Source file src/go/types/resolver.go

Documentation: go/types

     1  // Copyright 2013 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.
     4  
     5  package types
     6  
     7  import (
     8  	"fmt"
     9  	"go/ast"
    10  	"go/constant"
    11  	"go/internal/typeparams"
    12  	"go/token"
    13  	"sort"
    14  	"strconv"
    15  	"strings"
    16  	"unicode"
    17  )
    18  
    19  // A declInfo describes a package-level const, type, var, or func declaration.
    20  type declInfo struct {
    21  	file      *Scope        // scope of file containing this declaration
    22  	lhs       []*Var        // lhs of n:1 variable declarations, or nil
    23  	vtyp      ast.Expr      // type, or nil (for const and var declarations only)
    24  	init      ast.Expr      // init/orig expression, or nil (for const and var declarations only)
    25  	inherited bool          // if set, the init expression is inherited from a previous constant declaration
    26  	tdecl     *ast.TypeSpec // type declaration, or nil
    27  	fdecl     *ast.FuncDecl // func declaration, or nil
    28  
    29  	// The deps field tracks initialization expression dependencies.
    30  	deps map[Object]bool // lazily initialized
    31  }
    32  
    33  // hasInitializer reports whether the declared object has an initialization
    34  // expression or function body.
    35  func (d *declInfo) hasInitializer() bool {
    36  	return d.init != nil || d.fdecl != nil && d.fdecl.Body != nil
    37  }
    38  
    39  // addDep adds obj to the set of objects d's init expression depends on.
    40  func (d *declInfo) addDep(obj Object) {
    41  	m := d.deps
    42  	if m == nil {
    43  		m = make(map[Object]bool)
    44  		d.deps = m
    45  	}
    46  	m[obj] = true
    47  }
    48  
    49  // arityMatch checks that the lhs and rhs of a const or var decl
    50  // have the appropriate number of names and init exprs. For const
    51  // decls, init is the value spec providing the init exprs; for
    52  // var decls, init is nil (the init exprs are in s in this case).
    53  func (check *Checker) arityMatch(s, init *ast.ValueSpec) {
    54  	l := len(s.Names)
    55  	r := len(s.Values)
    56  	if init != nil {
    57  		r = len(init.Values)
    58  	}
    59  
    60  	const code = _WrongAssignCount
    61  	switch {
    62  	case init == nil && r == 0:
    63  		// var decl w/o init expr
    64  		if s.Type == nil {
    65  			check.errorf(s, code, "missing type or init expr")
    66  		}
    67  	case l < r:
    68  		if l < len(s.Values) {
    69  			// init exprs from s
    70  			n := s.Values[l]
    71  			check.errorf(n, code, "extra init expr %s", n)
    72  			// TODO(gri) avoid declared but not used error here
    73  		} else {
    74  			// init exprs "inherited"
    75  			check.errorf(s, code, "extra init expr at %s", check.fset.Position(init.Pos()))
    76  			// TODO(gri) avoid declared but not used error here
    77  		}
    78  	case l > r && (init != nil || r != 1):
    79  		n := s.Names[r]
    80  		check.errorf(n, code, "missing init expr for %s", n)
    81  	}
    82  }
    83  
    84  func validatedImportPath(path string) (string, error) {
    85  	s, err := strconv.Unquote(path)
    86  	if err != nil {
    87  		return "", err
    88  	}
    89  	if s == "" {
    90  		return "", fmt.Errorf("empty string")
    91  	}
    92  	const illegalChars = `!"#$%&'()*,:;<=>?[\]^{|}` + "`\uFFFD"
    93  	for _, r := range s {
    94  		if !unicode.IsGraphic(r) || unicode.IsSpace(r) || strings.ContainsRune(illegalChars, r) {
    95  			return s, fmt.Errorf("invalid character %#U", r)
    96  		}
    97  	}
    98  	return s, nil
    99  }
   100  
   101  // declarePkgObj declares obj in the package scope, records its ident -> obj mapping,
   102  // and updates check.objMap. The object must not be a function or method.
