go/types: don't associate methods with alias type names

R=go1.11

The existing code associated methods with receiver base type
names before knowing if a type name denoted a locally defined
type. Sometimes, methods would be incorrectly associated with
alias type names and consequently were lost down the road.

This change first collects all methods with non-blank names
and in a follow-up pass resolves receiver base type names to
valid non-alias type names with which the methods are then
associated.

Fixes #23042.

Change-Id: I7699e577b70aadef6a2997e882beb0644da89fa3
Reviewed-on: https://go-review.googlesource.com/83996Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/check.go

@@ -85,7 +85,7 @@ type Checker struct {
 	unusedDotImports map[*Scope]map[*Package]token.Pos // positions of unused dot-imported packages for each file scope
 
 	firstErr   error                    // first error encountered
-	methods    map[string][]*Func       // maps package scope type names to associated non-blank, non-interface methods
+	methods    map[*TypeName][]*Func    // maps package scope type names to associated non-blank, non-interface methods
 	interfaces map[*TypeName]*ifaceInfo // maps interface type names to corresponding interface infos
 	untyped    map[ast.Expr]exprInfo    // map of expressions without final type
 	delayed    []func()                 // stack of delayed actions
@@ -126,15 +126,6 @@ func (check *Checker) addDeclDep(to Object) {
 	from.addDep(to)
 }
 
-func (check *Checker) assocMethod(tname string, meth *Func) {
-	m := check.methods
-	if m == nil {
-		m = make(map[string][]*Func)
-		check.methods = m
-	}
-	m[tname] = append(m[tname], meth)
-}
-
 func (check *Checker) rememberUntyped(e ast.Expr, lhs bool, mode operandMode, typ *Basic, val constant.Value) {
 	m := check.untyped
 	if m == nil {

src/go/types/decl.go

@@ -57,7 +57,7 @@ func (check *Checker) objDecl(obj Object, def *Named, path []*TypeName) {
 	}
 
 	if trace {
-		check.trace(obj.Pos(), "-- declaring %s (path = %s)", obj.Name(), pathString(path))
+		check.trace(obj.Pos(), "-- checking %s (path = %s)", obj, pathString(path))
 		check.indent++
 		defer func() {
 			check.indent--
@@ -67,7 +67,7 @@ func (check *Checker) objDecl(obj Object, def *Named, path []*TypeName) {
 
 	d := check.objMap[obj]
 	if d == nil {
-		check.dump("%s: %s should have been declared", obj.Pos(), obj.Name())
+		check.dump("%s: %s should have been declared", obj.Pos(), obj)
 		unreachable()
 	}
 
@@ -271,18 +271,22 @@ func (check *Checker) typeDecl(obj *TypeName, typ ast.Expr, def *Named, path []*
 
 func (check *Checker) addMethodDecls(obj *TypeName) {
 	// get associated methods
-	methods := check.methods[obj.name]
-	if len(methods) == 0 {
-		return // no methods
+	// (Checker.collectObjects only collects methods with non-blank names;
+	// Checker.resolveBaseTypeName ensures that obj is not an alias name
+	// if it has attached methods.)
+	methods := check.methods[obj]
+	if methods == nil {
+		return
 	}
-	delete(check.methods, obj.name)
+	delete(check.methods, obj)
+	assert(!obj.IsAlias())
 
 	// use an objset to check for name conflicts
 	var mset objset
 
 	// spec: "If the base type is a struct type, the non-blank method
 	// and field names must be distinct."
-	base, _ := obj.typ.(*Named) // nil if receiver base type is type alias
+	base, _ := obj.typ.(*Named) // shouldn't fail but be conservative
 	if base != nil {
 		if t, _ := base.underlying.(*Struct); t != nil {
 			for _, fld := range t.fields {
@@ -305,26 +309,24 @@ func (check *Checker) addMethodDecls(obj *TypeName) {
 	for _, m := range methods {
 		// spec: "For a base type, the non-blank names of methods bound
 		// to it must be unique."
-		if m.name != "_" {
-			if alt := mset.insert(m); alt != nil {
-				switch alt.(type) {
-				case *Var:
-					check.errorf(m.pos, "field and method with the same name %s", m.name)
-				case *Func:
-					check.errorf(m.pos, "method %s already declared for %s", m.name, obj)
-				default:
-					unreachable()
-				}
-				check.reportAltDecl(alt)
-				continue
+		assert(m.name != "_")
+		if alt := mset.insert(m); alt != nil {
+			switch alt.(type) {
+			case *Var:
+				check.errorf(m.pos, "field and method with the same name %s", m.name)
+			case *Func:
+				check.errorf(m.pos, "method %s already declared for %s", m.name, obj)
+			default:
+				unreachable()
 			}
+			check.reportAltDecl(alt)
+			continue
 		}
 
 		// type-check
 		check.objDecl(m, nil, nil)
 
-		// methods with blank _ names cannot be found - don't keep them
-		if base != nil && m.name != "_" {
+		if base != nil {
 			base.methods = append(base.methods, m)
 		}
 	}

src/go/types/resolver.go

@@ -217,6 +217,7 @@ func (check *Checker) collectObjects() {
 		pkgImports[imp] = true
 	}
 
