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
@@ -6123,8 +6123,8 @@
   "array subscript is not an integer">;
 def err_subscript_function_type : Error<
   "subscript of pointer to function type %0">;
-def err_subscript_incomplete_type : Error<
-  "subscript of pointer to incomplete type %0">;
+def err_subscript_incomplete_or_sizeless_type : Error<
+  "subscript of pointer to %select{incomplete|sizeless}0 type %1">;
 def err_dereference_incomplete_type : Error<
   "dereference of pointer to incomplete type %0">;
 def ext_gnu_subscript_void_type : Extension<
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -4885,8 +4885,9 @@
     // See IsCForbiddenLValueType.
     if (!ResultType.hasQualifiers()) VK = VK_RValue;
   } else if (!ResultType->isDependentType() &&
-      RequireCompleteType(LLoc, ResultType,
-                          diag::err_subscript_incomplete_type, BaseExpr))
+             RequireCompleteSizedType(
+                 LLoc, ResultType,
+                 diag::err_subscript_incomplete_or_sizeless_type, BaseExpr))
     return ExprError();
 
   assert(VK == VK_RValue || LangOpts.CPlusPlus ||
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
@@ -142,6 +142,10 @@
   *&volatile_int8 = local_int8;
   *&const_volatile_int8 = local_int8; // expected-error {{read-only variable is not assignable}}
 
+  global_int8_ptr[0] = local_int8;       // expected-error {{subscript of pointer to sizeless type 'svint8_t'}}
+  global_int8_ptr[1] = local_int8;       // expected-error {{subscript of pointer to sizeless type 'svint8_t'}}
+  global_int8_ptr = &global_int8_ptr[2]; // expected-error {{subscript of pointer to sizeless type 'svint8_t'}}
+
   overf(local_int8);
   overf(local_int16);
 
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
@@ -163,6 +163,10 @@
   *&volatile_int8 = local_int8;
   *&const_volatile_int8 = local_int8; // expected-error {{read-only variable is not assignable}}
 
+  global_int8_ptr[0] = local_int8;       // expected-error {{subscript of pointer to sizeless type 'svint8_t'}}
+  global_int8_ptr[1] = local_int8;       // expected-error {{subscript of pointer to sizeless type 'svint8_t'}}
+  global_int8_ptr = &global_int8_ptr[2]; // expected-error {{subscript of pointer to sizeless type 'svint8_t'}}
+
   overf(local_int8);
   overf(local_int16);