   103  func (check *Checker) declarePkgObj(ident *ast.Ident, obj Object, d *declInfo) {
   104  	assert(ident.Name == obj.Name())
   105  
   106  	// spec: "A package-scope or file-scope identifier with name init
   107  	// may only be declared to be a function with this (func()) signature."
   108  	if ident.Name == "init" {
   109  		check.errorf(ident, _InvalidInitDecl, "cannot declare init - must be func")
   110  		return
   111  	}
   112  
   113  	// spec: "The main package must have package name main and declare
   114  	// a function main that takes no arguments and returns no value."
   115  	if ident.Name == "main" && check.pkg.name == "main" {
   116  		check.errorf(ident, _InvalidMainDecl, "cannot declare main - must be func")
   117  		return
   118  	}
   119  
   120  	check.declare(check.pkg.scope, ident, obj, token.NoPos)
   121  	check.objMap[obj] = d
   122  	obj.setOrder(uint32(len(check.objMap)))
   123  }
   124  
   125  // filename returns a filename suitable for debugging output.
   126  func (check *Checker) filename(fileNo int) string {
   127  	file := check.files[fileNo]
   128  	if pos := file.Pos(); pos.IsValid() {
   129  		return check.fset.File(pos).Name()
   130  	}
   131  	return fmt.Sprintf("file[%d]", fileNo)
   132  }
   133  
   134  func (check *Checker) importPackage(at positioner, path, dir string) *Package {
   135  	// If we already have a package for the given (path, dir)
   136  	// pair, use it instead of doing a full import.
   137  	// Checker.impMap only caches packages that are marked Complete
   138  	// or fake (dummy packages for failed imports). Incomplete but
   139  	// non-fake packages do require an import to complete them.
   140  	key := importKey{path, dir}
   141  	imp := check.impMap[key]
   142  	if imp != nil {
   143  		return imp
   144  	}
   145  
   146  	// no package yet => import it
   147  	if path == "C" && (check.conf.FakeImportC || check.conf.go115UsesCgo) {
   148  		imp = NewPackage("C", "C")
   149  		imp.fake = true // package scope is not populated
   150  		imp.cgo = check.conf.go115UsesCgo
   151  	} else {
   152  		// ordinary import
   153  		var err error
   154  		if importer := check.conf.Importer; importer == nil {
   155  			err = fmt.Errorf("Config.Importer not installed")
   156  		} else if importerFrom, ok := importer.(ImporterFrom); ok {
   157  			imp, err = importerFrom.ImportFrom(path, dir, 0)
   158  			if imp == nil && err == nil {
   159  				err = fmt.Errorf("Config.Importer.ImportFrom(%s, %s, 0) returned nil but no error", path, dir)
   160  			}
   161  		} else {
   162  			imp, err = importer.Import(path)
   163  			if imp == nil && err == nil {
   164  				err = fmt.Errorf("Config.Importer.Import(%s) returned nil but no error", path)
   165  			}
   166  		}
   167  		// make sure we have a valid package name
   168  		// (errors here can only happen through manipulation of packages after creation)
   169  		if err == nil && imp != nil && (imp.name == "_" || imp.name == "") {
   170  			err = fmt.Errorf("invalid package name: %q", imp.name)
   171  			imp = nil // create fake package below
   172  		}
   173  		if err != nil {
   174  			check.errorf(at, _BrokenImport, "could not import %s (%s)", path, err)
   175  			if imp == nil {
   176  				// create a new fake package
   177  				// come up with a sensible package name (heuristic)
   178  				name := path
   179  				if i := len(name); i > 0 && name[i-1] == '/' {
   180  					name = name[:i-1]
   181  				}
   182  				if i := strings.LastIndex(name, "/"); i >= 0 {
   183  					name = name[i+1:]
   184  				}
   185  				imp = NewPackage(path, name)
   186  			}
   187  			// continue to use the package as best as we can
   188  			imp.fake = true // avoid follow-up lookup failures
   189  		}
   190  	}
   191  
   192  	// package should be complete or marked fake, but be cautious
   193  	if imp.complete || imp.fake {
   194  		check.impMap[key] = imp
   195  		// Once we've formatted an error message, keep the pkgPathMap
   196  		// up-to-date on subsequent imports. It is used for package
   197  		// qualification in error messages.
