[clang][Interp] Fix checking unions for initialization

Author: tbaederr

clang/lib/AST/Interp/EvaluationResult.cpp

@@ -115,6 +115,10 @@ static bool CheckFieldsInitialized(InterpState &S, SourceLocation Loc,
       DiagnoseUninitializedSubobject(S, Loc, F.Decl);
       Result = false;
     }
+
+    // Only the first member of a union needs to be initialized.
+    if (R->isUnion())
+      break;
   }
 
   // Check Fields in all bases

clang/test/AST/Interp/unions.cpp

@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -fexperimental-new-constant-interpreter -verify=expected,both %s
+// RUN: %clang_cc1 -verify=ref,both %s
+
+// both-no-diagnostics
+
+union U {
+  int a;
+  int b;
+};
+
+constexpr U a = {12};
+static_assert(a.a == 12, "");
+
+