resolver.go

     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  	. "internal/types/errors"
    14  	"sort"
    15  	"strconv"
    16  	"strings"
    17  	"unicode"
    18  )
    19  
    20  // A declInfo describes a package-level const, type, var, or func declaration.
    21  type declInfo struct {
    22  	file      *Scope        // scope of file containing this declaration
    23  	lhs       []*Var        // lhs of n:1 variable declarations, or nil
    24  	vtyp      ast.Expr      // type, or nil (for const and var declarations only)
    25  	init      ast.Expr      // init/orig expression, or nil (for const and var declarations only)
    26  	inherited bool          // if set, the init expression is inherited from a previous constant declaration
    27  	tdecl     *ast.TypeSpec // type declaration, or nil
    28  	fdecl     *ast.FuncDecl // func declaration, or nil
    29  
    30  	// The deps field tracks initialization expression dependencies.
    31  	deps map[Object]bool // lazily initialized
    32  }
    33  
    34  // hasInitializer reports whether the declared object has an initialization
    35  // expression or function body.
    36  func (d *declInfo) hasInitializer() bool {
    37  	return d.init != nil || d.fdecl != nil && d.fdecl.Body != nil
    38  }
    39  
    40  // addDep adds obj to the set of objects d's init expression depends on.
    41  func (d *declInfo) addDep(obj Object) {
    42  	m := d.deps
    43  	if m == nil {
    44  		m = make(map[Object]bool)
    45  		d.deps = m
    46  	}
    47  	m[obj] = true
    48  }
    49  
    50  // arityMatch checks that the lhs and rhs of a const or var decl
    51  // have the appropriate number of names and init exprs. For const
    52  // decls, init is the value spec providing the init exprs; for
    53  // var decls, init is nil (the init exprs are in s in this case).
    54  func (check *Checker) arityMatch(s, init *ast.ValueSpec) {
    55  	l := len(s.Names)
    56  	r := len(s.Values)
    57  	if init != nil {
    58  		r = len(init.Values)
    59  	}
    60  
    61  	const code = WrongAssignCount
    62  	switch {
    63  	case init == nil && r == 0:
    64  		// var decl w/o init expr
    65  		if s.Type == nil {
    66  			check.error(s, code, "missing type or init expr")
    67  		}
    68  	case l < r:
    69  		if l < len(s.Values) {
    70  			// init exprs from s
    71  			n := s.Values[l]
    72  			check.errorf(n, code, "extra init expr %s", n)
    73  			// TODO(gri) avoid declared and not used error here
    74  		} else {
    75  			// init exprs "inherited"
    76  			check.errorf(s, code, "extra init expr at %s", check.fset.Position(init.Pos()))
    77  			// TODO(gri) avoid declared and not used error here
    78  		}
    79  	case l > r && (init != nil || r != 1):
    80  		n := s.Names[r]
    81  		check.errorf(n, code, "missing init expr for %s", n)
    82  	}
    83  }
    84  
    85  func validatedImportPath(path string) (string, error) {
    86  	s, err := strconv.Unquote(path)
    87  	if err != nil {
    88  		return "", err
    89  	}
    90  	if s == "" {
    91  		return "", fmt.Errorf("empty string")
    92  	}
    93  	const illegalChars = `!"#$%&'()*,:;<=>?[\]^{|}` + "`\uFFFD"
    94  	for _, r := range s {
    95  		if !unicode.IsGraphic(r) || unicode.IsSpace(r) || strings.ContainsRune(illegalChars, r) {
    96  			return s, fmt.Errorf("invalid character %#U", r)
    97  		}
    98  	}
    99  	return s, nil
   100  }
   101  
   102  // declarePkgObj declares obj in the package scope, records its ident -> obj mapping,
   103  // and updates check.objMap. The object must not be a function or method.
   104  func (check *Checker) declarePkgObj(ident *ast.Ident, obj Object, d *declInfo) {
   105  	assert(ident.Name == obj.Name())
   106  
   107  	// spec: "A package-scope or file-scope identifier with name init
   108  	// may only be declared to be a function with this (func()) signature."
