diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -5925,7 +5925,7 @@
   "incomplete result type %0 in function definition">;
 def err_atomic_specifier_bad_type : Error<
   "_Atomic cannot be applied to "
-  "%select{incomplete |array |function |reference |atomic |qualified |}0type "
+  "%select{incomplete |array |function |reference |atomic |qualified |sizeless |}0type "
   "%1 %select{||||||which is not trivially copyable}0">;
 
 // Expressions.
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -8564,9 +8564,11 @@
       DisallowedKind = 4;
     else if (T.hasQualifiers())
       DisallowedKind = 5;
+    else if (T->isSizelessType())
+      DisallowedKind = 6;
     else if (!T.isTriviallyCopyableType(Context))
       // Some other non-trivially-copyable type (probably a C++ class)
-      DisallowedKind = 6;
+      DisallowedKind = 7;
 
     if (DisallowedKind != -1) {
       Diag(Loc, diag::err_atomic_specifier_bad_type) << DisallowedKind << T;
diff --git a/clang/test/Sema/sizeless-1.c b/clang/test/Sema/sizeless-1.c
--- a/clang/test/Sema/sizeless-1.c
+++ b/clang/test/Sema/sizeless-1.c
@@ -84,6 +84,7 @@
   const volatile svint8_t const_volatile_int8 = local_int8; // expected-note {{declared const here}}
   const volatile svint8_t uninit_const_volatile_int8;
 
+  _Atomic svint8_t atomic_int8;      // expected-error {{_Atomic cannot be applied to sizeless type 'svint8_t'}}
   __restrict svint8_t restrict_int8; // expected-error {{requires a pointer or reference}}
 
   _Bool test_int8 = init_int8; // expected-error {{initializing '_Bool' with an expression of incompatible type 'svint8_t'}}
diff --git a/clang/test/SemaCXX/sizeless-1.cpp b/clang/test/SemaCXX/sizeless-1.cpp
--- a/clang/test/SemaCXX/sizeless-1.cpp
+++ b/clang/test/SemaCXX/sizeless-1.cpp
@@ -98,6 +98,7 @@
   const volatile svint8_t const_volatile_int8 = local_int8; // expected-note {{declared const here}}
   const volatile svint8_t uninit_const_volatile_int8;       // expected-error {{default initialization of an object of const type 'const volatile svint8_t'}}
 
+  _Atomic svint8_t atomic_int8;      // expected-error {{_Atomic cannot be applied to sizeless type 'svint8_t'}}
   __restrict svint8_t restrict_int8; // expected-error {{requires a pointer or reference}}
 
   bool test_int8 = init_int8; // expected-error {{cannot initialize a variable of type 'bool' with an lvalue of type 'svint8_t'}}