go/types: collect type info for type ...T in variadic functions

Because the code type-checks T rather than ...T (and then corrects
the type to []T "manually"), it didn't automatically record the
type for the ast.Expr corresponding to ...T. Do it manually.

Fixes #28277.

Change-Id: I3d9aae310c90b01f52d189e70c48dd9007f72207
Reviewed-on: https://go-review.googlesource.com/c/143317Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/api_test.go

@@ -257,6 +257,16 @@ func TestTypesInfo(t *testing.T) {
 			`(string, bool)`,
 		},
 
+		// issue 28277
+		{`package issue28277_a; func f(...int)`,
+			`...int`,
+			`[]int`,
+		},
+		{`package issue28277_b; func f(a, b, c ...[]struct{})`,
+			`...[]struct{}`,
+			`[][]struct{}`,
+		},
+
 		// tests for broken code that doesn't parse or type-check
 		{`package x0; func _() { var x struct {f string}; x.f := 0 }`, `x.f`, `string`},
 		{`package x1; func _() { var z string; type x struct {f string}; y := &x{q: z}}`, `z`, `string`},
@@ -389,6 +399,8 @@ func TestPredicatesInfo(t *testing.T) {
 		{`package t0; type _ int`, `int`, `type`},
 		{`package t1; type _ []int`, `[]int`, `type`},
 		{`package t2; type _ func()`, `func()`, `type`},
+		{`package t3; type _ func(int)`, `int`, `type`},
+		{`package t3; type _ func(...int)`, `...int`, `type`},
 
 		// built-ins
 		{`package b0; var _ = len("")`, `len`, `builtin`},

src/go/types/typexpr.go

@@ -451,9 +451,12 @@ func (check *Checker) collectParams(scope *Scope, list *ast.FieldList, variadicO
 	}
 
 	// For a variadic function, change the last parameter's type from T to []T.
-	if variadic && len(params) > 0 {
+	// Since we type-checked T rather than ...T, we also need to retro-actively
+	// record the type for ...T.
+	if variadic {
 		last := params[len(params)-1]
 		last.typ = &Slice{elem: last.typ}
+		check.recordTypeAndValue(list.List[len(list.List)-1].Type, typexpr, last.typ, nil)
 	}
 
 	return