   109  	if ident.Name == "init" {
   110  		check.error(ident, InvalidInitDecl, "cannot declare init - must be func")
   111  		return
   112  	}
   113  
   114  	// spec: "The main package must have package name main and declare
   115  	// a function main that takes no arguments and returns no value."
   116  	if ident.Name == "main" && check.pkg.name == "main" {
   117  		check.error(ident, InvalidMainDecl, "cannot declare main - must be func")
   118  		return
   119  	}
   120  
   121  	check.declare(check.pkg.scope, ident, obj, nopos)
   122  	check.objMap[obj] = d
   123  	obj.setOrder(uint32(len(check.objMap)))
   124  }
   125  
   126  // filename returns a filename suitable for debugging output.
   127  func (check *Checker) filename(fileNo int) string {
   128  	file := check.files[fileNo]
   129  	if pos := file.Pos(); pos.IsValid() {
   130  		return check.fset.File(pos).Name()
   131  	}
   132  	return fmt.Sprintf("file[%d]", fileNo)
   133  }
   134  
   135  func (check *Checker) importPackage(at positioner, path, dir string) *Package {
   136  	// If we already have a package for the given (path, dir)
   137  	// pair, use it instead of doing a full import.
   138  	// Checker.impMap only caches packages that are marked Complete
   139  	// or fake (dummy packages for failed imports). Incomplete but
   140  	// non-fake packages do require an import to complete them.
   141  	key := importKey{path, dir}
   142  	imp := check.impMap[key]
   143  	if imp != nil {
   144  		return imp
   145  	}
   146  
   147  	// no package yet => import it
   148  	if path == "C" && (check.conf.FakeImportC || check.conf.go115UsesCgo) {
   149  		if check.conf.FakeImportC && check.conf.go115UsesCgo {
   150  			check.error(at, BadImportPath, "cannot use FakeImportC and go115UsesCgo together")
   151  		}
   152  		imp = NewPackage("C", "C")
   153  		imp.fake = true // package scope is not populated
   154  		imp.cgo = check.conf.go115UsesCgo
   155  	} else {
   156  		// ordinary import
   157  		var err error
   158  		if importer := check.conf.Importer; importer == nil {
   159  			err = fmt.Errorf("Config.Importer not installed")
   160  		} else if importerFrom, ok := importer.(ImporterFrom); ok {
   161  			imp, err = importerFrom.ImportFrom(path, dir, 0)
   162  			if imp == nil && err == nil {
   163  				err = fmt.Errorf("Config.Importer.ImportFrom(%s, %s, 0) returned nil but no error", path, dir)
   164  			}
   165  		} else {
   166  			imp, err = importer.Import(path)
   167  			if imp == nil && err == nil {
   168  				err = fmt.Errorf("Config.Importer.Import(%s) returned nil but no error", path)
   169  			}
   170  		}
   171  		// make sure we have a valid package name
   172  		// (errors here can only happen through manipulation of packages after creation)
   173  		if err == nil && imp != nil && (imp.name == "_" || imp.name == "") {
   174  			err = fmt.Errorf("invalid package name: %q", imp.name)
   175  			imp = nil // create fake package below
   176  		}
   177  		if err != nil {
   178  			check.errorf(at, BrokenImport, "could not import %s (%s)", path, err)
   179  			if imp == nil {
   180  				// create a new fake package
   181  				// come up with a sensible package name (heuristic)
   182  				name := path
   183  				if i := len(name); i > 0 && name[i-1] == '/' {
   184  					name = name[:i-1]
   185  				}
   186  				if i := strings.LastIndex(name, "/"); i >= 0 {
   187  					name = name[i+1:]
   188  				}
   189  				imp = NewPackage(path, name)
   190  			}
   191  			// continue to use the package as best as we can
   192  			imp.fake = true // avoid follow-up lookup failures
   193  		}
   194  	}
   195  
   196  	// package should be complete or marked fake, but be cautious
   197  	if imp.complete || imp.fake {
   198  		check.impMap[key] = imp
   199  		// Once we've formatted an error message, keep the pkgPathMap
   200  		// up-to-date on subsequent imports. It is used for package
   201  		// qualification in error messages.