   198  		if check.pkgPathMap != nil {
   199  			check.markImports(imp)
   200  		}
   201  		return imp
   202  	}
   203  
   204  	// something went wrong (importer may have returned incomplete package without error)
   205  	return nil
   206  }
   207  
   208  // collectObjects collects all file and package objects and inserts them
   209  // into their respective scopes. It also performs imports and associates
   210  // methods with receiver base type names.
   211  func (check *Checker) collectObjects() {
   212  	pkg := check.pkg
   213  
   214  	// pkgImports is the set of packages already imported by any package file seen
   215  	// so far. Used to avoid duplicate entries in pkg.imports. Allocate and populate
   216  	// it (pkg.imports may not be empty if we are checking test files incrementally).
   217  	// Note that pkgImports is keyed by package (and thus package path), not by an
   218  	// importKey value. Two different importKey values may map to the same package
   219  	// which is why we cannot use the check.impMap here.
   220  	var pkgImports = make(map[*Package]bool)
   221  	for _, imp := range pkg.imports {
   222  		pkgImports[imp] = true
   223  	}
   224  
   225  	type methodInfo struct {
   226  		obj  *Func      // method
   227  		ptr  bool       // true if pointer receiver
   228  		recv *ast.Ident // receiver type name
   229  	}
   230  	var methods []methodInfo // collected methods with valid receivers and non-blank _ names
   231  	var fileScopes []*Scope
   232  	for fileNo, file := range check.files {
   233  		// The package identifier denotes the current package,
   234  		// but there is no corresponding package object.
   235  		check.recordDef(file.Name, nil)
   236  
   237  		// Use the actual source file extent rather than *ast.File extent since the
   238  		// latter doesn't include comments which appear at the start or end of the file.
   239  		// Be conservative and use the *ast.File extent if we don't have a *token.File.
   240  		pos, end := file.Pos(), file.End()
   241  		if f := check.fset.File(file.Pos()); f != nil {
   242  			pos, end = token.Pos(f.Base()), token.Pos(f.Base()+f.Size())
   243  		}
   244  		fileScope := NewScope(check.pkg.scope, pos, end, check.filename(fileNo))
   245  		fileScopes = append(fileScopes, fileScope)
   246  		check.recordScope(file, fileScope)
   247  
   248  		// determine file directory, necessary to resolve imports
   249  		// FileName may be "" (typically for tests) in which case
   250  		// we get "." as the directory which is what we would want.
