t/issue745/: Add test case for #745

This currently fails on gcc 15 but succeeds on gcc 14, as expected.

Makefile.am

@@ -329,6 +329,23 @@ BUILT_SOURCES += \
 EXTRA_DIST += \
 	t/issue440/issue440.proto
 
+# Issue #745
+check_PROGRAMS += \
+	t/issue745/issue745
+TESTS += \
+	t/issue745/issue745
+t_issue745_issue745_SOURCES = \
+	t/issue745/issue745.c \
+	t/issue745/issue745.pb-c.c
+t_issue745_issue745_LDADD = \
+	protobuf-c/libprotobuf-c.la
+t/issue745/issue745.pb-c.c t/issue745/issue745.pb-c.h: $(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) $(top_srcdir)/t/issue745/issue745.proto
+	$(AM_V_GEN)@PROTOC@ --plugin=protoc-gen-c=$(top_builddir)/protoc-c/protoc-gen-c$(EXEEXT) -I$(top_srcdir) --c_out=$(top_builddir) $(top_srcdir)/t/issue745/issue745.proto
+BUILT_SOURCES += \
+	t/issue745/issue745.pb-c.c t/issue745/issue745.pb-c.h
+EXTRA_DIST += \
+	t/issue745/issue745.proto
+
 endif # CROSS_COMPILING
 
 endif # BUILD_COMPILER

t/issue745/.gitignore

@@ -0,0 +1 @@
+/issue745

t/issue745/issue745.c

@@ -0,0 +1,41 @@
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "t/issue745/issue745.pb-c.h"
+
+int main(void)
+{
+	T t = T__INIT;
+
+	size_t offset_to_union = offsetof(T, test_bool);
+	size_t size_of_union = sizeof(T) - offset_to_union;
+	unsigned char *ptr_to_union = ((unsigned char *)&t) + offset_to_union;
+
+	assert(offsetof(T, test_bool) == offsetof(T, test_enum));
+	assert(offsetof(T, test_enum) == offsetof(T, test_float));
+	assert(offsetof(T, test_float) == offsetof(T, test_uint32));
+	assert(offsetof(T, test_uint32) == offsetof(T, test_message));
+	assert(offsetof(T, test_message) == offsetof(T, test_string));
+	assert(offsetof(T, test_string) == offsetof(T, test_double));
+	assert(offsetof(T, test_double) == offsetof(T, test_uint64));
+	assert(offsetof(T, test_uint64) == offsetof(T, test_bytes));
+
+	for (size_t i = 0; i < size_of_union; i++) {
+		fprintf(stderr, "ptr_to_union[%zd] = %02x\n", i, ptr_to_union[i]);
+	}
+
+	// The following will probably crash on gcc >= 15 under its default
+	// `-fzero-init-padding-bits=standard` behavior, if the ordering of oneof union
+	// members in T are as performed by protobuf-c <= 1.5.0.
+	//
+	// The code generator in protobuf-c >= 1.5.1 should order union members from
+	// largest to smallest, which should correctly zero all the bits used by the
+	// object representations of the members of the oneof union even on gcc >= 15.
+
+	for (size_t i = 0; i < size_of_union; i++) {
+		assert(ptr_to_union[i] == 0);
+	}
+
+	return EXIT_SUCCESS;
+}

t/issue745/issue745.proto

@@ -0,0 +1,32 @@
+syntax = "proto3";
+
+enum E {
+	FIRST_VALUE = 0;
+	SECOND_VALUE = 1;
+}
+
+message M {
+	int32 test = 1;
+}
+
+message T {
+	oneof test_oneof {
+		bool test_bool = 1;
+		E test_enum = 2;
+		float test_float = 3;
+		fixed32 test_fixed32 = 4;
+		int32 test_int32 = 5;
+		sfixed32 test_sfixed32 = 6;
+		sint32 test_sint32 = 7;
+		uint32 test_uint32 = 8;
+		M test_message = 9;
+		string test_string = 10;
+		double test_double = 11;
+		fixed64 test_fixed64 = 12;
+		int64 test_int64 = 13;
+		sfixed64 test_sfixed64 = 14;
+		sint64 test_sint64 = 15;
+		uint64 test_uint64 = 16;
+		bytes test_bytes = 17;
+	}
+}