change fallback

Author: teunbrand

R/stat-smooth.R

@@ -110,6 +110,16 @@ StatSmooth <- ggproto("StatSmooth", Stat,
       msg <- c(msg, paste0("method = '", method, "'"))
     }
 
+    if (identical(method, "gam") &&
+        !prompt_install("mgcv", "for using {.code method = \"gam\"}")) {
+      cli::cli_inform(c(
+        "The {.arg method} was set to {.val gam}, but {.pkg mgcv} is not installed.",
+        "!" = "Falling back to {.code method = \"lm\"}.",
+        i = "Change the {.arg method} argument to silence this message."
+      ))
+      method <- "lm"
+    }
+
     if (is.null(params$formula)) {
       if (identical(method, "gam")) {
         params$formula <- y ~ s(x, bs = "cs")

tests/testthat/test-geom-smooth.R

@@ -98,13 +98,13 @@ test_that("geom_smooth() works when one group fails", {
   expect_gte(nrow(ld), 2)
 })
 
-test_that("a warning is thrown when `method = 'gam'` and {mgcv} is absent", {
+test_that("a fallback message is thrown when `method = 'gam'` and {mgcv} is absent", {
   p <- ggplot(mpg, aes(displ, hwy)) +
     geom_smooth(method = "gam", formula = y ~ x)
 
   with_mocked_bindings(
-    expect_warning(
-      ggplot_build(p), regexp = "package must be installed"
+    expect_message(
+      ggplot_build(p), regexp = "Falling back to `method = \"lm\"`"
     ),
     is_installed = function(...) FALSE
   )