device.h: make it compatible with both C++20 and pre-C11 gcc

Use braces conditionally for designated initializers of anonymous
unions. This is for device.h what what commit 19a33c7 ("init.h:
restore designated initializers in SYS_INIT_NAMED()") did for init.h

See long discussion in zephyrproject-rtos#69411 for more obscure C/C++ compatibility
details.

Signed-off-by: Marc Herbert <[email protected]>

include/zephyr/device.h

@@ -906,9 +906,24 @@ static inline bool z_impl_device_is_ready(const struct device *dev)
 		.state = (state_),                                             \
 		.data = (data_),                                               \
 		IF_ENABLED(CONFIG_DEVICE_DEPS, (.deps = (deps_),)) /**/        \
-		IF_ENABLED(CONFIG_PM_DEVICE, ({ .pm_base = (pm_),})) /**/         \
+			Z_DEVICE_INIT_PM_BASE(pm_)                             \
 	}
 
+/*
+ * Anonymous unions require C11. Some pre-C11 gcc versions have early
+ * support for anonymous unions but they require these braces when
+ * combined with C99 designated initializers. These braces are
+ * compatible with any C version but not with C++20.  For more obscure
+ * C/C++ compatibility details see commit c15f029a7108 and PR #69411.
+ */
+#if defined(__STDC_VERSION__) && (__STDC_VERSION__) < 201100
+#  define Z_DEVICE_INIT_PM_BASE(pm_) \
+		IF_ENABLED(CONFIG_PM_DEVICE, ({ .pm_base = (pm_),}))
+#else
+#  define Z_DEVICE_INIT_PM_BASE(pm_) \
+		IF_ENABLED(CONFIG_PM_DEVICE,   (.pm_base = (pm_),))
+#endif
+
 /**
  * @brief Device section name (used for sorting purposes).
  *
@@ -980,12 +995,25 @@ static inline bool z_impl_device_is_ready(const struct device *dev)
 		Z_INIT_ENTRY_NAME(DEVICE_NAME_GET(dev_id)) = {                                     \
 			.init_fn = {COND_CODE_1(Z_DEVICE_IS_MUTABLE(node_id), (.dev_rw), (.dev)) = \
 					    (init_fn_)},                                           \
-			{                                                                          \
-				COND_CODE_1(Z_DEVICE_IS_MUTABLE(node_id), (.dev_rw), (.dev)) =     \
-					&DEVICE_NAME_GET(dev_id),                                  \
-			},                                                                         \
+			Z_DEVICE_INIT_ENTRY_DEV(node_id, dev_id),                                  \
 	}
 
+/*
+ * Anonymous unions require C11. Some pre-C11 gcc versions have early
+ * support for anonymous unions but they require these braces when
+ * combined with C99 designated initializers. These braces are
+ * compatible with any C version but not with C++20.  For more obscure
+ * C/C++ compatibility details see commit c15f029a7108 and PR #69411.
+ */
+#if defined(__STDC_VERSION__) && (__STDC_VERSION__) < 201100
+#  define Z_DEVICE_INIT_ENTRY_DEV(node_id, dev_id) { Z_DEV_ENTRY_DEV(node_id, dev_id) }
+#else
+#  define Z_DEVICE_INIT_ENTRY_DEV(node_id, dev_id)   Z_DEV_ENTRY_DEV(node_id, dev_id)
+#endif
+
+#define Z_DEV_ENTRY_DEV(node_id, dev_id)                                                           \
+	COND_CODE_1(Z_DEVICE_IS_MUTABLE(node_id), (.dev_rw), (.dev)) =  &DEVICE_NAME_GET(dev_id)
+
 /**
  * @brief Define a @ref device and all other required objects.
  *