   251  		fileDir := dir(check.fset.Position(file.Name.Pos()).Filename)
   252  
   253  		check.walkDecls(file.Decls, func(d decl) {
   254  			switch d := d.(type) {
   255  			case importDecl:
   256  				// import package
   257  				path, err := validatedImportPath(d.spec.Path.Value)
   258  				if err != nil {
   259  					check.errorf(d.spec.Path, _BadImportPath, "invalid import path (%s)", err)
   260  					return
   261  				}
   262  
   263  				imp := check.importPackage(d.spec.Path, path, fileDir)
   264  				if imp == nil {
   265  					return
   266  				}
   267  
   268  				// local name overrides imported package name
   269  				name := imp.name
   270  				if d.spec.Name != nil {
   271  					name = d.spec.Name.Name
   272  					if path == "C" {
   273  						// match 1.17 cmd/compile (not prescribed by spec)
   274  						check.errorf(d.spec.Name, _ImportCRenamed, `cannot rename import "C"`)
   275  						return
   276  					}
   277  				}
   278  
   279  				if name == "init" {
   280  					check.errorf(d.spec, _InvalidInitDecl, "cannot import package as init - init must be a func")
   281  					return
   282  				}
   283  
   284  				// add package to list of explicit imports
   285  				// (this functionality is provided as a convenience
   286  				// for clients; it is not needed for type-checking)
   287  				if !pkgImports[imp] {
   288  					pkgImports[imp] = true
   289  					pkg.imports = append(pkg.imports, imp)
   290  				}
   291  
   292  				pkgName := NewPkgName(d.spec.Pos(), pkg, name, imp)
   293  				if d.spec.Name != nil {
   294  					// in a dot-import, the dot represents the package
   295  					check.recordDef(d.spec.Name, pkgName)
   296  				} else {
   297  					check.recordImplicit(d.spec, pkgName)
   298  				}
   299  
   300  				if imp.fake {
   301  					// match 1.17 cmd/compile (not prescribed by spec)
   302  					pkgName.used = true
   303  				}
   304  
   305  				// add import to file scope
   306  				check.imports = append(check.imports, pkgName)
   307  				if name == "." {
   308  					// dot-import
   309  					if check.dotImportMap == nil {
   310  						check.dotImportMap = make(map[dotImportKey]*PkgName)
   311  					}
   312  					// merge imported scope with file scope
   313  					for name, obj := range imp.scope.elems {
   314  						// Note: Avoid eager resolve(name, obj) here, so we only
   315  						// resolve dot-imported objects as needed.
   316  
   317  						// A package scope may contain non-exported objects,
   318  						// do not import them!
   319  						if token.IsExported(name) {
   320  							// declare dot-imported object
   321  							// (Do not use check.declare because it modifies the object
   322  							// via Object.setScopePos, which leads to a race condition;
   323  							// the object may be imported into more than one file scope
   324  							// concurrently. See issue #32154.)
   325  							if alt := fileScope.Lookup(name); alt != nil {
   326  								check.errorf(d.spec.Name, _DuplicateDecl, "%s redeclared in this block", alt.Name())
   327  								check.reportAltDecl(alt)
   328  							} else {
   329  								fileScope.insert(name, obj)
   330  								check.dotImportMap[dotImportKey{fileScope, name}] = pkgName
   331  							}
   332  						}
   333  					}
   334  				} else {
   335  					// declare imported package object in file scope
   336  					// (no need to provide s.Name since we called check.recordDef earlier)
   337  					check.declare(fileScope, nil, pkgName, token.NoPos)
   338  				}
   339  			case constDecl:
   340  				// declare all constants
   341  				for i, name := range d.spec.Names {
   342  					obj := NewConst(name.Pos(), pkg, name.Name, nil, constant.MakeInt64(int64(d.iota)))
   343  
   344  					var init ast.Expr
   345  					if i < len(d.init) {
   346  						init = d.init[i]
   347  					}
   348  
   349  					d := &declInfo{file: fileScope, vtyp: d.typ, init: init, inherited: d.inherited}
   350  					check.declarePkgObj(name, obj, d)
   351  				}
   352  
   353  			case varDecl:
   354  				lhs := make([]*Var, len(d.spec.Names))
   355  				// If there's exactly one rhs initializer, use
   356  				// the same declInfo d1 for all lhs variables
   357  				// so that each lhs variable depends on the same
   358  				// rhs initializer (n:1 var declaration).
   359  				var d1 *declInfo
   360  				if len(d.spec.Values) == 1 {
   361  					// The lhs elements are only set up after the for loop below,
   362  					// but that's ok because declareVar only collects the declInfo
   363  					// for a later phase.
