gistfile1.txt

> tribble(~"X0.0 cm", ~"X1.5 cm", 1, 2) %>%
+   pivot_longer(
+     cols = everything(),
+     names_pattern = "X([\\d.]+) cm",
+     names_ptypes = list(name=double()))
Error: Lossy cast from `x` <character> to `to` <double>.
Locations: 1
Run `rlang::last_error()` to see where the error occurred.

From ?pivot_longer

names_ptypes, values_ptypes: A list of of column name-prototype pairs. A prototype (or ptype for short) is a zero-length vector (like integer() or numeric()) that defines the type, class, and attributes of a vector.

If vctrs is going to complain about any char-to-double conversion, I don't see how numeric can work as documented.