--- gcc/doc/extend.texi.jj	2022-02-09 15:16:03.336783697 +0100
+++ gcc/doc/extend.texi	2022-02-11 13:22:39.846157538 +0100
@@ -13993,6 +13993,11 @@ bits that are padding bits for all the u
 This built-in-function is useful if the padding bits of an object might
 have intederminate values and the object representation needs to be
 bitwise compared to some other object, for example for atomic operations.
+
+For C++, @var{ptr} argument type should be pointer to trivially-copyable
+type, unless the argument is address of a variable or parameter, because
+otherwise it isn't known if the type isn't just a base class whose padding
+bits are reused or laid out differently in a derived class.
 @end deftypefn
 
 @deftypefn {Built-in Function} @var{type} __builtin_bit_cast (@var{type}, @var{arg})
--- gcc/cp/call.cc.jj	2022-02-09 20:13:51.523305107 +0100
+++ gcc/cp/call.cc	2022-02-11 12:58:19.168301395 +0100
@@ -10398,6 +10398,27 @@ build_cxx_call (tree fn, int nargs, tree
       if (!check_builtin_function_arguments (EXPR_LOCATION (fn), vNULL, fndecl,
 					     orig_fndecl, nargs, argarray))
 	return error_mark_node;
+      else if (fndecl_built_in_p (fndecl, BUILT_IN_CLEAR_PADDING))
+	{
+	  tree arg0 = argarray[0];
+	  STRIP_NOPS (arg0);
+	  if (TREE_CODE (arg0) == ADDR_EXPR
+	      && DECL_P (TREE_OPERAND (arg0, 0))
+	      && same_type_ignoring_top_level_qualifiers_p
+			(TREE_TYPE (TREE_TYPE (argarray[0])),
+			 TREE_TYPE (TREE_TYPE (arg0))))
+	    /* For __builtin_clear_padding (&var) we know the type
+	       is for a complete object, so there is no risk in clearing
+	       padding that is reused in some derived class member.  */;
+	  else if (!trivially_copyable_p (TREE_TYPE (TREE_TYPE (argarray[0]))))
+	    {
+	      error_at (EXPR_LOC_OR_LOC (argarray[0], input_location),
+			"argument %u in call to function %qE "
+			"has pointer to a non-trivially-copyable type (%qT)",
+			1, fndecl, TREE_TYPE (argarray[0]));
+	      return error_mark_node;
+	    }
+	}
     }
 
   if (VOID_TYPE_P (TREE_TYPE (fn)))
--- gcc/testsuite/g++.dg/cpp2a/builtin-clear-padding1.C.jj	2022-02-11 13:13:49.125471991 +0100
+++ gcc/testsuite/g++.dg/cpp2a/builtin-clear-padding1.C	2022-02-11 13:13:43.403550851 +0100
@@ -0,0 +1,50 @@
+// PR tree-optimization/102586
+// { dg-do compile }
+// { dg-options "-Wno-inaccessible-base" }
+
+struct C0 {};
+struct C1 {};
+struct C2 : C1, virtual C0 {};
+struct C3 : virtual C2, C1 {};
+struct C4 : virtual C3, C1 {};
+struct C5 : C4 {};
+struct C6 { char c; };
+struct C7 : virtual C6, virtual C3, C1 {};
+struct C8 : C7 {};
+
+void
+foo (C0 *c0, C1 *c1, C2 *c2, C3 *c3, C4 *c4, C5 *c5, C6 *c6, C7 *c7, C8 *c8)
+{
+  __builtin_clear_padding (c0);
+  __builtin_clear_padding (c1);
+  __builtin_clear_padding (c2);	// { dg-error "argument 1 in call to function '__builtin_clear_padding' has pointer to a non-trivially-copyable type \\\('C2\\\*'\\\)" }
+  __builtin_clear_padding (c3);	// { dg-error "argument 1 in call to function '__builtin_clear_padding' has pointer to a non-trivially-copyable type \\\('C3\\\*'\\\)" }
+  __builtin_clear_padding (c4);	// { dg-error "argument 1 in call to function '__builtin_clear_padding' has pointer to a non-trivially-copyable type \\\('C4\\\*'\\\)" }
+  __builtin_clear_padding (c5);	// { dg-error "argument 1 in call to function '__builtin_clear_padding' has pointer to a non-trivially-copyable type \\\('C5\\\*'\\\)" }
+  __builtin_clear_padding (c6);
+  __builtin_clear_padding (c7);	// { dg-error "argument 1 in call to function '__builtin_clear_padding' has pointer to a non-trivially-copyable type \\\('C7\\\*'\\\)" }
+  __builtin_clear_padding (c8);	// { dg-error "argument 1 in call to function '__builtin_clear_padding' has pointer to a non-trivially-copyable type \\\('C8\\\*'\\\)" }
+}
+
+void
+bar ()
+{
+  C0 c0;
+  C1 c1;
+  C2 c2;
+  C3 c3;
+  C4 c4;
+  C5 c5;
+  C6 c6;
+  C7 c7;
+  C8 c8;
+  __builtin_clear_padding (&c0);
+  __builtin_clear_padding (&c1);
+  __builtin_clear_padding (&c2);
+  __builtin_clear_padding (&c3);
+  __builtin_clear_padding (&c4);
+//  __builtin_clear_padding (&c5);
+  __builtin_clear_padding (&c6);
+  __builtin_clear_padding (&c7);
+  __builtin_clear_padding (&c8);
+}