   364  					d1 = &declInfo{file: fileScope, lhs: lhs, vtyp: d.spec.Type, init: d.spec.Values[0]}
   365  				}
   366  
   367  				// declare all variables
   368  				for i, name := range d.spec.Names {
   369  					obj := NewVar(name.Pos(), pkg, name.Name, nil)
   370  					lhs[i] = obj
   371  
   372  					di := d1
   373  					if di == nil {
   374  						// individual assignments
   375  						var init ast.Expr
   376  						if i < len(d.spec.Values) {
   377  							init = d.spec.Values[i]
   378  						}
   379  						di = &declInfo{file: fileScope, vtyp: d.spec.Type, init: init}
   380  					}
   381  
   382  					check.declarePkgObj(name, obj, di)
   383  				}
   384  			case typeDecl:
   385  				if d.spec.TypeParams.NumFields() != 0 && !check.allowVersion(pkg, 1, 18) {
   386  					check.softErrorf(d.spec.TypeParams.List[0], _UnsupportedFeature, "type parameters require go1.18 or later")
   387  				}
   388  				obj := NewTypeName(d.spec.Name.Pos(), pkg, d.spec.Name.Name, nil)
   389  				check.declarePkgObj(d.spec.Name, obj, &declInfo{file: fileScope, tdecl: d.spec})
   390  			case funcDecl:
   391  				name := d.decl.Name.Name
   392  				obj := NewFunc(d.decl.Name.Pos(), pkg, name, nil)
   393  				hasTParamError := false // avoid duplicate type parameter errors
   394  				if d.decl.Recv.NumFields() == 0 {
   395  					// regular function
   396  					if d.decl.Recv != nil {
   397  						check.error(d.decl.Recv, _BadRecv, "method is missing receiver")
   398  						// treat as function
   399  					}
   400  					if name == "init" || (name == "main" && check.pkg.name == "main") {
   401  						code := _InvalidInitDecl
   402  						if name == "main" {
   403  							code = _InvalidMainDecl
   404  						}
   405  						if d.decl.Type.TypeParams.NumFields() != 0 {
   406  							check.softErrorf(d.decl.Type.TypeParams.List[0], code, "func %s must have no type parameters", name)
   407  							hasTParamError = true
   408  						}
   409  						if t := d.decl.Type; t.Params.NumFields() != 0 || t.Results != nil {
   410  							// TODO(rFindley) Should this be a hard error?
   411  							check.softErrorf(d.decl, code, "func %s must have no arguments and no return values", name)
   412  						}
   413  					}
   414  					if name == "init" {
   415  						// don't declare init functions in the package scope - they are invisible
   416  						obj.parent = pkg.scope
   417  						check.recordDef(d.decl.Name, obj)
   418  						// init functions must have a body
   419  						if d.decl.Body == nil {
   420  							// TODO(gri) make this error message consistent with the others above
   421  							check.softErrorf(obj, _MissingInitBody, "missing function body")
   422  						}
   423  					} else {
   424  						check.declare(pkg.scope, d.decl.Name, obj, token.NoPos)
   425  					}
   426  				} else {
   427  					// method
   428  
   429  					// TODO(rFindley) earlier versions of this code checked that methods
   430  					//                have no type parameters, but this is checked later
   431  					//                when type checking the function type. Confirm that
   432  					//                we don't need to check tparams here.
   433  
   434  					ptr, recv, _ := check.unpackRecv(d.decl.Recv.List[0].Type, false)
   435  					// (Methods with invalid receiver cannot be associated to a type, and
   436  					// methods with blank _ names are never found; no need to collect any
   437  					// of them. They will still be type-checked with all the other functions.)
