Revise function arity mismatch errors involving TypeApplications

Summary:
Currently, whenever you apply a function to too many arguments and
some of those arguments happen to be visible type applications, the error
message that GHC gives is rather confusing. Consider the message you receive
when typechecking `id @Int 1 2`:

```
The function `id` is applied to three arguments,
but its type `Int -> Int` has only one
```

This is baffling, since the two lines treat the visible type argument `@Int`
differently. The top line ("applied to three arguments") includes `@Int`,
whereas the bottom line ("has only one") excludes `@Int` from consideration.

There are multiple ways one could fix this, which I explain in an addendum to
`Note [Herald for matchExpectedFunTys]`. The approach adopted here is to change
the herald of this error message to include visible type arguments, and to
avoid counting them in the "applied to n arguments" part of the error. The end
result is that the new error message for `id @Int 1 2` is now:

```
The expression `id @Int` is applied to two arguments,
but its type `Int -> Int` has only one
```

Test Plan: make test TEST=T13902

Reviewers: goldfire, austin, bgamari

Reviewed By: goldfire

Subscribers: rwbarton, thomie

GHC Trac Issues: #13902

Differential Revision: https://phabricator.haskell.org/D3868

1 files changed, 8 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_fail/T13902.stderr b/testsuite/tests/typecheck/should_fail/T13902.stderr
new file mode 100644
index 0000000000..c3d07edfd1
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T13902.stderr
@@ -0,0 +1,8 @@
+
+T13902.hs:8:5: error:
+    • Couldn't match expected type ‘Integer -> Int’
+                  with actual type ‘Int’
+    • The expression ‘f @Int’ is applied to two arguments,
+      but its type ‘Int -> Int’ has only one
+      In the expression: f @Int 42 5
+      In an equation for ‘g’: g = f @Int 42 5