go/types: add lookup method to context and factor out LookupParent calls

R=go1.11

Also: Moved Checker.pos field into context where it belongs.

This is a cleanup/code factoring.

For #22992.

Change-Id: If9d4f0af537cb181f73735e709ebc8258b2a1378
Reviewed-on: https://go-review.googlesource.com/83017Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/call.go

@@ -314,7 +314,7 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr) {
 	// can only appear in qualified identifiers which are mapped to
 	// selector expressions.
 	if ident, ok := e.X.(*ast.Ident); ok {
-		_, obj := check.scope.LookupParent(ident.Name, check.pos)
+		obj := check.lookup(ident.Name)
 		if pname, _ := obj.(*PkgName); pname != nil {
 			assert(pname.pkg == check.pkg)
 			check.recordUse(ident, pname)

src/go/types/check.go

@@ -51,12 +51,19 @@ type funcInfo struct {
 type context struct {
 	decl          *declInfo      // package-level declaration whose init expression/function body is checked
 	scope         *Scope         // top-most scope for lookups
+	pos           token.Pos      // if valid, identifiers are looked up as if at position pos (used by Eval)
 	iota          constant.Value // value of iota in a constant declaration; nil otherwise
 	sig           *Signature     // function signature if inside a function; nil otherwise
 	hasLabel      bool           // set if a function makes use of labels (only ~1% of functions); unused outside functions
 	hasCallOrRecv bool           // set if an expression contains a function call or channel receive operation
 }
 
+// lookup looks up name in the current context and returns the matching object, or nil.
+func (ctxt *context) lookup(name string) Object {
+	_, obj := ctxt.scope.LookupParent(name, ctxt.pos)
+	return obj
+}
+
 // An importKey identifies an imported package by import path and source directory
 // (directory containing the file containing the import). In practice, the directory
 // may always be the same, or may not matter. Given an (import path, directory), an
@@ -95,7 +102,6 @@ type Checker struct {
 	// context within which the current object is type-checked
 	// (valid only for the duration of type-checking a specific object)
 	context
-	pos token.Pos // if valid, identifiers are looked up as if at position pos (used by Eval)
 
 	// debugging
 	indent int // indentation for tracing

src/go/types/interfaces.go

@@ -288,7 +288,7 @@ func (check *Checker) infoFromTypeName(name *ast.Ident, path []*TypeName) *iface
 
 typenameLoop:
 	// name must be a type name denoting a type whose underlying type is an interface
-	_, obj := check.scope.LookupParent(name.Name, check.pos /* use Eval position, if any */)
+	obj := check.lookup(name.Name)
 	if obj == nil {
 		return nil
 	}
@@ -363,7 +363,7 @@ func (check *Checker) infoFromQualifiedTypeName(qname *ast.SelectorExpr) *ifaceI
 	if name == nil {
 		return nil
 	}
-	_, obj1 := check.scope.LookupParent(name.Name, check.pos /* use Eval position, if any */)
+	obj1 := check.lookup(name.Name)
 	if obj1 == nil {
 		return nil
 	}

src/go/types/stmt.go

@@ -440,7 +440,7 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 				// list in a "return" statement if a different entity (constant, type, or variable)
 				// with the same name as a result parameter is in scope at the place of the return."
 				for _, obj := range res.vars {
-					if _, alt := check.scope.LookupParent(obj.name, check.pos); alt != nil && alt != obj {
+					if alt := check.lookup(obj.name); alt != nil && alt != obj {
 						check.errorf(s.Pos(), "result parameter %s not in scope at return", obj.name)
 						check.errorf(alt.Pos(), "\tinner declaration of %s", obj)
 						// ok to continue

src/go/types/typexpr.go

@@ -22,6 +22,8 @@ func (check *Checker) ident(x *operand, e *ast.Ident, def *Named, path []*TypeNa
 	x.mode = invalid
 	x.expr = e
 
+	// Note that we cannot use check.lookup here because the returned scope
+	// may be different from obj.Parent(). See also Scope.LookupParent doc.
 	scope, obj := check.scope.LookupParent(e.Name, check.pos)
 	if obj == nil {
 		if e.Name == "_" {