   438  					if recv != nil && name != "_" {
   439  						methods = append(methods, methodInfo{obj, ptr, recv})
   440  					}
   441  					check.recordDef(d.decl.Name, obj)
   442  				}
   443  				if d.decl.Type.TypeParams.NumFields() != 0 && !check.allowVersion(pkg, 1, 18) && !hasTParamError {
   444  					check.softErrorf(d.decl.Type.TypeParams.List[0], _UnsupportedFeature, "type parameters require go1.18 or later")
   445  				}
   446  				info := &declInfo{file: fileScope, fdecl: d.decl}
   447  				// Methods are not package-level objects but we still track them in the
   448  				// object map so that we can handle them like regular functions (if the
   449  				// receiver is invalid); also we need their fdecl info when associating
   450  				// them with their receiver base type, below.
   451  				check.objMap[obj] = info
   452  				obj.setOrder(uint32(len(check.objMap)))
   453  			}
   454  		})
   455  	}
   456  
   457  	// verify that objects in package and file scopes have different names
   458  	for _, scope := range fileScopes {
   459  		for name, obj := range scope.elems {
   460  			if alt := pkg.scope.Lookup(name); alt != nil {
   461  				obj = resolve(name, obj)
   462  				if pkg, ok := obj.(*PkgName); ok {
   463  					check.errorf(alt, _DuplicateDecl, "%s already declared through import of %s", alt.Name(), pkg.Imported())
   464  					check.reportAltDecl(pkg)
   465  				} else {
   466  					check.errorf(alt, _DuplicateDecl, "%s already declared through dot-import of %s", alt.Name(), obj.Pkg())
   467  					// TODO(gri) dot-imported objects don't have a position; reportAltDecl won't print anything
   468  					check.reportAltDecl(obj)
   469  				}
   470  			}
   471  		}
   472  	}
   473  
   474  	// Now that we have all package scope objects and all methods,
   475  	// associate methods with receiver base type name where possible.
   476  	// Ignore methods that have an invalid receiver. They will be
   477  	// type-checked later, with regular functions.
   478  	if methods == nil {
   479  		return // nothing to do
   480  	}
   481  	check.methods = make(map[*TypeName][]*Func)
   482  	for i := range methods {
   483  		m := &methods[i]
   484  		// Determine the receiver base type and associate m with it.
   485  		ptr, base := check.resolveBaseTypeName(m.ptr, m.recv)
   486  		if base != nil {
   487  			m.obj.hasPtrRecv_ = ptr
   488  			check.methods[base] = append(check.methods[base], m.obj)
   489  		}
   490  	}
   491  }
   492  
   493  // unpackRecv unpacks a receiver type and returns its components: ptr indicates whether
   494  // rtyp is a pointer receiver, rname is the receiver type name, and tparams are its
   495  // type parameters, if any. The type parameters are only unpacked if unpackParams is
   496  // set. If rname is nil, the receiver is unusable (i.e., the source has a bug which we
   497  // cannot easily work around).
   498  func (check *Checker) unpackRecv(rtyp ast.Expr, unpackParams bool) (ptr bool, rname *ast.Ident, tparams []*ast.Ident) {
   499  L: // unpack receiver type
   500  	// This accepts invalid receivers such as ***T and does not
   501  	// work for other invalid receivers, but we don't care. The
   502  	// validity of receiver expressions is checked elsewhere.
