diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -1143,7 +1143,7 @@
 def fapple_kext : Flag<["-"], "fapple-kext">, Group<f_Group>, Flags<[CC1Option]>,
   HelpText<"Use Apple's kernel extensions ABI">,
   MarshallingInfoFlag<LangOpts<"AppleKext">>;
-def fstrict_flex_arrays_EQ : Joined<["-"], "fstrict-flex-arrays=">,Group<f_Group>,
+def fstrict_flex_arrays_EQ : Joined<["-"], "fstrict-flex-arrays=">, Group<f_Group>,
   MetaVarName<"<n>">, Values<"0,1,2">,
   LangOpts<"StrictFlexArrays">,
   Flags<[CC1Option]>,
diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -906,10 +906,8 @@
     if (const auto *FD = dyn_cast<FieldDecl>(ME->getMemberDecl())) {
       // FIXME: Sema doesn't treat a T[1] union member as a flexible array
       // member, only a T[0] or T[] member gets that treatment.
-      // Under StrictFlexArraysLevel, obey c99+ that disallows FAM in union, see
-      // C11 6.7.2.1 §18
       if (FD->getParent()->isUnion())
-        return StrictFlexArraysLevel < 2;
+        return true;
       RecordDecl::field_iterator FI(
           DeclContext::decl_iterator(const_cast<FieldDecl *>(FD)));
       return ++FI == FD->getParent()->field_end();
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -15888,16 +15888,10 @@
   if (!ND)
     return false;
 
-  if (StrictFlexArraysLevel >= 2 && Size != 0)
-    return false;
-
-  if (StrictFlexArraysLevel == 1 && Size.uge(2))
-    return false;
-
   // FIXME: While the default -fstrict-flex-arrays=0 permits Size>1 trailing
   // arrays to be treated as flexible-array-members, we still emit diagnostics
   // as if they are not. Pending further discussion...
-  if (StrictFlexArraysLevel == 0 && Size != 1)
+  if (StrictFlexArraysLevel >= 2 || Size.uge(2))
     return false;
 
   const FieldDecl *FD = dyn_cast<FieldDecl>(ND);
@@ -16065,8 +16059,8 @@
     if (BaseType->isIncompleteType())
       return;
 
-    // FIXME: this check should belong to the IsTailPaddedMemberArray call
-    // below.
+    // FIXME: this check should be used to set IsUnboundedArray from the
+    // beginning.
     llvm::APInt size = ArrayTy->getSize();
     if (!size.isStrictlyPositive())
       return;
diff --git a/clang/test/CodeGen/bounds-checking-fam.c b/clang/test/CodeGen/bounds-checking-fam.c
--- a/clang/test/CodeGen/bounds-checking-fam.c
+++ b/clang/test/CodeGen/bounds-checking-fam.c
@@ -35,6 +35,51 @@
   return p->a[i] + (p->a)[i];
 }
 
+union uZero {
+  int a[0];
+};
+union uOne {
+  int a[1];
+};
+union uTwo {
+  int a[2];
+};
+union uThree {
+  int a[3];
+};
+
+// CHECK-LABEL: define {{.*}} @{{.*}}test_uzero{{.*}}(
+int test_uzero(union uZero *p, int i) {
+  // CHECK-STRICT-0-NOT: @__ubsan
+  // CHECK-STRICT-1-NOT: @__ubsan
+  // CHECK-STRICT-2-NOT: @__ubsan
+  return p->a[i] + (p->a)[i];
+}
+
+// CHECK-LABEL: define {{.*}} @{{.*}}test_uone{{.*}}(
+int test_uone(union uOne *p, int i) {
+  // CHECK-STRICT-0-NOT: @__ubsan
+  // CHECK-STRICT-1-NOT: @__ubsan
+  // CHECK-STRICT-2: @__ubsan
+  return p->a[i] + (p->a)[i];
+}
+
+// CHECK-LABEL: define {{.*}} @{{.*}}test_utwo{{.*}}(
+int test_utwo(union uTwo *p, int i) {
+  // CHECK-STRICT-0: @__ubsan
+  // CHECK-STRICT-1: @__ubsan
+  // CHECK-STRICT-2: @__ubsan
+  return p->a[i] + (p->a)[i];
+}
+
+// CHECK-LABEL: define {{.*}} @{{.*}}test_uthree{{.*}}(
+int test_uthree(union uThree *p, int i) {
+  // CHECK-STRICT-0: @__ubsan
+  // CHECK-STRICT-1: @__ubsan
+  // CHECK-STRICT-2: @__ubsan
+  return p->a[i] + (p->a)[i];
+}
+
 // CHECK-LABEL: define {{.*}} @{{.*}}test_three{{.*}}(
 int test_three(struct Three *p, int i) {
   // CHECK-STRICT-0:     call void @__ubsan_handle_out_of_bounds_abort(
diff --git a/clang/test/CodeGen/object-size-flex-array.c b/clang/test/CodeGen/object-size-flex-array.c
--- a/clang/test/CodeGen/object-size-flex-array.c
+++ b/clang/test/CodeGen/object-size-flex-array.c
@@ -24,7 +24,7 @@
   double c[2];
 } foo2_t;
 
-// CHECK-LABEL: @bar
+// CHECK-LABEL: @bar(
 unsigned bar(foo_t *f) {
   // CHECK-STRICT-0: ret i32 %
   // CHECK-STRICT-1: ret i32 %
@@ -32,7 +32,7 @@
   return OBJECT_SIZE_BUILTIN(f->c, 1);
 }
 
-// CHECK-LABEL: @bar0
+// CHECK-LABEL: @bar0(
 unsigned bar0(foo0_t *f) {
   // CHECK-STRICT-0: ret i32 %
   // CHECK-STRICT-1: ret i32 %
@@ -40,7 +40,7 @@
   return OBJECT_SIZE_BUILTIN(f->c, 1);
 }
 
-// CHECK-LABEL: @bar1
+// CHECK-LABEL: @bar1(
 unsigned bar1(foo1_t *f) {
   // CHECK-STRICT-0: ret i32 %
   // CHECK-STRICT-1: ret i32 %
@@ -48,7 +48,7 @@
   return OBJECT_SIZE_BUILTIN(f->c, 1);
 }
 
-// CHECK-LABEL: @bar2
+// CHECK-LABEL: @bar2(
 unsigned bar2(foo2_t *f) {
   // CHECK-STRICT-0: ret i32 %
   // CHECK-STRICT-1: ret i32 16
@@ -73,7 +73,7 @@
   float f;
 } foofoo2_t;
 
-// CHECK-LABEL: @babar0
+// CHECK-LABEL: @babar0(
 unsigned babar0(foofoo0_t *f) {
   // CHECK-STRICT-0: ret i32 0
   // CHECK-STRICT-1: ret i32 0
@@ -81,7 +81,7 @@
   return OBJECT_SIZE_BUILTIN(f->c, 1);
 }
 
-// CHECK-LABEL: @babar1
+// CHECK-LABEL: @babar1(
 unsigned babar1(foofoo1_t *f) {
   // CHECK-STRICT-0: ret i32 8
   // CHECK-STRICT-1: ret i32 8
@@ -89,7 +89,7 @@
   return OBJECT_SIZE_BUILTIN(f->c, 1);
 }
 
-// CHECK-LABEL: @babar2
+// CHECK-LABEL: @babar2(
 unsigned babar2(foofoo2_t *f) {
   // CHECK-STRICT-0: ret i32 16
   // CHECK-STRICT-1: ret i32 16