This may be more of an error when detecting the input when layer blending is being used (as we've seen in other functions), rather than it necessarily being an error with the compilation of the standard material version.
Download the licensee repro from here - [Link Removed]
Open M_Landscape material
The red section, when connected to the base colour, shows no error.
Connect the "Multiply" node from the green section directly to the Base Colour.
Compiler error will appear: "[SM5] Function MF_Test: Missing function input 'In'"
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-134331 in the post.