   202  		if check.pkgPathMap != nil {
   203  			check.markImports(imp)
   204  		}
   205  		return imp
   206  	}
   207  
   208  	// something went wrong (importer may have returned incomplete package without error)
   209  	return nil
   210  }
   211  
   212  // collectObjects collects all file and package objects and inserts them
   213  // into their respective scopes. It also performs imports and associates
   214  // methods with receiver base type names.
   215  func (check *Checker) collectObjects() {
   216  	pkg := check.pkg
   217  
   218  	// pkgImports is the set of packages already imported by any package file seen
   219  	// so far. Used to avoid duplicate entries in pkg.imports. Allocate and populate
   220  	// it (pkg.imports may not be empty if we are checking test files incrementally).
   221  	// Note that pkgImports is keyed by package (and thus package path), not by an
   222  	// importKey value. Two different importKey values may map to the same package
   223  	// which is why we cannot use the check.impMap here.
   224  	var pkgImports = make(map[*Package]bool)
   225  	for _, imp := range pkg.imports {
   226  		pkgImports[imp] = true
   227  	}
   228  
   229  	type methodInfo struct {
   230  		obj  *Func      // method
   231  		ptr  bool       // true if pointer receiver
   232  		recv *ast.Ident // receiver type name
   233  	}
   234  	var methods []methodInfo // collected methods with valid receivers and non-blank _ names
   235  	var fileScopes []*Scope
   236  	for fileNo, file := range check.files {
   237  		// The package identifier denotes the current package,
   238  		// but there is no corresponding package object.
   239  		check.recordDef(file.Name, nil)
   240  
   241  		// Use the actual source file extent rather than *ast.File extent since the
   242  		// latter doesn't include comments which appear at the start or end of the file.
   243  		// Be conservative and use the *ast.File extent if we don't have a *token.File.
   244  		pos, end := file.Pos(), file.End()
   245  		if f := check.fset.File(file.Pos()); f != nil {
   246  			pos, end = token.Pos(f.Base()), token.Pos(f.Base()+f.Size())
   247  		}
   248  		fileScope := NewScope(pkg.scope, pos, end, check.filename(fileNo))
   249  		fileScopes = append(fileScopes, fileScope)
   250  		check.recordScope(file, fileScope)
   251  
   252  		// determine file directory, necessary to resolve imports
   253  		// FileName may be "" (typically for tests) in which case
   254  		// we get "." as the directory which is what we would want.
   255  		fileDir := dir(check.fset.Position(file.Name.Pos()).Filename)
   256  
   257  		check.walkDecls(file.Decls, func(d decl) {
   258  			switch d := d.(type) {
   259  			case importDecl:
   260  				// import package
   261  				if d.spec.Path.Value == "" {
   262  					return // error reported by parser
   263  				}
   264  				path, err := validatedImportPath(d.spec.Path.Value)
   265  				if err != nil {
   266  					check.errorf(d.spec.Path, BadImportPath, "invalid import path (%s)", err)
   267  					return
   268  				}
   269  
   270  				imp := check.importPackage(d.spec.Path, path, fileDir)
   271  				if imp == nil {
   272  					return
   273  				}
   274  
   275  				// local name overrides imported package name
   276  				name := imp.name
   277  				if d.spec.Name != nil {
   278  					name = d.spec.Name.Name
   279  					if path == "C" {
   280  						// match 1.17 cmd/compile (not prescribed by spec)
   281  						check.error(d.spec.Name, ImportCRenamed, `cannot rename import "C"`)
   282  						return
   283  					}
   284  				}
   285  
   286  				if name == "init" {
   287  					check.error(d.spec, InvalidInitDecl, "cannot import package as init - init must be a func")
   288  					return
   289  				}
   290  
   291  				// add package to list of explicit imports
   292  				// (this functionality is provided as a convenience
   293  				// for clients; it is not needed for type-checking)
   294  				if !pkgImports[imp] {
   295  					pkgImports[imp] = true
   296  					pkg.imports = append(pkg.imports, imp)
   297  				}
   298  
   299  				pkgName := NewPkgName(d.spec.Pos(), pkg, name, imp)
   300  				if d.spec.Name != nil {
   301  					// in a dot-import, the dot represents the package
   302  					check.recordDef(d.spec.Name, pkgName)
   303  				} else {
   304  					check.recordImplicit(d.spec, pkgName)
   305  				}
   306  
   307  				if imp.fake {
   308  					// match 1.17 cmd/compile (not prescribed by spec)
   309  					pkgName.used = true
   310  				}
   311  
   312  				// add import to file scope
   313  				check.imports = append(check.imports, pkgName)
   314  				if name == "." {
   315  					// dot-import
   316  					if check.dotImportMap == nil {
   317  						check.dotImportMap = make(map[dotImportKey]*PkgName)
   318  					}
   319  					// merge imported scope with file scope
   320  					for name, obj := range imp.scope.elems {
   321  						// Note: Avoid eager resolve(name, obj) here, so we only
   322  						// resolve dot-imported objects as needed.
