From 90ae274d0a54b0cae1770729569ab1a50c54ffc9 Mon Sep 17 00:00:00 2001
From: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Date: Mon, 16 Oct 2023 10:33:27 +0200
Subject: [PATCH 1/3] subclass inheritance

---
 R/theme.R | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/R/theme.R b/R/theme.R
index 33cf0244cd..42fcb1363d 100644
--- a/R/theme.R
+++ b/R/theme.R
@@ -767,9 +767,21 @@ combine_elements <- function(e1, e2) {
     e1$linewidth <- e2$linewidth * unclass(e1$linewidth)
   }
 
+  # If e2 is 'richer' than e1, fill e2 with e1 parameters
+  if (is.subclass(e2, e1)) {
+    new <- defaults(e1, e2)
+    e2[names(new)] <- new
+    return(e2)
+  }
+
   e1
 }
 
+is.subclass <- function(x, y) {
+  inheritance <- inherits(x, class(y), which = TRUE)
+  !any(inheritance == 0) && length(setdiff(class(x), class(y))) > 0
+}
+
 #' Reports whether x is a theme object
 #' @param x An object to test
 #' @export

From bcf2155d9681c5b691ad92ebe3e1f2b38ef5f84d Mon Sep 17 00:00:00 2001
From: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Date: Mon, 16 Oct 2023 10:33:33 +0200
Subject: [PATCH 2/3] add tests

---
 tests/testthat/test-theme.R | 45 +++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/tests/testthat/test-theme.R b/tests/testthat/test-theme.R
index 7bf37c90e1..db239b710f 100644
--- a/tests/testthat/test-theme.R
+++ b/tests/testthat/test-theme.R
@@ -509,6 +509,51 @@ test_that("Theme validation behaves as expected", {
   expect_snapshot_error(validate_element("A", "aspect.ratio", tree))
 })
 
+test_that("Element subclasses are inherited", {
+
+  # `rich` is subclass of `poor`
+  poor <- element_line(colour = "red", linetype = 3)
+  rich <- element_line(linetype = 2, linewidth = 2)
+  class(rich) <- c("element_rich", class(rich))
+
+  # `poor` should acquire `rich`
+  test <- combine_elements(poor, rich)
+  expect_s3_class(test, "element_rich")
+  expect_equal(
+    test[c("colour", "linetype", "linewidth")],
+    list(colour = "red", linetype = 3, linewidth = 2)
+  )
+
+  # `rich` should stay `rich`
+  test <- combine_elements(rich, poor)
+  expect_s3_class(test, "element_rich")
+  expect_equal(
+    test[c("colour", "linetype", "linewidth")],
+    list(colour = "red", linetype = 2, linewidth = 2)
+  )
+
+  # `sibling` is not strict subclass of `rich`
+  sibling <- poor
+  class(sibling) <- c("element_sibling", class(sibling))
+
+  # `sibling` should stay `sibling`
+  test <- combine_elements(sibling, rich)
+  expect_s3_class(test, "element_sibling")
+  expect_equal(
+    test[c("colour", "linetype", "linewidth")],
+    list(colour = "red", linetype = 3, linewidth = 2)
+  )
+
+  # `rich` should stay `rich`
+  test <- combine_elements(rich, sibling)
+  expect_s3_class(test, "element_rich")
+  expect_equal(
+    test[c("colour", "linetype", "linewidth")],
+    list(colour = "red", linetype = 2, linewidth = 2)
+  )
+
+})
+
 # Visual tests ------------------------------------------------------------
 
 test_that("aspect ratio is honored", {

From bef9bb37291058fffaf3e6a0fa57eb7e4e9e7972 Mon Sep 17 00:00:00 2001
From: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Date: Mon, 16 Oct 2023 11:01:03 +0200
Subject: [PATCH 3/3] add news bullet

---
 NEWS.md | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/NEWS.md b/NEWS.md
index 254c49abd1..33c894caae 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,5 +1,8 @@
 # ggplot2 (development version)
 
+* In the theme element hierarchy, parent elements that are a strict subclass
+  of child elements now confer their subclass upon the children (#5457).
+
 * Legend titles no longer take up space if they've been removed by setting 
   `legend.title = element_blank()` (@teunbrand, #3587).