   503  	for {
   504  		switch t := rtyp.(type) {
   505  		case *ast.ParenExpr:
   506  			rtyp = t.X
   507  		case *ast.StarExpr:
   508  			ptr = true
   509  			rtyp = t.X
   510  		default:
   511  			break L
   512  		}
   513  	}
   514  
   515  	// unpack type parameters, if any
   516  	switch rtyp.(type) {
   517  	case *ast.IndexExpr, *ast.IndexListExpr:
   518  		ix := typeparams.UnpackIndexExpr(rtyp)
   519  		rtyp = ix.X
   520  		if unpackParams {
   521  			for _, arg := range ix.Indices {
   522  				var par *ast.Ident
   523  				switch arg := arg.(type) {
   524  				case *ast.Ident:
   525  					par = arg
   526  				case *ast.BadExpr:
   527  					// ignore - error already reported by parser
   528  				case nil:
   529  					check.invalidAST(ix.Orig, "parameterized receiver contains nil parameters")
   530  				default:
   531  					check.errorf(arg, _BadDecl, "receiver type parameter %s must be an identifier", arg)
   532  				}
   533  				if par == nil {
   534  					par = &ast.Ident{NamePos: arg.Pos(), Name: "_"}
   535  				}
   536  				tparams = append(tparams, par)
   537  			}
   538  		}
   539  	}
   540  
   541  	// unpack receiver name
   542  	if name, _ := rtyp.(*ast.Ident); name != nil {
   543  		rname = name
   544  	}
   545  
   546  	return
   547  }
   548  
   549  // resolveBaseTypeName returns the non-alias base type name for typ, and whether
   550  // there was a pointer indirection to get to it. The base type name must be declared
   551  // in package scope, and there can be at most one pointer indirection. If no such type
   552  // name exists, the returned base is nil.
   553  func (check *Checker) resolveBaseTypeName(seenPtr bool, name *ast.Ident) (ptr bool, base *TypeName) {
   554  	// Algorithm: Starting from a type expression, which may be a name,
   555  	// we follow that type through alias declarations until we reach a
   556  	// non-alias type name. If we encounter anything but pointer types or
   557  	// parentheses we're done. If we encounter more than one pointer type
   558  	// we're done.
   559  	ptr = seenPtr
   560  	var seen map[*TypeName]bool
   561  	var typ ast.Expr = name
   562  	for {
   563  		typ = unparen(typ)
   564  
   565  		// check if we have a pointer type
   566  		if pexpr, _ := typ.(*ast.StarExpr); pexpr != nil {
   567  			// if we've already seen a pointer, we're done
   568  			if ptr {
   569  				return false, nil
   570  			}
   571  			ptr = true
   572  			typ = unparen(pexpr.X) // continue with pointer base type
   573  		}
   574  
   575  		// typ must be a name
   576  		name, _ := typ.(*ast.Ident)
   577  		if name == nil {
   578  			return false, nil
   579  		}
   580  
   581  		// name must denote an object found in the current package scope
   582  		// (note that dot-imported objects are not in the package scope!)
   583  		obj := check.pkg.scope.Lookup(name.Name)
   584  		if obj == nil {
   585  			return false, nil
   586  		}
   587  
   588  		// the object must be a type name...
   589  		tname, _ := obj.(*TypeName)
   590  		if tname == nil {
   591  			return false, nil
   592  		}
   593  
   594  		// ... which we have not seen before
   595  		if seen[tname] {
   596  			return false, nil
   597  		}
   598  
   599  		// we're done if tdecl defined tname as a new type
   600  		// (rather than an alias)
   601  		tdecl := check.objMap[tname].tdecl // must exist for objects in package scope
   602  		if !tdecl.Assign.IsValid() {
   603  			return ptr, tname
   604  		}
   605  
   606  		// otherwise, continue resolving
   607  		typ = tdecl.Type
   608  		if seen == nil {
   609  			seen = make(map[*TypeName]bool)
   610  		}
   611  		seen[tname] = true
   612  	}
   613  }
   614  
   615  // packageObjects typechecks all package objects, but not function bodies.
   616  func (check *Checker) packageObjects() {
   617  	// process package objects in source order for reproducible results
   618  	objList := make([]Object, len(check.objMap))
   619  	i := 0
   620  	for obj := range check.objMap {
   621  		objList[i] = obj
   622  		i++
   623  	}
   624  	sort.Sort(inSourceOrder(objList))
   625  
   626  	// add new methods to already type-checked types (from a prior Checker.Files call)
   627  	for _, obj := range objList {
   628  		if obj, _ := obj.(*TypeName); obj != nil && obj.typ != nil {
   629  			check.collectMethods(obj)
   630  		}
   631  	}
   632  
   633  	// We process non-alias type declarations first, followed by alias declarations,
   634  	// and then everything else. This appears to avoid most situations where the type
   635  	// of an alias is needed before it is available.
