go/types: temporarily pin the Checker to Interface during checking

While type checking expressions involving interface types, it is
possible that their type set is used before delayed actions are
processed. As a result, computeInterfaceTypeSet is called with a nil
checker, and errors in the interface type definition result in panics
(see #48234).

To avoid the panics, store a *Checker on Interface for use in between
checking of the interface type expression and processing of delayed
actions.

Fixes #48234

Change-Id: I5509bc1c01b55edac52446b9e075fbe8fcc01874
Reviewed-on: https://go-review.googlesource.com/c/go/+/348371
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/interface.go

@@ -14,6 +14,7 @@ import (
 
 // An Interface represents an interface type.
 type Interface struct {
+	check     *Checker     // for error reporting; nil once type set is computed
 	obj       *TypeName    // type name object defining this interface; or nil (for better error messages)
 	methods   []*Func      // ordered list of explicitly declared methods
 	embeddeds []Type       // ordered list of explicitly embedded elements
@@ -24,7 +25,7 @@ type Interface struct {
 }
 
 // typeSet returns the type set for interface t.
-func (t *Interface) typeSet() *_TypeSet { return computeInterfaceTypeSet(nil, token.NoPos, t) }
+func (t *Interface) typeSet() *_TypeSet { return computeInterfaceTypeSet(t.check, token.NoPos, t) }
 
 // emptyInterface represents the empty (completed) interface
 var emptyInterface = Interface{complete: true, tset: &topTypeSet}
@@ -220,7 +221,7 @@ func (check *Checker) interfaceType(ityp *Interface, iface *ast.InterfaceType, d
 	}
 
 	// All methods and embedded elements for this interface are collected;
-	// i.e., this interface is may be used in a type set computation.
+	// i.e., this interface may be used in a type set computation.
 	ityp.complete = true
 
 	if len(ityp.methods) == 0 && len(ityp.embeddeds) == 0 {
@@ -236,7 +237,15 @@ func (check *Checker) interfaceType(ityp *Interface, iface *ast.InterfaceType, d
 	// Compute type set with a non-nil *Checker as soon as possible
 	// to report any errors. Subsequent uses of type sets will use
 	// this computed type set and won't need to pass in a *Checker.
-	check.later(func() { computeInterfaceTypeSet(check, iface.Pos(), ityp) })
+	//
+	// Pin the checker to the interface type in the interim, in case the type set
+	// must be used before delayed funcs are processed (see issue #48234).
+	// TODO(rfindley): clean up use of *Checker with computeInterfaceTypeSet
+	ityp.check = check
+	check.later(func() {
+		computeInterfaceTypeSet(check, iface.Pos(), ityp)
+		ityp.check = nil
+	})
 }
 
 func flattenUnion(list []ast.Expr, x ast.Expr) []ast.Expr {

src/go/types/sizeof_test.go

@@ -27,7 +27,7 @@ func TestSizeof(t *testing.T) {
 		{Tuple{}, 12, 24},
 		{Signature{}, 28, 56},
 		{Union{}, 16, 32},
-		{Interface{}, 40, 80},
+		{Interface{}, 44, 88},
 		{Map{}, 16, 32},
 		{Chan{}, 12, 24},
 		{Named{}, 72, 136},

src/go/types/testdata/fixedbugs/issue48234.go2

@@ -0,0 +1,10 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+var _ = interface{
+	m()
+	m /* ERROR "duplicate method" */ ()
+}(nil)

src/go/types/universe.go

@@ -89,7 +89,7 @@ func defPredeclaredTypes() {
 		res := NewVar(token.NoPos, nil, "", Typ[String])
 		sig := NewSignature(nil, nil, NewTuple(res), false)
 		err := NewFunc(token.NoPos, nil, "Error", sig)
-		ityp := &Interface{obj, []*Func{err}, nil, nil, true, nil}
+		ityp := &Interface{nil, obj, []*Func{err}, nil, nil, true, nil}
 		computeInterfaceTypeSet(nil, token.NoPos, ityp) // prevent races due to lazy computation of tset
 		typ := NewNamed(obj, ityp, nil)
 		sig.recv = NewVar(token.NoPos, nil, "", typ)
@@ -100,7 +100,7 @@ func defPredeclaredTypes() {
 	{
 		obj := NewTypeName(token.NoPos, nil, "comparable", nil)
 		obj.setColor(black)
-		ityp := &Interface{obj, nil, nil, nil, true, &_TypeSet{true, nil, allTermlist}}
+		ityp := &Interface{nil, obj, nil, nil, nil, true, &_TypeSet{true, nil, allTermlist}}
 		NewNamed(obj, ityp, nil)
 		def(obj)
 	}