References below are to the Scala Language Specification
Consider the following method:
def foo(a: Int, b: Int) = 0
Eta Expansion can convert this to a value of type (Int, Int) => Int. This expansion is invoked if:
a) _ is used in place of the argument list (Method Value (§6.7))
val f = foo _
b) the argument list is omitted, and expected type of expression is a function type (§6.25.2):
val f: (Int, Int) => Int = foo
c) each of the arguments is _ (a special case of the ‘Placeholder Syntax for Anonymous Functions’ (§6.23))
val f = foo(_, _)
The expression, foo(_, 1) doesn’t qualify for Eta Expansion; it just expands to (a) => foo(a, 1) (§6.23). Regular type inference doesn’t attempt to figure out that a: Int.