   323  
   324  						// A package scope may contain non-exported objects,
   325  						// do not import them!
   326  						if token.IsExported(name) {
   327  							// declare dot-imported object
   328  							// (Do not use check.declare because it modifies the object
   329  							// via Object.setScopePos, which leads to a race condition;
   330  							// the object may be imported into more than one file scope
   331  							// concurrently. See go.dev/issue/32154.)
   332  							if alt := fileScope.Lookup(name); alt != nil {
   333  								err := check.newError(DuplicateDecl)
   334  								err.addf(d.spec.Name, "%s redeclared in this block", alt.Name())
   335  								err.addAltDecl(alt)
   336  								err.report()
   337  							} else {
   338  								fileScope.insert(name, obj)
   339  								check.dotImportMap[dotImportKey{fileScope, name}] = pkgName
   340  							}
   341  						}
   342  					}
   343  				} else {
   344  					// declare imported package object in file scope
   345  					// (no need to provide s.Name since we called check.recordDef earlier)
   346  					check.declare(fileScope, nil, pkgName, nopos)
   347  				}
   348  			case constDecl:
   349  				// declare all constants
   350  				for i, name := range d.spec.Names {
   351  					obj := NewConst(name.Pos(), pkg, name.Name, nil, constant.MakeInt64(int64(d.iota)))
   352  
   353  					var init ast.Expr
   354  					if i < len(d.init) {
   355  						init = d.init[i]
   356  					}
   357  
   358  					d := &declInfo{file: fileScope, vtyp: d.typ, init: init, inherited: d.inherited}
   359  					check.declarePkgObj(name, obj, d)
   360  				}
   361  
   362  			case varDecl:
   363  				lhs := make([]*Var, len(d.spec.Names))
   364  				// If there's exactly one rhs initializer, use
   365  				// the same declInfo d1 for all lhs variables
   366  				// so that each lhs variable depends on the same
   367  				// rhs initializer (n:1 var declaration).
   368  				var d1 *declInfo
   369  				if len(d.spec.Values) == 1 {
   370  					// The lhs elements are only set up after the for loop below,
   371  					// but that's ok because declareVar only collects the declInfo
   372  					// for a later phase.
