[clang][Interp] Don't zero-init unions

Zero-initializing them would accidentally activate the members.

Author: tbaederr

clang/lib/AST/Interp/Compiler.cpp

@@ -2523,7 +2523,7 @@ bool Compiler<Emitter>::VisitCXXConstructExpr(const CXXConstructExpr *E) {
     if (E->requiresZeroInitialization()) {
       const Record *R = getRecord(E->getType());
 
-      if (!this->visitZeroRecordInitializer(R, E))
+      if (!R->isUnion() && !this->visitZeroRecordInitializer(R, E))
         return false;
 
       // If the constructor is trivial anyway, we're done.

clang/test/AST/Interp/unions.cpp

@@ -253,4 +253,19 @@ namespace Nested {
                               // both-note {{in call to}}
 
 }
+
+
+namespace Zeroing {
+  struct non_trivial_constructor {
+      constexpr non_trivial_constructor() : x(100) {}
+      int x;
+  };
+  union U2 {
+      int a{1000};
+      non_trivial_constructor b;
+  };
+
+  static_assert(U2().b.x == 100, ""); // both-error {{not an integral constant expression}} \
+                                      // both-note {{read of member 'b' of union with active member 'a'}}
+}
 #endif