_copy_to_output!! error handling for mismatched types (#858)

* _copy_to_output!! error handling for mismatched types

* update project.toml

* comments

* Improve error handling in _copy_to_output!! function

Enhance error message in _copy_to_output!! function to provide more context about type mismatch.

Signed-off-by: Hong Ge <[email protected]>

* format.

* Apply suggestions from code review

Co-authored-by: Xianda Sun <[email protected]>
Signed-off-by: Hong Ge <[email protected]>

* minor tweaks

---------

Signed-off-by: Hong Ge <[email protected]>
Co-authored-by: Hong Ge <[email protected]>
Co-authored-by: Hong Ge <[email protected]>
Co-authored-by: Xianda Sun <[email protected]>

Author: 3 people

Project.toml

@@ -1,7 +1,7 @@
 name = "Mooncake"
 uuid = "da2b9cff-9c12-43a0-ae48-6db2b0edb7d6"
 authors = ["Will Tebbutt, Hong Ge, and contributors"]
-version = "0.4.177"
+version = "0.4.178"
 
 [deps]
 ADTypes = "47edcb42-4c32-4615-8424-f2b9edc5f35b"

src/interface.jl

@@ -317,6 +317,19 @@ function _copy_to_output!!(dst::P, src::P) where {P}
     end
 end
 
+# fallback for invalid type combinations
+function _copy_to_output!!(dst::T, src::P) where {T,P}
+    throw(
+        ArgumentError(
+            "Mooncake.jl does not currently have a method " *
+            "`_copy_to_output!!` to handle this type combination: " *
+            "dst passed is of type $T, while src is a $P. " *
+            "This often happens when differentiating over " *
+            "non-differentiable types (e.g. integers or booleans).",
+        ),
+    )
+end
+
 """
     _copy_output(x::T)