   373  					d1 = &declInfo{file: fileScope, lhs: lhs, vtyp: d.spec.Type, init: d.spec.Values[0]}
   374  				}
   375  
   376  				// declare all variables
   377  				for i, name := range d.spec.Names {
   378  					obj := NewVar(name.Pos(), pkg, name.Name, nil)
   379  					lhs[i] = obj
   380  
   381  					di := d1
   382  					if di == nil {
   383  						// individual assignments
   384  						var init ast.Expr
   385  						if i < len(d.spec.Values) {
   386  							init = d.spec.Values[i]
   387  						}
   388  						di = &declInfo{file: fileScope, vtyp: d.spec.Type, init: init}
   389  					}
   390  
   391  					check.declarePkgObj(name, obj, di)
   392  				}
   393  			case typeDecl:
   394  				obj := NewTypeName(d.spec.Name.Pos(), pkg, d.spec.Name.Name, nil)
   395  				check.declarePkgObj(d.spec.Name, obj, &declInfo{file: fileScope, tdecl: d.spec})
   396  			case funcDecl:
   397  				name := d.decl.Name.Name
   398  				obj := NewFunc(d.decl.Name.Pos(), pkg, name, nil) // signature set later
   399  				hasTParamError := false                           // avoid duplicate type parameter errors
   400  				if d.decl.Recv.NumFields() == 0 {
   401  					// regular function
   402  					if d.decl.Recv != nil {
   403  						check.error(d.decl.Recv, BadRecv, "method has no receiver")
   404  						// treat as function
   405  					}
   406  					if name == "init" || (name == "main" && check.pkg.name == "main") {
   407  						code := InvalidInitDecl
   408  						if name == "main" {
   409  							code = InvalidMainDecl
   410  						}
   411  						if d.decl.Type.TypeParams.NumFields() != 0 {
   412  							check.softErrorf(d.decl.Type.TypeParams.List[0], code, "func %s must have no type parameters", name)
   413  							hasTParamError = true
   414  						}
   415  						if t := d.decl.Type; t.Params.NumFields() != 0 || t.Results != nil {
   416  							// TODO(rFindley) Should this be a hard error?
   417  							check.softErrorf(d.decl.Name, code, "func %s must have no arguments and no return values", name)
   418  						}
   419  					}
   420  					if name == "init" {
   421  						// don't declare init functions in the package scope - they are invisible
   422  						obj.parent = pkg.scope
   423  						check.recordDef(d.decl.Name, obj)
   424  						// init functions must have a body
   425  						if d.decl.Body == nil {
   426  							// TODO(gri) make this error message consistent with the others above
   427  							check.softErrorf(obj, MissingInitBody, "missing function body")
   428  						}
   429  					} else {
   430  						check.declare(pkg.scope, d.decl.Name, obj, nopos)
   431  					}
   432  				} else {
   433  					// method
   434  
   435  					// TODO(rFindley) earlier versions of this code checked that methods
   436  					//                have no type parameters, but this is checked later
   437  					//                when type checking the function type. Confirm that
   438  					//                we don't need to check tparams here.
   439  
   440  					ptr, recv, _ := check.unpackRecv(d.decl.Recv.List[0].Type, false)
   441  					// (Methods with invalid receiver cannot be associated to a type, and
   442  					// methods with blank _ names are never found; no need to collect any
   443  					// of them. They will still be type-checked with all the other functions.)
   444  					if recv != nil && name != "_" {
   445  						methods = append(methods, methodInfo{obj, ptr, recv})
   446  					}
   447  					check.recordDef(d.decl.Name, obj)
   448  				}
   449  				_ = d.decl.Type.TypeParams.NumFields() != 0 && !hasTParamError && check.verifyVersionf(d.decl.Type.TypeParams.List[0], go1_18, "type parameter")
   450  				info := &declInfo{file: fileScope, fdecl: d.decl}
   451  				// Methods are not package-level objects but we still track them in the
   452  				// object map so that we can handle them like regular functions (if the
   453  				// receiver is invalid); also we need their fdecl info when associating
   454  				// them with their receiver base type, below.
   455  				check.objMap[obj] = info
   456  				obj.setOrder(uint32(len(check.objMap)))
   457  			}
   458  		})
   459  	}
   460  
   461  	// verify that objects in package and file scopes have different names
   462  	for _, scope := range fileScopes {
   463  		for name, obj := range scope.elems {
   464  			if alt := pkg.scope.Lookup(name); alt != nil {
   465  				obj = resolve(name, obj)
   466  				err := check.newError(DuplicateDecl)
   467  				if pkg, ok := obj.(*PkgName); ok {
   468  					err.addf(alt, "%s already declared through import of %s", alt.Name(), pkg.Imported())
   469  					err.addAltDecl(pkg)
   470  				} else {
   471  					err.addf(alt, "%s already declared through dot-import of %s", alt.Name(), obj.Pkg())
   472  					// TODO(gri) dot-imported objects don't have a position; addAltDecl won't print anything
   473  					err.addAltDecl(obj)
   474  				}
   475  				err.report()
   476  			}
   477  		}
   478  	}
   479  
   480  	// Now that we have all package scope objects and all methods,
   481  	// associate methods with receiver base type name where possible.
