go/types, types2: permit partially instantiated functions as function arguments

This CL changes Checker.genericExprList such that it collects partially
instantiated generic functions together with their (partial) type
argument (and corresponding) expression lists, instead of trying to
infer the missing type arguments in place or to report an error.
Special care is being taken to explictly record expression types where
needed (because we can't use one of the usual expr evaluators which
takes care of that), or to track the correct instance expression for
later recording with Checker.arguments.

The resulting generic expression list is passed to Checker.arguments
which is changed to accept explicit partial type argument (and
corresponding) expression lists. The provided type arguments are fed
into type inference, matching up with their respective type parameters
(which were collected already, before this CL). If type inference is
successful, the instantiated functions are recorded as needed.

For now, the type argument expression lists are collected and passed
along but not yet used. We may use them eventually for better error
reporting.

Fixes #59958.
For #59338.

Change-Id: I26db47ef3546e64553da49d62b23cd3ef9e2b549
Reviewed-on: https://go-review.googlesource.com/c/go/+/494116
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/src/internal/types/testdata/examples/inference2.go b/src/internal/types/testdata/examples/inference2.go
index 4eeb6d1b05..aa2475b741 100644
--- a/src/internal/types/testdata/examples/inference2.go
+++ b/src/internal/types/testdata/examples/inference2.go
@@ -88,3 +88,13 @@ func _() {
 	g5(f6, f7)
 	g6(f1, f1)
 }
+
+// Argument passing of partially instantiated functions
+func h(func(int, string), func(string, int)) {}
+
+func p[P, Q any](P, Q) {}
+
+func _() {
+	h(p, p)
+	h(p[int], p[string])
+}