Variable-becomes in combination with a syntactic type is rejected by the checker

[rodinaarssen]

Describe the bug
Consider the following module (which is a severely reduced example from actual code).

syntax B = "b";
B a(int _) = (B)`b`;
map[int, B] b = (n : f | n <- [0], f:_ := a(n));

The type checker incorrectly assigns the type value to f.

Observations:

• The "variable-becomes" f: is essential (i.e., f := a(n) does not exhibit this behavior)
• The return type of a must be a syntax type (any kind)

Expected behavior
This fragment should type check without issues

Desktop (please complete the following information):

• Context: VS Code 0.13.5