   482  	// Ignore methods that have an invalid receiver. They will be
   483  	// type-checked later, with regular functions.
   484  	if methods == nil {
   485  		return // nothing to do
   486  	}
   487  	check.methods = make(map[*TypeName][]*Func)
   488  	for i := range methods {
   489  		m := &methods[i]
   490  		// Determine the receiver base type and associate m with it.
   491  		ptr, base := check.resolveBaseTypeName(m.ptr, m.recv, fileScopes)
   492  		if base != nil {
   493  			m.obj.hasPtrRecv_ = ptr
   494  			check.methods[base] = append(check.methods[base], m.obj)
   495  		}
   496  	}
   497  }
   498  
   499  // unpackRecv unpacks a receiver type and returns its components: ptr indicates whether
   500  // rtyp is a pointer receiver, rname is the receiver type name, and tparams are its
   501  // type parameters, if any. The type parameters are only unpacked if unpackParams is
   502  // set. If rname is nil, the receiver is unusable (i.e., the source has a bug which we
   503  // cannot easily work around).
   504  func (check *Checker) unpackRecv(rtyp ast.Expr, unpackParams bool) (ptr bool, rname *ast.Ident, tparams []*ast.Ident) {
   505  L: // unpack receiver type
   506  	// This accepts invalid receivers such as ***T and does not
   507  	// work for other invalid receivers, but we don't care. The
   508  	// validity of receiver expressions is checked elsewhere.
   509  	for {
   510  		switch t := rtyp.(type) {
   511  		case *ast.ParenExpr:
   512  			rtyp = t.X
   513  		case *ast.StarExpr:
   514  			ptr = true
   515  			rtyp = t.X
   516  		default:
   517  			break L
   518  		}
   519  	}
   520  
   521  	// unpack type parameters, if any
   522  	switch rtyp.(type) {
   523  	case *ast.IndexExpr, *ast.IndexListExpr:
   524  		ix := typeparams.UnpackIndexExpr(rtyp)
   525  		rtyp = ix.X
   526  		if unpackParams {
   527  			for _, arg := range ix.Indices {
   528  				var par *ast.Ident
   529  				switch arg := arg.(type) {
   530  				case *ast.Ident:
   531  					par = arg
   532  				case *ast.BadExpr:
   533  					// ignore - error already reported by parser
   534  				case nil:
   535  					check.error(ix.Orig, InvalidSyntaxTree, "parameterized receiver contains nil parameters")
   536  				default:
   537  					check.errorf(arg, BadDecl, "receiver type parameter %s must be an identifier", arg)
   538  				}
   539  				if par == nil {
   540  					par = &ast.Ident{NamePos: arg.Pos(), Name: "_"}
   541  				}
   542  				tparams = append(tparams, par)
   543  			}
   544  		}
   545  	}
   546  
   547  	// unpack receiver name
   548  	if name, _ := rtyp.(*ast.Ident); name != nil {
   549  		rname = name
   550  	}
   551  
   552  	return
   553  }
   554  
   555  // resolveBaseTypeName returns the non-alias base type name for typ, and whether
   556  // there was a pointer indirection to get to it. The base type name must be declared
   557  // in package scope, and there can be at most one pointer indirection. If no such type
   558  // name exists, the returned base is nil.
   559  func (check *Checker) resolveBaseTypeName(seenPtr bool, typ ast.Expr, fileScopes []*Scope) (ptr bool, base *TypeName) {
   560  	// Algorithm: Starting from a type expression, which may be a name,
   561  	// we follow that type through alias declarations until we reach a
   562  	// non-alias type name. If we encounter anything but pointer types or
   563  	// parentheses we're done. If we encounter more than one pointer type
   564  	// we're done.
