go/types: added test case for Checker.CheckExpr

For #65898.

Change-Id: I495e53060ac56b88a551ccd9901f25bbce97c714
Reviewed-on: https://go-review.googlesource.com/c/go/+/567215
Reviewed-by: Alan Donovan <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Robert Griesemer <[email protected]>

Author: griesemer

src/go/types/eval_test.go

@@ -12,6 +12,7 @@ import (
 	"go/importer"
 	"go/parser"
 	"go/token"
+	"go/types"
 	"internal/testenv"
 	"strings"
 	"testing"
@@ -295,3 +296,32 @@ func f(a int, s string) S {
 		}
 	}
 }
+
+func TestIssue65898(t *testing.T) {
+	const src = `
+package p
+func _[A any](A) {}
+`
+
+	fset := token.NewFileSet()
+	f := mustParse(fset, src)
+
+	var conf types.Config
+	pkg, err := conf.Check(pkgName(src), fset, []*ast.File{f}, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for _, d := range f.Decls {
+		if fun, _ := d.(*ast.FuncDecl); fun != nil {
+			// type parameter A is not found at the start of the function type
+			if err := types.CheckExpr(fset, pkg, fun.Type.Pos(), fun.Type, nil); err == nil || !strings.Contains(err.Error(), "undefined") {
+				t.Fatalf("got %s, want undefined error", err)
+			}
+			// type parameter A must be found at the end of the function type
+			if err := types.CheckExpr(fset, pkg, fun.Type.End(), fun.Type, nil); err != nil {
+				t.Fatal(err)
+			}
+		}
+	}
+}