   636  	// There may still be cases where this is not good enough (see also issue #25838).
   637  	// In those cases Checker.ident will report an error ("invalid use of type alias").
   638  	var aliasList []*TypeName
   639  	var othersList []Object // everything that's not a type
   640  	// phase 1: non-alias type declarations
   641  	for _, obj := range objList {
   642  		if tname, _ := obj.(*TypeName); tname != nil {
   643  			if check.objMap[tname].tdecl.Assign.IsValid() {
   644  				aliasList = append(aliasList, tname)
   645  			} else {
   646  				check.objDecl(obj, nil)
   647  			}
   648  		} else {
   649  			othersList = append(othersList, obj)
   650  		}
   651  	}
   652  	// phase 2: alias type declarations
   653  	for _, obj := range aliasList {
   654  		check.objDecl(obj, nil)
   655  	}
   656  	// phase 3: all other declarations
   657  	for _, obj := range othersList {
   658  		check.objDecl(obj, nil)
   659  	}
   660  
   661  	// At this point we may have a non-empty check.methods map; this means that not all
   662  	// entries were deleted at the end of typeDecl because the respective receiver base
   663  	// types were not found. In that case, an error was reported when declaring those
   664  	// methods. We can now safely discard this map.
   665  	check.methods = nil
   666  }
   667  
   668  // inSourceOrder implements the sort.Sort interface.
   669  type inSourceOrder []Object
   670  
   671  func (a inSourceOrder) Len() int           { return len(a) }
   672  func (a inSourceOrder) Less(i, j int) bool { return a[i].order() < a[j].order() }
   673  func (a inSourceOrder) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
   674  
   675  // unusedImports checks for unused imports.
   676  func (check *Checker) unusedImports() {
   677  	// If function bodies are not checked, packages' uses are likely missing - don't check.
   678  	if check.conf.IgnoreFuncBodies {
   679  		return
   680  	}
   681  
   682  	// spec: "It is illegal (...) to directly import a package without referring to
   683  	// any of its exported identifiers. To import a package solely for its side-effects
   684  	// (initialization), use the blank identifier as explicit package name."
   685  
   686  	for _, obj := range check.imports {
   687  		if !obj.used && obj.name != "_" {
   688  			check.errorUnusedPkg(obj)
   689  		}
   690  	}
   691  }
   692  
   693  func (check *Checker) errorUnusedPkg(obj *PkgName) {
   694  	// If the package was imported with a name other than the final
   695  	// import path element, show it explicitly in the error message.
   696  	// Note that this handles both renamed imports and imports of
   697  	// packages containing unconventional package declarations.
   698  	// Note that this uses / always, even on Windows, because Go import
   699  	// paths always use forward slashes.
   700  	path := obj.imported.path
   701  	elem := path
   702  	if i := strings.LastIndex(elem, "/"); i >= 0 {
   703  		elem = elem[i+1:]
   704  	}
   705  	if obj.name == "" || obj.name == "." || obj.name == elem {
   706  		check.softErrorf(obj, _UnusedImport, "%q imported but not used", path)
   707  	} else {
   708  		check.softErrorf(obj, _UnusedImport, "%q imported but not used as %s", path, obj.name)
   709  	}
   710  }
   711  
   712  // dir makes a good-faith attempt to return the directory
   713  // portion of path. If path is empty, the result is ".".
   714  // (Per the go/build package dependency tests, we cannot import
   715  // path/filepath and simply use filepath.Dir.)
   716  func dir(path string) string {
   717  	if i := strings.LastIndexAny(path, `/\`); i > 0 {
   718  		return path[:i]
   719  	}
   720  	// i <= 0
   721  	return "."
   722  }
   723  

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