Unions are always static since Issue 9244.

dmd2/struct.c

@@ -515,17 +515,6 @@ void StructDeclaration::semantic(Scope *sc)
     protection = sc->protection;
     alignment = sc->structalign;
     storage_class |= sc->stc;
-#if IN_LLVM
-    // DMD allows nested unions with functions that access the outer scope, but
-    // generates invalid code for them (the context pointer is stored at the
-    // same offset as all the union fields, just as if it was a regular member).
-    // LDC would assert on this instead due to a type mismatch when trying to
-    // store the context pointer. This change mitigates the wrong-code/crash
-    // bug – see "[dmd-internals] Nested Unions?" for a discussion on whether
-    // this should be legal or not.
-    if (isUnionDeclaration())
-        storage_class |= STCstatic;
-#endif
     if (sc->stc & STCdeprecated)
         isdeprecated = true;
     assert(!isAnonymous());