   565  	ptr = seenPtr
   566  	var seen map[*TypeName]bool
   567  	for {
   568  		// Note: this differs from types2, but is necessary. The syntax parser
   569  		// strips unnecessary parens.
   570  		typ = ast.Unparen(typ)
   571  
   572  		// check if we have a pointer type
   573  		if pexpr, _ := typ.(*ast.StarExpr); pexpr != nil {
   574  			// if we've already seen a pointer, we're done
   575  			if ptr {
   576  				return false, nil
   577  			}
   578  			ptr = true
   579  			typ = ast.Unparen(pexpr.X) // continue with pointer base type
   580  		}
   581  
   582  		// typ must be a name, or a C.name cgo selector.
   583  		var name string
   584  		switch typ := typ.(type) {
   585  		case *ast.Ident:
   586  			name = typ.Name
   587  		case *ast.SelectorExpr:
   588  			// C.struct_foo is a valid type name for packages using cgo.
   589  			//
   590  			// Detect this case, and adjust name so that the correct TypeName is
   591  			// resolved below.
   592  			if ident, _ := typ.X.(*ast.Ident); ident != nil && ident.Name == "C" {
   593  				// Check whether "C" actually resolves to an import of "C", by looking
   594  				// in the appropriate file scope.
   595  				var obj Object
   596  				for _, scope := range fileScopes {
   597  					if scope.Contains(ident.Pos()) {
   598  						obj = scope.Lookup(ident.Name)
   599  					}
   600  				}
   601  				// If Config.go115UsesCgo is set, the typechecker will resolve Cgo
   602  				// selectors to their cgo name. We must do the same here.
   603  				if pname, _ := obj.(*PkgName); pname != nil {
   604  					if pname.imported.cgo { // only set if Config.go115UsesCgo is set
   605  						name = "_Ctype_" + typ.Sel.Name
   606  					}
   607  				}
   608  			}
   609  			if name == "" {
   610  				return false, nil
   611  			}
   612  		default:
   613  			return false, nil
   614  		}
   615  
   616  		// name must denote an object found in the current package scope
   617  		// (note that dot-imported objects are not in the package scope!)
   618  		obj := check.pkg.scope.Lookup(name)
   619  		if obj == nil {
   620  			return false, nil
   621  		}
   622  
   623  		// the object must be a type name...
   624  		tname, _ := obj.(*TypeName)
   625  		if tname == nil {
   626  			return false, nil
   627  		}
   628  
   629  		// ... which we have not seen before
   630  		if seen[tname] {
   631  			return false, nil
   632  		}
   633  
   634  		// we're done if tdecl defined tname as a new type
   635  		// (rather than an alias)
   636  		tdecl := check.objMap[tname].tdecl // must exist for objects in package scope
   637  		if !tdecl.Assign.IsValid() {
   638  			return ptr, tname
   639  		}
   640  
   641  		// otherwise, continue resolving
   642  		typ = tdecl.Type
   643  		if seen == nil {
   644  			seen = make(map[*TypeName]bool)
   645  		}
   646  		seen[tname] = true
   647  	}
   648  }
   649  
   650  // packageObjects typechecks all package objects, but not function bodies.
   651  func (check *Checker) packageObjects() {
   652  	// process package objects in source order for reproducible results
   653  	objList := make([]Object, len(check.objMap))
   654  	i := 0
   655  	for obj := range check.objMap {
   656  		objList[i] = obj
   657  		i++
   658  	}
   659  	sort.Sort(inSourceOrder(objList))
   660  
   661  	// add new methods to already type-checked types (from a prior Checker.Files call)
   662  	for _, obj := range objList {
   663  		if obj, _ := obj.(*TypeName); obj != nil && obj.typ != nil {
   664  			check.collectMethods(obj)
   665  		}
   666  	}
   667  
   668  	if check.conf._EnableAlias {
   669  		// With Alias nodes we can process declarations in any order.
