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
@@ -2801,6 +2801,8 @@
   "redeclaration has different alignment requirement (%1 vs %0)">;
 def err_alignas_underaligned : Error<
   "requested alignment is less than minimum alignment of %1 for type %0">;
+def err_attribute_sizeless_type : Error<
+  "%0 attribute cannot be applied to sizeless type %1">;
 def err_attribute_argument_n_type : Error<
   "%0 attribute requires parameter %1 to be %select{int or bool|an integer "
   "constant|a string|an identifier}2">;
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -3868,6 +3868,7 @@
   //   not specify an alignment that is less strict than the alignment that
   //   would otherwise be required for the entity being declared.
   AlignedAttr *AlignasAttr = nullptr;
+  AlignedAttr *LastAlignedAttr = nullptr;
   unsigned Align = 0;
   for (auto *I : D->specific_attrs<AlignedAttr>()) {
     if (I->isAlignmentDependent())
@@ -3875,9 +3876,13 @@
     if (I->isAlignas())
       AlignasAttr = I;
     Align = std::max(Align, I->getAlignment(Context));
+    LastAlignedAttr = I;
   }
 
-  if (AlignasAttr && Align) {
+  if (Align && DiagTy->isSizelessType()) {
+    Diag(LastAlignedAttr->getLocation(), diag::err_attribute_sizeless_type)
+      << LastAlignedAttr << DiagTy;
+  } else if (AlignasAttr && Align) {
     CharUnits RequestedAlign = Context.toCharUnitsFromBits(Align);
     CharUnits NaturalAlign = Context.getTypeAlignInChars(UnderlyingTy);
     if (NaturalAlign > RequestedAlign)
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
@@ -54,6 +54,9 @@
   svint8_t local_int8;
   svint16_t local_int16;
 
+  svint8_t __attribute__((aligned)) aligned_int8_1; // expected-error {{'aligned' attribute cannot be applied to sizeless type 'svint8_t'}}
+  svint8_t __attribute__((aligned(4))) aligned_int8_2; // expected-error {{'aligned' attribute cannot be applied to sizeless type 'svint8_t'}}
+  svint8_t _Alignas(int) aligned_int8_3; // expected-error {{'_Alignas' attribute cannot be applied to sizeless type 'svint8_t'}}
   int _Alignas(svint8_t) aligned_int; // expected-error {{invalid application of 'alignof' to sizeless type 'svint8_t'}}
 
   // Using pointers to sizeless data isn't wrong here, but because the
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
@@ -63,6 +63,9 @@
   svint8_t local_int8;
   svint16_t local_int16;
 
+  svint8_t __attribute__((aligned)) aligned_int8_1; // expected-error {{'aligned' attribute cannot be applied to sizeless type 'svint8_t'}}
+  svint8_t __attribute__((aligned(4))) aligned_int8_2; // expected-error {{'aligned' attribute cannot be applied to sizeless type 'svint8_t'}}
+  svint8_t _Alignas(int) aligned_int8_3; // expected-error {{'_Alignas' attribute cannot be applied to sizeless type 'svint8_t'}}
   int _Alignas(svint8_t) aligned_int; // expected-error {{invalid application of 'alignof' to sizeless type 'svint8_t'}}
 
   // Using pointers to sizeless data isn't wrong here, but because the