior initialization}}
-}; // wmissing-warning {{missing field 'x' initializer}}
+}; // wmissing-designated-warning {{missing field 'x' initializer}}
 B b2 = {.a = 1}; // pedantic-error {{brace elision for designated initializer is a C99 extension}}
                  // wmissing-warning@-1 {{missing field 'y' initializer}}
 B b3 = {.a = 1, 2}; // pedantic-error {{mixture of designated and non-designated}} pedantic-note {{first non-designated}} pedantic-error {{brace elision}}
@@ -74,8 +75,8 @@ C c = {
 struct Foo { int a, b; };
 
 struct Foo foo0 = { 1 }; // wmissing-warning {{missing field 'b' initializer}}
-struct Foo foo1 = { .a = 1 }; // wmissing-warning {{missing field 'b' initializer}}
-struct Foo foo2 = { .b = 1 }; // wmissing-warning {{missing field 'a' initializer}}
+struct Foo foo1 = { .a = 1 }; // wmissing-designated-warning {{missing field 'b' initializer}}
+struct Foo foo2 = { .b = 1 }; // wmissing-designated-warning {{missing field 'a' initializer}}
 
 }