   670  		for _, obj := range objList {
   671  			check.objDecl(obj, nil)
   672  		}
   673  	} else {
   674  		// Without Alias nodes, we process non-alias type declarations first, followed by
   675  		// alias declarations, and then everything else. This appears to avoid most situations
   676  		// where the type of an alias is needed before it is available.
   677  		// There may still be cases where this is not good enough (see also go.dev/issue/25838).
   678  		// In those cases Checker.ident will report an error ("invalid use of type alias").
   679  		var aliasList []*TypeName
   680  		var othersList []Object // everything that's not a type
   681  		// phase 1: non-alias type declarations
   682  		for _, obj := range objList {
   683  			if tname, _ := obj.(*TypeName); tname != nil {
   684  				if check.objMap[tname].tdecl.Assign.IsValid() {
   685  					aliasList = append(aliasList, tname)
   686  				} else {
   687  					check.objDecl(obj, nil)
   688  				}
   689  			} else {
   690  				othersList = append(othersList, obj)
   691  			}
   692  		}
   693  		// phase 2: alias type declarations
   694  		for _, obj := range aliasList {
   695  			check.objDecl(obj, nil)
   696  		}
   697  		// phase 3: all other declarations
   698  		for _, obj := range othersList {
   699  			check.objDecl(obj, nil)
   700  		}
   701  	}
   702  
   703  	// At this point we may have a non-empty check.methods map; this means that not all
   704  	// entries were deleted at the end of typeDecl because the respective receiver base
   705  	// types were not found. In that case, an error was reported when declaring those
   706  	// methods. We can now safely discard this map.
   707  	check.methods = nil
   708  }
   709  
   710  // inSourceOrder implements the sort.Sort interface.
   711  type inSourceOrder []Object
   712  
   713  func (a inSourceOrder) Len() int           { return len(a) }
   714  func (a inSourceOrder) Less(i, j int) bool { return a[i].order() < a[j].order() }
   715  func (a inSourceOrder) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
   716  
   717  // unusedImports checks for unused imports.
   718  func (check *Checker) unusedImports() {
   719  	// If function bodies are not checked, packages' uses are likely missing - don't check.
   720  	if check.conf.IgnoreFuncBodies {
   721  		return
   722  	}
   723  
   724  	// spec: "It is illegal (...) to directly import a package without referring to
   725  	// any of its exported identifiers. To import a package solely for its side-effects
   726  	// (initialization), use the blank identifier as explicit package name."
   727  
   728  	for _, obj := range check.imports {
   729  		if !obj.used && obj.name != "_" {
   730  			check.errorUnusedPkg(obj)
   731  		}
   732  	}
   733  }
   734  
   735  func (check *Checker) errorUnusedPkg(obj *PkgName) {
   736  	// If the package was imported with a name other than the final
   737  	// import path element, show it explicitly in the error message.
   738  	// Note that this handles both renamed imports and imports of
   739  	// packages containing unconventional package declarations.
   740  	// Note that this uses / always, even on Windows, because Go import
   741  	// paths always use forward slashes.
   742  	path := obj.imported.path
   743  	elem := path
   744  	if i := strings.LastIndex(elem, "/"); i >= 0 {
   745  		elem = elem[i+1:]
   746  	}
   747  	if obj.name == "" || obj.name == "." || obj.name == elem {
   748  		check.softErrorf(obj, UnusedImport, "%q imported and not used", path)
   749  	} else {
   750  		check.softErrorf(obj, UnusedImport, "%q imported as %s and not used", path, obj.name)
   751  	}
   752  }
   753  
   754  // dir makes a good-faith attempt to return the directory
   755  // portion of path. If path is empty, the result is ".".
   756  // (Per the go/build package dependency tests, we cannot import
   757  // path/filepath and simply use filepath.Dir.)
   758  func dir(path string) string {
   759  	if i := strings.LastIndexAny(path, `/\`); i > 0 {
   760  		return path[:i]
   761  	}
   762  	// i <= 0
   763  	return "."
   764  }
   765
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