[clang][bytecode] Fix reading union template parameter object (#179899)

Before this patch, reading union template parameter object will trigger
diagnostics saying it's not initialized. This patch fixes this issue.

Reading union template parameter with no active fields, class type
fields, or bit-fields is handled as a drive-by.

AI usage: The implementation was generated by AI and modified by me
afterwards.

Assisted-by: GPT-5.2

---------

Co-authored-by: Timm Baeder <tbaeder@redhat.com>

clang/lib/AST/ByteCode/Compiler.cpp

diff --git a/clang/lib/AST/ByteCode/Compiler.cpp b/clang/lib/AST/ByteCode/Compiler.cpp
index eb52112..a0138c4 100644
--- a/clang/lib/AST/ByteCode/Compiler.cpp
+++ b/clang/lib/AST/ByteCode/Compiler.cpp
@@ -5069,14 +5069,29 @@
   }
   if (Val.isUnion()) {
     const FieldDecl *UnionField = Val.getUnionField();
-    const Record *R = this->getRecord(UnionField->getParent());
+    if (!UnionField)
+      return true;
+    const Record *R = this->getRecord(T);
     assert(R);
     const APValue &F = Val.getUnionValue();
     const Record::Field *RF = R->getField(UnionField);
-    PrimType T = classifyPrim(RF->Decl->getType());
-    if (!this->visitAPValue(F, T, E))
+    QualType FieldType = RF->Decl->getType();
+
+    if (OptPrimType PT = classify(FieldType)) {
+      if (!this->visitAPValue(F, *PT, E))
+        return false;
+      if (RF->isBitField())
+        return this->emitInitBitFieldActivate(*PT, RF, E);
+      return this->emitInitFieldActivate(*PT, RF->Offset, E);
+    }
+
+    if (!this->emitGetPtrField(RF->Offset, E))
       return false;
-    return this->emitInitField(T, RF->Offset, E);
+    if (!this->emitActivate(E))
+      return false;
+    if (!this->visitAPValueInitializer(F, E, FieldType))
+      return false;
+    return this->emitPopPtr(E);
   }
   if (Val.isArray()) {
     const auto *ArrType = T->getAsArrayTypeUnsafe();
@@ -7126,8 +7141,9 @@
           return false;
         return this->emitInitGlobal(*T, *Index, E);
       }
-      return this->visitAPValueInitializer(TPOD->getValue(), E,
-                                           TPOD->getType());
+      if (!this->visitAPValueInitializer(TPOD->getValue(), E, TPOD->getType()))
+        return false;
+      return this->emitFinishInit(E);
     }
     return false;
   }

clang/test/AST/ByteCode/cxx20.cpp

diff --git a/clang/test/AST/ByteCode/cxx20.cpp b/clang/test/AST/ByteCode/cxx20.cpp
index 2abe8dd..139b6c8 100644
--- a/clang/test/AST/ByteCode/cxx20.cpp
+++ b/clang/test/AST/ByteCode/cxx20.cpp
@@ -785,6 +785,30 @@
   constexpr const A &w = get<A{1, &g, &A::n, "hello"}>;
 }
 
+namespace InitFromAPValues {
+  template <auto a> struct S {
+    static constexpr auto &ref = a;
+  };
+
+  union U1 { int x, y; };
+  static_assert(S<U1{1}>::ref.x == 1);
+  static_assert(S<U1{1}>::ref.y == 1); // both-error {{static assertion expression is not an integral constant expression}} \
+                                       // both-note {{read of member 'y' of union with active member 'x' is not allowed in a constant expression}}
+
+  union U2 {
+    bool x;
+    constexpr U2() {}
+  };
+  static_assert(S<U2{}>::ref.x); // both-error {{static assertion expression is not an integral constant expression}} \
+                                 // both-note {{read of member 'x' of union with no active member is not allowed in a constant expression}}
+
+  union U3 {
+    struct S { int x; };
+    S s;
+  };
+  static_assert(S<U3{2}>::ref.s.x == 2);
+}
+
 namespace self_referencing {
   struct S {
     S* ptr = nullptr;