c++: Fix up __builtin_{bit_cast,convertvector} parsing

Jonathan reported on IRC that we don't parse
__builtin_bit_cast (type, val).field
etc.
The problem is that for these 2 builtins we return from
cp_parser_postfix_expression instead of setting postfix_expression
to the cp_build_* value and falling through into the postfix regression
suffix handling loop.

2022-03-26  Jakub Jelinek  <jakub@redhat.com>

	* parser.cc (cp_parser_postfix_expression)
	<case RID_BILTIN_CONVERTVECTOR, case RID_BUILTIN_BIT_CAST>: Don't
	return cp_build_{vec,convert,bit_cast} result right away, instead
	set postfix_expression to it and break.

	* c-c++-common/builtin-convertvector-3.c: New test.
	* g++.dg/cpp2a/bit-cast15.C: New test.

gcc/cp/parser.cc

@@ -7525,8 +7525,10 @@ cp_parser_postfix_expression (cp_parser *parser, bool address_p, bool cast_p,
 	}
 	/* Look for the closing `)'.  */
 	parens.require_close (parser);
-	return cp_build_vec_convert (expression, type_location, type,
-				     tf_warning_or_error);
+	postfix_expression
+	  = cp_build_vec_convert (expression, type_location, type,
+				  tf_warning_or_error);
+	break;
       }
 
     case RID_BUILTIN_BIT_CAST:
@@ -7551,8 +7553,10 @@ cp_parser_postfix_expression (cp_parser *parser, bool address_p, bool cast_p,
 	expression = cp_parser_assignment_expression (parser);
 	/* Look for the closing `)'.  */
 	parens.require_close (parser);
-	return cp_build_bit_cast (type_location, type, expression,
-				  tf_warning_or_error);
+	postfix_expression
+	  = cp_build_bit_cast (type_location, type, expression,
+			       tf_warning_or_error);
+	break;
       }
 
     default:

gcc/testsuite/c-c++-common/builtin-convertvector-3.c

@@ -0,0 +1,11 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+typedef int v4si __attribute__((vector_size (4 * sizeof (int))));
+typedef double v4df __attribute__((vector_size (4 * sizeof (double))));
+double
+foo (void)
+{
+  v4si a = { 1, 2, 3, 4 };
+  return __builtin_convertvector (a, v4df)[1];
+}

gcc/testsuite/g++.dg/cpp2a/bit-cast15.C

@@ -0,0 +1,19 @@
+// { dg-do compile }
+
+struct S { short a, b; };
+struct T { float a[4]; };
+struct U { int b[4]; };
+
+#if __SIZEOF_FLOAT__ == __SIZEOF_INT__
+int
+f1 (T &x)
+{
+  return __builtin_bit_cast (U, x).b[1];
+}
+
+float
+f2 (int (&x)[4])
+{
+  return __builtin_bit_cast (T, x).a[2];
+}
+#endif