diff --git a/src/lower_il.c b/src/lower_il.c
index 34ee33cb6f..4493746e7b 100644
--- a/src/lower_il.c
+++ b/src/lower_il.c
@@ -1182,20 +1182,19 @@ offset for the field.
   /* Store the base class name. */
   (void)strcpy(name_ptr+prefix_length, temp_name);
   /* Create the field. */
-#if IA64_ABI
   field_ptr = add_field(name_ptr, field_type, field_offset, struct_type,
                         /*for_base_class=*/TRUE);
-  if (targ_reuse_tail_padding && is_immediate_class_type(field_type) &&
+  if (
+#if IA64_ABI
+      targ_reuse_tail_padding &&
+#endif /* IA64_ABI */
+      is_immediate_class_type(field_type) &&
       class_type_supp(field_type)->compiler_generated &&
       (field_type->size % field_type->alignment) != 0) {
     /* There is padding at the end of the base class subobject in which
        derived class members may be allocated. */
     field_ptr->class_subobject_with_tail_padding = TRUE;
   }  /* if */
-#else /* !IA64_ABI */
-  field_ptr = add_field(name_ptr, field_type, field_offset, struct_type,
-                        /*for_base_class=*/TRUE);
-#endif /* IA64_ABI */
   field_ptr->is_lowered_base_class = TRUE;
 }  /* add_base_class_dummy_field */
 
@@ -8534,13 +8533,21 @@ this routine to do a relatively simple copy of all the fields.
 
   ctsp = class_type->variant.class_struct_union.extra_info;
   if ((!class_type->variant.class_struct_union.any_virtual_base_classes
+#if !IA64_ABI
+       /* The size as a base can still be smaller than the size of the class
+          (in Microsoft mode, which ignores some alignment requirements when
+          computing the size of a base class). */
+       && (type_is(class_type, tk_union) ||
+           ctsp->size_without_virtual_base_classes == class_type->size)
+#endif /* !IA64_ABI */
 #if CFRONT_OBJECT_CODE_COMPATIBILITY
-      || !class_has_independently_allocated_virtual_base_classes(class_type)
+      || (class_type->variant.class_struct_union.any_virtual_base_classes &&
+          !class_has_independently_allocated_virtual_base_classes(class_type))
 #endif /* CFRONT_OBJECT_CODE_COMPATIBILITY */
                                                                             )
 #if IA64_ABI
       && (!targ_reuse_tail_padding ||
-          class_type->kind == (a_type_kind)tk_union ||
+          type_is(class_type, tk_union) ||
           ctsp->size_without_virtual_base_classes == class_type->size)
 #endif /* IA64_ABI */
                                                                       ) {
@@ -10020,7 +10027,7 @@ Do IL lowering of the indicated field and everything under it.
     mark_as_visited(field);
     lower_source_correspondence(&field->source_corresp);
     lower_os_type(field->type);
-#if BACK_END_IS_C_GEN_BE && IA64_ABI && MAINTAIN_NEEDED_FLAGS
+#if BACK_END_IS_C_GEN_BE && MAINTAIN_NEEDED_FLAGS
     if (field->class_subobject_with_tail_padding) {
       /* Ensure that the corresponding subobject type definition is not
          removed from the IL, as it will be needed by the C-generating back
@@ -10031,7 +10038,7 @@ Do IL lowering of the indicated field and everything under it.
       class_type_supp(ftp)->subobject_partner->
                        variant.class_struct_union.keep_definition_in_il = TRUE;
     }  /* if */
-#endif /* BACK_END_IS_C_GEN_BE && IA64_ABI && MAINTAIN_NEEDED_FLAGS */
+#endif /* BACK_END_IS_C_GEN_BE && MAINTAIN_NEEDED_FLAGS */
   }  /* if */
 }  /* lower_field */
 