+	var methods []*Func // list of methods with non-blank _ names
 	for fileNo, file := range check.files {
 		// The package identifier denotes the current package,
 		// but there is no corresponding package object.
@@ -412,20 +413,13 @@ func (check *Checker) collectObjects() {
 					}
 				} else {
 					// method
-					check.recordDef(d.Name, obj)
-					// Associate method with receiver base type name, if possible.
-					// Ignore methods that have an invalid receiver, or a blank _
-					// receiver name. They will be type-checked later, with regular
-					// functions.
-					if list := d.Recv.List; len(list) > 0 {
-						typ := unparen(list[0].Type)
-						if ptr, _ := typ.(*ast.StarExpr); ptr != nil {
-							typ = unparen(ptr.X)
-						}
-						if base, _ := typ.(*ast.Ident); base != nil && base.Name != "_" {
-							check.assocMethod(base.Name, obj)
-						}
+					// (Methods with blank _ names are never found; no need to collect
+					// them for later type association. They will still be type-checked
+					// with all the other functions.)
+					if name != "_" {
+						methods = append(methods, obj)
 					}
+					check.recordDef(d.Name, obj)
 				}
 				info := &declInfo{file: fileScope, fdecl: d}
 				check.objMap[obj] = info
@@ -452,6 +446,83 @@ func (check *Checker) collectObjects() {
 			}
 		}
 	}
+
+	// Now that we have all package scope objects and all methods,
+	// associate methods with receiver base type name where possible.
+	// Ignore methods that have an invalid receiver. They will be
+	// type-checked later, with regular functions.
+	if methods == nil {
+		return // nothing to do
+	}
+	check.methods = make(map[*TypeName][]*Func)
+	for _, f := range methods {
+		fdecl := check.objMap[f].fdecl
+		if list := fdecl.Recv.List; len(list) > 0 {
+			// f is a method
+			// receiver may be of the form T or *T, possibly with parentheses
+			typ := unparen(list[0].Type)
+			if ptr, _ := typ.(*ast.StarExpr); ptr != nil {
+				typ = unparen(ptr.X)
+			}
+			if base, _ := typ.(*ast.Ident); base != nil {
+				// base is a potential base type name; determine
+				// "underlying" defined type and associate f with it
+				if tname := check.resolveBaseTypeName(base); tname != nil {
+					check.methods[tname] = append(check.methods[tname], f)
+				}
+			}
+		}
+	}
+}
+
+// resolveBaseTypeName returns the non-alias receiver base type name,
+// explicitly declared in the package scope, for the given receiver
+// type name; or nil.
+func (check *Checker) resolveBaseTypeName(name *ast.Ident) *TypeName {
+	var path []*TypeName
+	for {
+		// name must denote an object found in the current package
+		// (it could be explicitly declared or dot-imported)
+		obj := check.pkg.scope.Lookup(name.Name)
+		if obj == nil {
+			return nil
+		}
+		// the object must be a type name...
+		tname, _ := obj.(*TypeName)
+		if tname == nil {
+			return nil
+		}
+
+		// ... which we have not seen before
+		if check.cycle(tname, path, false) {
+			return nil
+		}
+
+		// tname must have been explicitly declared
+		// (dot-imported objects are not in objMap)
+		tdecl := check.objMap[tname]
+		if tdecl == nil {
+			return nil
+		}
+
+		// we're done if tdecl defined tname as a new type
+		// (rather than an alias)
+		if !tdecl.alias {
+			return tname
+		}
+
+		// Otherwise, if tdecl defined an alias for a (possibly parenthesized)
+		// type which is not an (unqualified) named type, we're done because
+		// receiver base types must be named types declared in this package.
+		typ := unparen(tdecl.typ) // a type may be parenthesized
+		name, _ = typ.(*ast.Ident)
+		if name == nil {
+			return nil
+		}
+
+		// continue resolving name
+		path = append(path, tname)
+	}
 }
 
 // packageObjects typechecks all package objects, but not function bodies.

src/go/types/testdata/decls4.src

@@ -69,6 +69,55 @@ func (A10 /* ERROR invalid receiver */ ) m1() {}
 // x0 has methods m1, m2 declared via receiver type names T0 and A0
 var _ interface{ m1(); m2() } = x0
 
+// alias receiver types (test case for issue #23042)
+type T struct{}
+
+var (
+	_ = T.m
+	_ = T{}.m
+	_ interface{m()} = T{}
+)
+
+var (
+	_ = T.n
+	_ = T{}.n
+	_ interface{m(); n()} = T{}
+)
+
+type U = T
+func (U) m() {}
+
+// alias receiver types (long type declaration chains)
+type (
+	V0 = V1
+	V1 = (V2)
+	V2 = ((V3))
+	V3 = T
+)
+
+func (V0) m /* ERROR already declared */ () {}
+func (V1) n() {}
+
+// alias receiver types (invalid due to cycles)
+type (
+	W0 /* ERROR illegal cycle */ = W1
+	W1 = (W2)
+	W2 = ((W0))
+)
+
+func (W0) m() {} // no error expected (due to above cycle error)
+func (W1) n() {}
+
+// alias receiver types (invalid due to builtin underlying type)
+type (
+	B0 = B1
+	B1 = B2
+	B2 = int
+)
+
+func (B0 /* ERROR invalid receiver */ ) m() {}
+func (B1 /* ERROR invalid receiver */ ) n() {}
+
 // cycles
 type (
 	C2 /* ERROR illegal cycle */ = C2