diff --git a/gdk/gdkgltexture.c b/gdk/gdkgltexture.c
index 618aa45f4c..e110b6670b 100644
--- a/gdk/gdkgltexture.c
+++ b/gdk/gdkgltexture.c
@@ -91,10 +91,10 @@ gdk_gl_texture_download (GdkTexture         *texture,
     }
   else
     {
-      GdkSurface *surface;
+      GdkSurface *gl_surface;
 
-      surface = gdk_gl_context_get_surface (self->context);
-      gdk_cairo_draw_from_gl (cr, surface, self->id, GL_TEXTURE, 1, 
+      gl_surface = gdk_gl_context_get_surface (self->context);
+      gdk_cairo_draw_from_gl (cr, gl_surface, self->id, GL_TEXTURE, 1, 
                               area->x, area->y,
                               area->width, area->height);
     }
diff --git a/gdk/gdkvulkancontext.c b/gdk/gdkvulkancontext.c
index 3aebd5717d..28a135ad06 100644
--- a/gdk/gdkvulkancontext.c
+++ b/gdk/gdkvulkancontext.c
@@ -786,12 +786,12 @@ gdk_display_create_vulkan_device (GdkDisplay  *display,
       else
         {
           gint64 device_idx;
-          GError *error = NULL;
+          GError *error2 = NULL;
 
-          if (!g_ascii_string_to_signed (override, 10, 0, G_MAXINT, &device_idx, &error))
+          if (!g_ascii_string_to_signed (override, 10, 0, G_MAXINT, &device_idx, &error2))
             {
-              g_warning ("Failed to parse %s: %s", "GDK_VULKAN_DEVICE", error->message);
-              g_error_free (error);
+              g_warning ("Failed to parse %s: %s", "GDK_VULKAN_DEVICE", error2->message);
+              g_error_free (error2);
               device_idx = -1;
             }
 
diff --git a/gdk/wayland/gdkdisplay-wayland.c b/gdk/wayland/gdkdisplay-wayland.c
index 0c870d8722..e6609559af 100644
--- a/gdk/wayland/gdkdisplay-wayland.c
+++ b/gdk/wayland/gdkdisplay-wayland.c
@@ -1789,7 +1789,7 @@ init_settings (GdkDisplay *display)
     {
       GVariant *ret;
       GError *error = NULL;
-      const char *schema;
+      const char *schema_str;
       GVariant *val;
       GVariantIter *iter;
       const char *patterns[] = { "org.gnome.*", NULL }; 
@@ -1829,7 +1829,7 @@ init_settings (GdkDisplay *display)
 
       g_variant_get (ret, "(a{sa{sv}})", &iter);
 
-      while (g_variant_iter_loop (iter, "{s@a{sv}}", &schema, &val))
+      while (g_variant_iter_loop (iter, "{s@a{sv}}", &schema_str, &val))
         {
           GVariantIter *iter2 = g_variant_iter_new (val);
           const char *key;
@@ -1837,17 +1837,17 @@ init_settings (GdkDisplay *display)
 
           while (g_variant_iter_loop (iter2, "{sv}", &key, &v))
             {
-              TranslationEntry *entry = find_translation_entry_by_schema (schema, key);
+              TranslationEntry *entry = find_translation_entry_by_schema (schema_str, key);
               if (entry)
                 {
                   char *a = g_variant_print (v, FALSE);
-                  g_debug ("Using portal setting for %s %s: %s\n", schema, key, a);
+                  g_debug ("Using portal setting for %s %s: %s\n", schema_str, key, a);
                   g_free (a);
                   apply_portal_setting (entry, v, display);
                 }
               else
                 {
-                  g_debug ("Ignoring portal setting for %s %s", schema, key);
+                  g_debug ("Ignoring portal setting for %s %s", schema_str, key);
                 }
             }
           g_variant_iter_free (iter2);
diff --git a/gdk/x11/gdkclipboard-x11.c b/gdk/x11/gdkclipboard-x11.c
index 55d0440e1e..aeea1af1fe 100644
--- a/gdk/x11/gdkclipboard-x11.c
+++ b/gdk/x11/gdkclipboard-x11.c
@@ -259,10 +259,11 @@ gdk_x11_clipboard_request_targets_finish (GObject      *source_object,
   GBytes *bytes;
   GError *error = NULL;
 