diff --git a/src/pending_changes/gh-479-lowered-base-class-size-without-ia64-abi.txt b/src/pending_changes/gh-479-lowered-base-class-size-without-ia64-abi.txt
new file mode 100644
index 0000000000..96b89a879c
--- /dev/null
+++ b/src/pending_changes/gh-479-lowered-base-class-size-without-ia64-abi.txt
@@ -0,0 +1,18 @@
+[GH #479]
+Lowered layout of Microsoft-mode base classes without the IA-64 ABI
+
+In front ends configured with IA64_ABI set to FALSE, IL lowering did not
+account for the padding at the end of a base class subobject.  In Microsoft
+mode, a base class whose alignment is stricter than the platform default
+can occupy less space as a base class subobject than its full size, so
+later members of the derived class are allocated in what would otherwise be
+its tail padding.  IL lowering nonetheless emitted such a base class as a
+field of the complete base class type, so the C compiler placed the derived
+class members at larger offsets than the front end had computed (e.g., than
+those used for sizeof and offsetof).  Such a base class is now
+lowered to a type-as-subobject whose members are emitted inline, as is done
+for IA-64 ABI configurations.  For example, with --ms_c++20:
+
+  struct B { alignas(16) char arr[2]; };
+  struct D : B { char d; };  // d is at offset 8.
+  void f(D *p) { p->d = 'x'; }  // Previously accessed offset 16.
diff --git a/tests/tests/regressions/.gh-479.rto/default.1.1.txt b/tests/tests/regressions/.gh-479.rto/default.1.1.txt
new file mode 100644
index 0000000000..9de71c7b9b
--- /dev/null
+++ b/tests/tests/regressions/.gh-479.rto/default.1.1.txt
@@ -0,0 +1,5 @@
+eccp -DTEST_NUMBER=1 --c++11 --g++ --gnu_version=150200 Test_name.c
+"Test_name.c", line 13: warning: offsetof applied to a type other than a standard-layout class
+    long fe_offset = __builtin_offsetof(D, d);  // Front end's layout.
+                                        ^
+
diff --git a/tests/tests/regressions/.gh-479.rto/default.1.2.txt b/tests/tests/regressions/.gh-479.rto/default.1.2.txt
new file mode 100644
index 0000000000..ff2642c6d5
--- /dev/null
+++ b/tests/tests/regressions/.gh-479.rto/default.1.2.txt
@@ -0,0 +1 @@
+./a.out
diff --git a/tests/tests/regressions/.gh-479.rto/default.2.1.txt b/tests/tests/regressions/.gh-479.rto/default.2.1.txt
new file mode 100644
index 0000000000..c915171df0
--- /dev/null
+++ b/tests/tests/regressions/.gh-479.rto/default.2.1.txt
@@ -0,0 +1,5 @@
+eccp -DTEST_NUMBER=2 --c++20 --clang --clang_version=210000 Test_name.c
+"Test_name.c", line 13: warning: offsetof applied to a type other than a standard-layout class
+    long fe_offset = __builtin_offsetof(D, d);  // Front end's layout.
+                                        ^
+
diff --git a/tests/tests/regressions/.gh-479.rto/default.2.2.txt b/tests/tests/regressions/.gh-479.rto/default.2.2.txt
new file mode 100644
index 0000000000..ff2642c6d5
--- /dev/null
+++ b/tests/tests/regressions/.gh-479.rto/default.2.2.txt
@@ -0,0 +1 @@
+./a.out
diff --git a/tests/tests/regressions/.gh-479.rto/default.3.1.txt b/tests/tests/regressions/.gh-479.rto/default.3.1.txt
new file mode 100644
index 0000000000..d4af7abaea
--- /dev/null
+++ b/tests/tests/regressions/.gh-479.rto/default.3.1.txt
@@ -0,0 +1,5 @@
+eccp -DTEST_NUMBER=3 --ms_c++20 --microsoft_version 1950 Test_name.c
+"Test_name.c", line 13: warning: offsetof applied to a type other than a standard-layout class
+    long fe_offset = __builtin_offsetof(D, d);  // Front end's layout.
+                                        ^
+
diff --git a/tests/tests/regressions/.gh-479.rto/default.3.2.txt b/tests/tests/regressions/.gh-479.rto/default.3.2.txt
new file mode 100644
index 0000000000..ff2642c6d5
--- /dev/null
+++ b/tests/tests/regressions/.gh-479.rto/default.3.2.txt
@@ -0,0 +1 @@
+./a.out
diff --git a/tests/tests/regressions/gh-479.sft.cpp b/tests/tests/regressions/gh-479.sft.cpp
new file mode 100644
index 0000000000..18fd0cc5a1
--- /dev/null
+++ b/tests/tests/regressions/gh-479.sft.cpp
@@ -0,0 +1,16 @@
+//type:rp
+//options:--c++11 --g++ --gnu_version=150200:--c++20 --clang --clang_version=210000:--ms_c++20 --microsoft_version 1950
+// A base class whose alignment requirement leaves padding at the end of the
+// base class subobject must have that padding accounted for when IL lowering
+// places the members of a derived class.  The offset of "d" in the lowered
+// code must match the offset computed by the front end.
+
+struct B { alignas(16) char c; };
+struct D : B { char d; };
+
+int main() {
+  D x;
+  long fe_offset = __builtin_offsetof(D, d);  // Front end's layout.
+  long be_offset = (char *)&x.d - (char *)&x;  // Lowered C's layout.
+  return fe_offset != be_offset;
+}