1 // Copyright 2020 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
5 // Package ifaceassert defines an Analyzer that flags
6 // impossible interface-interface type assertions.
13 "golang.org/x/tools/go/analysis"
14 "golang.org/x/tools/go/analysis/passes/inspect"
15 "golang.org/x/tools/go/ast/inspector"
18 const Doc = `detect impossible interface-to-interface type assertions
20 This checker flags type assertions v.(T) and corresponding type-switch cases
21 in which the static type V of v is an interface that cannot possibly implement
22 the target interface T. This occurs when V and T contain methods with the same
23 name but different signatures. Example:
30 The Read method in v has a different signature than the Read method in
31 io.Reader, so this assertion cannot succeed.
34 var Analyzer = &analysis.Analyzer{
37 Requires: []*analysis.Analyzer{inspect.Analyzer},
41 // assertableTo checks whether interface v can be asserted into t. It returns
42 // nil on success, or the first conflicting method on failure.
43 func assertableTo(v, t types.Type) *types.Func {
44 // ensure that v and t are interfaces
45 V, _ := v.Underlying().(*types.Interface)
46 T, _ := t.Underlying().(*types.Interface)
47 if V == nil || T == nil {
50 if f, wrongType := types.MissingMethod(V, T, false); wrongType {
56 func run(pass *analysis.Pass) (interface{}, error) {
57 inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
58 nodeFilter := []ast.Node{
59 (*ast.TypeAssertExpr)(nil),
60 (*ast.TypeSwitchStmt)(nil),
62 inspect.Preorder(nodeFilter, func(n ast.Node) {
64 assert *ast.TypeAssertExpr // v.(T) expression
65 targets []ast.Expr // interfaces T in v.(T)
67 switch n := n.(type) {
68 case *ast.TypeAssertExpr:
69 // take care of v.(type) in *ast.TypeSwitchStmt
74 targets = append(targets, n.Type)
75 case *ast.TypeSwitchStmt:
76 // retrieve type assertion from type switch's 'assign' field
77 switch t := n.Assign.(type) {
79 assert = t.X.(*ast.TypeAssertExpr)
81 assert = t.Rhs[0].(*ast.TypeAssertExpr)
83 // gather target types from case clauses
84 for _, c := range n.Body.List {
85 targets = append(targets, c.(*ast.CaseClause).List...)
88 V := pass.TypesInfo.TypeOf(assert.X)
89 for _, target := range targets {
90 T := pass.TypesInfo.TypeOf(target)
91 if f := assertableTo(V, T); f != nil {
94 "impossible type assertion: no type can implement both %v and %v (conflicting types for %v method)",