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
@@ -1474,8 +1474,8 @@
 def err_array_of_abstract_type : Error<"array of abstract class type %0">;
 def err_capture_of_abstract_type : Error<
   "by-copy capture of value of abstract type %0">;
-def err_capture_of_incomplete_type : Error<
-  "by-copy capture of variable %0 with incomplete type %1">;
+def err_capture_of_incomplete_or_sizeless_type : Error<
+  "by-copy capture of variable %0 with %select{incomplete|sizeless}1 type %2">;
 def err_capture_default_non_local : Error<
   "non-local lambda expression cannot have a capture-default">;
 
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
@@ -16341,9 +16341,10 @@
     // Make sure that by-copy captures are of a complete and non-abstract type.
     if (!Invalid && BuildAndDiagnose) {
       if (!CaptureType->isDependentType() &&
-          S.RequireCompleteType(Loc, CaptureType,
-                                diag::err_capture_of_incomplete_type,
-                                Var->getDeclName()))
+          S.RequireCompleteSizedType(
+              Loc, CaptureType,
+              diag::err_capture_of_incomplete_or_sizeless_type,
+              Var->getDeclName()))
         Invalid = true;
       else if (S.RequireNonAbstractType(Loc, CaptureType,
                                         diag::err_capture_of_abstract_type))
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
@@ -496,6 +496,7 @@
   local_int8 = ([]() -> svint8_t { return svint8_t(); })();
   auto fn1 = [&local_int8](svint8_t x) { local_int8 = x; };
   auto fn2 = [&local_int8](svint8_t *ptr) { *ptr = local_int8; };
+  auto fn3 = [local_int8](svint8_t *ptr) { *ptr = local_int8; }; // expected-error {{by-copy capture of variable 'local_int8' with sizeless type 'svint8_t'}}
 
   for (auto x : local_int8) { // expected-error {{no viable 'begin' function available}}
   }