Error messages on functions with generic type constraints can be quite misleading, especially when the solution is to mark a function as inline. Three different (but related) cases of poor messages are shown below:

Example 1

let averageBy (f : 'T -> ^U) (source : seq<'T>) : ^U =
    let inline acc (sum, count) el = (Checked.(+) sum (f el), count + 1)
    let (sum, count) = Seq.fold acc ((LanguagePrimitives.GenericZero< (^U) >), 0) source
    LanguagePrimitives.DivideByInt sum count

Expected behavior

Compiler warning: Checked.(+) is inferred to have default type of int rather than ^U because averageBy is not marked as inline.

Actual behavior

Compiler error on Checked.(+):

Type constraint mismatch when applying the default type 'int' for a type inference variable. The type 'int' does not support the operator 'DivideByInt' Consider adding further type constraints.

Example 2

let averageBy (f : 'T -> ^U) (source : seq<'T>) : ^U =
    let acc (sum, count) el = (Checked.(+) sum (f el), count + 1)
    let (sum, count) = Seq.fold acc ((LanguagePrimitives.GenericZero< (^U) >), 0) source
    LanguagePrimitives.DivideByInt sum count

Expected behavior

Compiler warning: Checked.(+) is inferred to have default type of int rather than ^U because averageBy and/or accare not marked as inline

Actual behavior

Compiler infers type ^U as obj without warnings or errors.

Example 3

let averageBy (f : 'T -> ^U) (source : seq<'T>) : ^U =
    let inline acc (sum, count) el = (Checked.(+) sum (f el), count + 1)
    let (sum, count) = Seq.fold acc ((LanguagePrimitives.GenericZero< (^U) >), 0) source
    Unchecked.defaultof<_> // TODO: finish implementation

Expected behavior

Compiler warning: Checked.(+) is inferred to have default type of int rather than ^U because averageBy and is not marked as inline

Actual behavior

Compiler warning on Checked.(+):

This construct causes code to be less generic than indicated by the type annotations. The type variable 'U has been constrained to be type 'int'.