diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -9798,7 +9798,10 @@
     ThisOverrideRAII ThisOverride(*Info.CurrentCall, &This,
                                   isa<CXXDefaultInitExpr>(InitExpr));
 
-    return EvaluateInPlace(Result.getUnionValue(), Info, Subobject, InitExpr);
+    return EvaluateInPlace(Result.getUnionValue(), Info, Subobject, InitExpr) ||
+           (Field->isBitField() &&
+            truncateBitfieldValue(Info, InitExpr, Result.getUnionValue(),
+                                  Field));
   }
 
   if (!Result.hasValue())
diff --git a/clang/test/CodeGenCXX/bitfield-layout.cpp b/clang/test/CodeGenCXX/bitfield-layout.cpp
--- a/clang/test/CodeGenCXX/bitfield-layout.cpp
+++ b/clang/test/CodeGenCXX/bitfield-layout.cpp
@@ -84,3 +84,12 @@
   // CHECK: ret i32 0
   return 0;
 }
+
+// CHECK: define i32 @_Z10test_truncv()
+int test_trunc() {
+  union {
+    int i : 4;
+  } U = {15};
+  return U.i;
+  // CHECK: ret i32 -1
+}