+  display = gdk_clipboard_get_display (GDK_CLIPBOARD (cb));
+
   bytes = g_input_stream_read_bytes_finish (stream, res, &error);
   if (bytes == NULL)
     {
-      GdkDisplay *display = gdk_clipboard_get_display (GDK_CLIPBOARD (cb));
       GDK_DISPLAY_NOTE (display, CLIPBOARD,
                 g_printerr ("%s: error reading TARGETS: %s\n", cb->selection, error->message));
       g_error_free (error);
@@ -292,7 +293,6 @@ gdk_x11_clipboard_request_targets_finish (GObject      *source_object,
                g_bytes_get_data (bytes, NULL),
                g_bytes_get_size (bytes) / sizeof (Atom));
 
-  display = gdk_clipboard_get_display (GDK_CLIPBOARD (cb));
   formats = gdk_x11_clipboard_formats_from_atoms (display,
                                                   g_bytes_get_data (bytes, NULL),
                                                   g_bytes_get_size (bytes) / sizeof (Atom));
diff --git a/gdk/x11/gdkdrop-x11.c b/gdk/x11/gdkdrop-x11.c
index 6a71b55cf3..050695bf9c 100644
--- a/gdk/x11/gdkdrop-x11.c
+++ b/gdk/x11/gdkdrop-x11.c
@@ -375,14 +375,14 @@ gdk_x11_drop_read_actions (GdkDrop *drop)
           if (GDK_DISPLAY_DEBUG_CHECK (display, DND))
             {
               GString *action_str = g_string_new (NULL);
-              GdkDragAction actions = gdk_drop_get_actions (drop);
-              if (actions & GDK_ACTION_MOVE)
+              GdkDragAction drop_actions = gdk_drop_get_actions (drop);
+              if (drop_actions & GDK_ACTION_MOVE)
                 g_string_append(action_str, "MOVE ");
-              if (actions & GDK_ACTION_COPY)
+              if (drop_actions & GDK_ACTION_COPY)
                 g_string_append(action_str, "COPY ");
-              if (actions & GDK_ACTION_LINK)
+              if (drop_actions & GDK_ACTION_LINK)
                 g_string_append(action_str, "LINK ");
-              if (actions & GDK_ACTION_ASK)
+              if (drop_actions & GDK_ACTION_ASK)
                 g_string_append(action_str, "ASK ");
 
               g_message("Xdnd actions = %s", action_str->str);
diff --git a/gdk/x11/gdkscreen-x11.c b/gdk/x11/gdkscreen-x11.c
index f83df090e8..70a10dbc33 100644
--- a/gdk/x11/gdkscreen-x11.c
+++ b/gdk/x11/gdkscreen-x11.c
@@ -676,7 +676,7 @@ init_no_multihead (GdkX11Screen *x11_screen, gboolean *changed)
 
   for (i = 0; i < x11_display->monitors->len; i++)
     {
-      GdkX11Monitor *monitor = x11_display->monitors->pdata[i];
+      monitor = x11_display->monitors->pdata[i];
       monitor->add = FALSE;
       monitor->remove = TRUE;
     }
@@ -720,7 +720,7 @@ init_no_multihead (GdkX11Screen *x11_screen, gboolean *changed)
 
   for (i = x11_display->monitors->len - 1; i >= 0; i--)
     {
-      GdkX11Monitor *monitor = x11_display->monitors->pdata[i];
+      monitor = x11_display->monitors->pdata[i];
       if (monitor->add)
         {
           gdk_display_monitor_added (GDK_DISPLAY (x11_display), GDK_MONITOR (monitor));
diff --git a/gdk/x11/gdkselectionoutputstream-x11.c b/gdk/x11/gdkselectionoutputstream-x11.c
index 5f35cdf6be..c43e513ab3 100644
--- a/gdk/x11/gdkselectionoutputstream-x11.c
+++ b/gdk/x11/gdkselectionoutputstream-x11.c
@@ -869,7 +869,6 @@ gdk_x11_selection_output_streams_request (GdkDisplay                   *display,
                                           gpointer                      user_data)
 {
   const char *mime_type, *selection, *target, *property;
-  gsize i;
 
