[Sema] Fix handling of fields with initializers in nested anonymous unions.

Make sure we count the anonymous union as an initialized field, so we
properly construct the AST.

Included bonus testcase Test3, which shows a remaining gap: an anonymous
union can contain a partially initialized anonymous struct, and we
handle that inconsistently.

Fixes #91257

Author: efriedma-quic

clang/lib/Sema/SemaInit.cpp

@@ -813,19 +813,13 @@ InitListChecker::FillInEmptyInitializations(const InitializedEntity &Entity,
 
   if (const RecordType *RType = ILE->getType()->getAs<RecordType>()) {
     const RecordDecl *RDecl = RType->getDecl();
-    if (RDecl->isUnion() && ILE->getInitializedFieldInUnion())
+    if (RDecl->isUnion() && ILE->getInitializedFieldInUnion()) {
       FillInEmptyInitForField(0, ILE->getInitializedFieldInUnion(),
                               Entity, ILE, RequiresSecondPass, FillWithNoInit);
-    else if (RDecl->isUnion() && isa<CXXRecordDecl>(RDecl) &&
-             cast<CXXRecordDecl>(RDecl)->hasInClassInitializer()) {
-      for (auto *Field : RDecl->fields()) {
-        if (Field->hasInClassInitializer()) {
-          FillInEmptyInitForField(0, Field, Entity, ILE, RequiresSecondPass,
-                                  FillWithNoInit);
-          break;
-        }
-      }
     } else {
+      assert((!RDecl->isUnion() || !isa<CXXRecordDecl>(RDecl) ||
+              !cast<CXXRecordDecl>(RDecl)->hasInClassInitializer()) &&
+             "We should have computed initialized fields already");
       // The fields beyond ILE->getNumInits() are default initialized, so in
       // order to leave them uninitialized, the ILE is expanded and the extra
       // fields are then filled with NoInitExpr.
@@ -2163,12 +2157,15 @@ void InitListChecker::CheckStructUnionTypes(
         return;
       for (RecordDecl::field_iterator FieldEnd = RD->field_end();
            Field != FieldEnd; ++Field) {
-        if (Field->hasInClassInitializer()) {
+        if (Field->hasInClassInitializer() ||
+            (Field->isAnonymousStructOrUnion() &&
+             Field->getType()->getAsCXXRecordDecl()->hasInClassInitializer())) {
           StructuredList->setInitializedFieldInUnion(*Field);
           // FIXME: Actually build a CXXDefaultInitExpr?
           return;
         }
       }
+      llvm_unreachable("Couldn't find in-class initializer");
     }
 
     // Value-initialize the first member of the union that isn't an unnamed

clang/test/AST/ast-dump-APValue-anon-union.cpp

@@ -36,7 +36,7 @@ void Test() {
 
   constexpr U0 u0a{};
   // CHECK:  | `-VarDecl {{.*}} <col:{{.*}}, col:{{.*}}> col:{{.*}} u0a 'const U0' constexpr listinit
-  // CHECK-NEXT:  |   |-value: Union None
+  // CHECK-NEXT:  |   |-value: Union .U0::(anonymous union at {{.*}}) Union .f Float 3.141500e+00
 
   constexpr U0 u0b{3.1415f};
   // CHECK:  | `-VarDecl {{.*}} <col:{{.*}}, col:{{.*}}> col:{{.*}} u0b 'const U0' constexpr listinit

clang/test/SemaCXX/cxx1y-initializer-aggregates.cpp

@@ -77,3 +77,38 @@ namespace use_self {
 
   int fib(int n) { return FibTree{n}.v; }
 }
+
+namespace nested_union {
+  union Test1 {
+    union {
+      int inner { 42 };
+    };
+    int outer;
+  };
+  static_assert(Test1{}.inner == 42, "");
+  struct Test2 {
+    union {
+      struct {
+        int inner : 32 { 42 }; // expected-warning {{C++20 extension}}
+      };
+      int outer;
+    };
+  };
+  static_assert(Test2{}.inner == 42, "");
+  struct Int { int x; };
+  struct Test3 {
+    int x;
+    union {
+      struct {
+        const int& y;
+        int inner : 32 { 42 }; // expected-warning {{C++20 extension}}
+      };
+      int outer;
+    };
+  };
+  constexpr char f(Test3) { return 1; } // expected-note {{candidate function}}
+  constexpr char f(Int) { return 2; } // expected-note {{candidate function}}
+  // FIXME: This shouldn't be ambiguous; either we should reject the declaration
+  // of Test3, or we should exclude f(Test3) as a candidate.
+  static_assert(f({1}) == 2, ""); // expected-error {{call to 'f' is ambiguous}}
+}