   selection = gdk_x11_get_xatom_name_for_display (display, xselection);
   target = gdk_x11_get_xatom_name_for_display (display, xtarget);
@@ -991,6 +990,8 @@ gdk_x11_selection_output_streams_request (GdkDisplay                   *display,
     }
   else
     {
+      gsize i;
+
       for (i = 0; i < G_N_ELEMENTS (special_targets); i++)
         {
           if (g_str_equal (target, special_targets[i].x_target) &&
diff --git a/gsk/gl/gskglrenderer.c b/gsk/gl/gskglrenderer.c
index be0f293add..6e7abbc3e1 100644
--- a/gsk/gl/gskglrenderer.c
+++ b/gsk/gl/gskglrenderer.c
@@ -653,7 +653,6 @@ render_border_node (GskGLRenderer   *self,
       }
     };
     int indices[4] = { 0, 1, 2, 3 };
-    int i;
 
     /* We sort them by color */
     sort_border_sides (colors, indices);
@@ -1067,7 +1066,7 @@ render_color_matrix_node (GskGLRenderer       *self,
 
   if (is_offscreen)
     {
-      GskQuadVertex vertex_data[GL_N_VERTICES] = {
+      GskQuadVertex offscreen_vertex_data[GL_N_VERTICES] = {
         { { min_x, min_y }, { 0, 1 }, },
         { { min_x, max_y }, { 0, 0 }, },
         { { max_x, min_y }, { 1, 1 }, },
@@ -1077,7 +1076,7 @@ render_color_matrix_node (GskGLRenderer       *self,
         { { max_x, min_y }, { 1, 1 }, },
       };
 
-      ops_draw (builder, vertex_data);
+      ops_draw (builder, offscreen_vertex_data);
     }
   else
     {
@@ -1120,7 +1119,7 @@ render_blur_node (GskGLRenderer       *self,
 
   if (is_offscreen)
     {
-      GskQuadVertex vertex_data[GL_N_VERTICES] = {
+      GskQuadVertex offscreen_vertex_data[GL_N_VERTICES] = {
         { { min_x, min_y }, { 0, 1 }, },
         { { min_x, max_y }, { 0, 0 }, },
         { { max_x, min_y }, { 1, 1 }, },
@@ -1130,7 +1129,7 @@ render_blur_node (GskGLRenderer       *self,
         { { max_x, min_y }, { 1, 1 }, },
       };
 
-      ops_draw (builder, vertex_data);
+      ops_draw (builder, offscreen_vertex_data);
     }
   else
     {
@@ -1635,7 +1634,7 @@ render_shadow_node (GskGLRenderer       *self,
       ops_set_texture (builder, texture_id);
       if (is_offscreen)
         {
-          const GskQuadVertex vertex_data[GL_N_VERTICES] = {
+          const GskQuadVertex offscreen_vertex_data[GL_N_VERTICES] = {
             { { dx + min_x, dy + min_y }, { 0, 1 }, },
             { { dx + min_x, dy + max_y }, { 0, 0 }, },
             { { dx + max_x, dy + min_y }, { 1, 1 }, },
@@ -1645,11 +1644,11 @@ render_shadow_node (GskGLRenderer       *self,
             { { dx + max_x, dy + min_y }, { 1, 1 }, },
           };
 
-          ops_draw (builder, vertex_data);
+          ops_draw (builder, offscreen_vertex_data);
         }
       else
         {
-          const GskQuadVertex vertex_data[GL_N_VERTICES] = {
+          const GskQuadVertex onscreen_vertex_data[GL_N_VERTICES] = {
             { { dx + min_x, dy + min_y }, { 0, 0 }, },
             { { dx + min_x, dy + max_y }, { 0, 1 }, },
             { { dx + max_x, dy + min_y }, { 1, 0 }, },
@@ -1659,7 +1658,7 @@ render_shadow_node (GskGLRenderer       *self,
             { { dx + max_x, dy + min_y }, { 1, 0 }, },
           };
 
-          ops_draw (builder, vertex_data);
+          ops_draw (builder, onscreen_vertex_data);
         }
 
       ops_offset (builder, - dx, - dy);
diff --git a/gsk/vulkan/gskvulkanrenderpass.c b/gsk/vulkan/gskvulkanrenderpass.c
index b49c57294c..fa3d10ada6 100644
--- a/gsk/vulkan/gskvulkanrenderpass.c
+++ b/gsk/vulkan/gskvulkanrenderpass.c
@@ -1787,10 +1787,10 @@ gsk_vulkan_render_pass_draw_rect (GskVulkanRenderPass     *self,
           break;
 
         case GSK_VULKAN_OP_PUSH_VERTEX_CONSTANTS:
-          for (int i = 0; i < layout_count; i++)
+          for (int j = 0; j < layout_count; j++)
             gsk_vulkan_push_constants_push (&op->constants.constants,
                                             command_buffer,
-                                            pipeline_layout[i]);
+                                            pipeline_layout[j]);
           break;
 
         case GSK_VULKAN_OP_CROSS_FADE:
diff --git a/gtk/gtkentry.c b/gtk/gtkentry.c
index 26a1b7ce7f..26fe36b100 100644
--- a/gtk/gtkentry.c
+++ b/gtk/gtkentry.c
@@ -6334,7 +6334,6 @@ paste_received (GObject      *clipboard,
 
   if (priv->insert_pos >= 0)
     {
-      gint pos, start, end;
       pos = priv->insert_pos;
       gtk_editable_get_selection_bounds (editable, &start, &end);
       if (!((start <= pos && pos <= end) || (end <= pos && pos <= start)))
diff --git a/gtk/gtkspinbutton.c b/gtk/gtkspinbutton.c
index 37fb282cf7..52c9a78df2 100644
--- a/gtk/gtkspinbutton.c
+++ b/gtk/gtkspinbutton.c
@@ -796,15 +796,15 @@ button_released_cb (GtkGestureMultiPress *gesture,
 
   if (button == GDK_BUTTON_SECONDARY)
     {
-      GtkWidget *button = gtk_event_controller_get_widget (GTK_EVENT_CONTROLLER (gesture));
+      GtkWidget *button_widget = gtk_event_controller_get_widget (GTK_EVENT_CONTROLLER (gesture));
       double diff;
-      if (button == priv->down_button)
+      if (button_widget == priv->down_button)
         {
           diff = gtk_adjustment_get_value (priv->adjustment) - gtk_adjustment_get_lower (priv->adjustment);
           if (diff > EPSILON)
             gtk_spin_button_real_spin (spin_button, -diff);
         }
-      else if (button == priv->up_button)
+      else if (button_widget == priv->up_button)
         {
           diff = gtk_adjustment_get_upper (priv->adjustment) - gtk_adjustment_get_value (priv->adjustment);
           if (diff > EPSILON)
diff --git a/meson.build b/meson.build
index afacc98635..0385d15b07 100644
--- a/meson.build
+++ b/meson.build
@@ -237,15 +237,15 @@ elif cc.get_id() == 'gcc' or cc.get_id() == 'clang'
     '-Wmissing-format-attribute',
     '-Wmissing-include-dirs',
     '-Wmissing-noreturn',
-    '-Wmissing-declarations',
-    '-Wmissing-prototypes',
     '-Wnested-externs',
     '-Wold-style-definition',
     '-Wpointer-arith',
+    '-Wshadow',
     '-Wstrict-prototypes',
     '-Wswitch-default',
     '-Wswitch-enum',
     '-Wundef',
+    '-Wuninitialized',
     '-Wunused',
     '-Werror=address',
     '-Werror=array-bounds',
@@ -255,6 +255,8 @@ elif cc.get_id() == 'gcc' or cc.get_id() == 'clang'
     '-Werror=int-to-pointer-cast',
     '-Werror=main',
     '-Werror=missing-braces',
+    '-Werror=missing-declarations',
+    '-Werror=missing-prototypes',
     '-Werror=nonnull',
     '-Werror=pointer-to-int-cast',
     '-Werror=redundant-decls',