signature.go

Documentation: go/types

     1  // Copyright 2021 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/token"
    11  )
    12  
    13  // ----------------------------------------------------------------------------
    14  // API
    15  
    16  // A Signature represents a (non-builtin) function or method type.
    17  // The receiver is ignored when comparing signatures for identity.
    18  type Signature struct {
    19  	// We need to keep the scope in Signature (rather than passing it around
    20  	// and store it in the Func Object) because when type-checking a function
    21  	// literal we call the general type checker which returns a general Type.
    22  	// We then unpack the *Signature and use the scope for the literal body.
    23  	rparams  *TypeParamList // receiver type parameters from left to right, or nil
    24  	tparams  *TypeParamList // type parameters from left to right, or nil
    25  	scope    *Scope         // function scope for package-local and non-instantiated signatures; nil otherwise
    26  	recv     *Var           // nil if not a method
    27  	params   *Tuple         // (incoming) parameters from left to right; or nil
    28  	results  *Tuple         // (outgoing) results from left to right; or nil
    29  	variadic bool           // true if the last parameter's type is of the form ...T (or string, for append built-in only)
    30  }
    31  
    32  // NewSignature returns a new function type for the given receiver, parameters,
    33  // and results, either of which may be nil. If variadic is set, the function
    34  // is variadic, it must have at least one parameter, and the last parameter
    35  // must be of unnamed slice type.
    36  //
    37  // Deprecated: Use NewSignatureType instead which allows for type parameters.
    38  func NewSignature(recv *Var, params, results *Tuple, variadic bool) *Signature {
    39  	return NewSignatureType(recv, nil, nil, params, results, variadic)
    40  }
    41  
    42  // NewSignatureType creates a new function type for the given receiver,
    43  // receiver type parameters, type parameters, parameters, and results. If
    44  // variadic is set, params must hold at least one parameter and the last
    45  // parameter's core type must be of unnamed slice or bytestring type.
    46  // If recv is non-nil, typeParams must be empty. If recvTypeParams is
    47  // non-empty, recv must be non-nil.
    48  func NewSignatureType(recv *Var, recvTypeParams, typeParams []*TypeParam, params, results *Tuple, variadic bool) *Signature {
    49  	if variadic {
    50  		n := params.Len()
    51  		if n == 0 {
    52  			panic("variadic function must have at least one parameter")
    53  		}
    54  		core := coreString(params.At(n - 1).typ)
    55  		if _, ok := core.(*Slice); !ok && !isString(core) {
    56  			panic(fmt.Sprintf("got %s, want variadic parameter with unnamed slice type or string as core type", core.String()))
    57  		}
    58  	}
    59  	sig := &Signature{recv: recv, params: params, results: results, variadic: variadic}
    60  	if len(recvTypeParams) != 0 {
    61  		if recv == nil {
    62  			panic("function with receiver type parameters must have a receiver")
    63  		}
    64  		sig.rparams = bindTParams(recvTypeParams)
    65  	}
    66  	if len(typeParams) != 0 {
    67  		if recv != nil {
    68  			panic("function with type parameters cannot have a receiver")
    69  		}
    70  		sig.tparams = bindTParams(typeParams)
    71  	}
    72  	return sig
    73  }
    74  
    75  // Recv returns the receiver of signature s (if a method), or nil if a
    76  // function. It is ignored when comparing signatures for identity.
    77  //
    78  // For an abstract method, Recv returns the enclosing interface either
    79  // as a *Named or an *Interface. Due to embedding, an interface may
    80  // contain methods whose receiver type is a different interface.
    81  func (s *Signature) Recv() *Var { return s.recv }
    82  
    83  // TypeParams returns the type parameters of signature s, or nil.
    84  func (s *Signature) TypeParams() *TypeParamList { return s.tparams }
    85  
    86  // RecvTypeParams returns the receiver type parameters of signature s, or nil.
    87  func (s *Signature) RecvTypeParams() *TypeParamList { return s.rparams }
    88  
    89  // Params returns the parameters of signature s, or nil.
    90  func (s *Signature) Params() *Tuple { return s.params }
    91  
    92  // Results returns the results of signature s, or nil.
    93  func (s *Signature) Results() *Tuple { return s.results }
    94  
    95  // Variadic reports whether the signature s is variadic.
    96  func (s *Signature) Variadic() bool { return s.variadic }
    97  
    98  func (t *Signature) Underlying() Type { return t }
    99  func (t *Signature) String() string   { return TypeString(t, nil) }
   100  
   101  // ----------------------------------------------------------------------------
   102  // Implementation
   103  
   104  // funcType type-checks a function or method type.
   105  func (check *Checker) funcType(sig *Signature, recvPar *ast.FieldList, ftyp *ast.FuncType) {
   106  	check.openScope(ftyp, "function")
   107  	check.scope.isFunc = true
   108  	check.recordScope(ftyp, check.scope)
   109  	sig.scope = check.scope
   110  	defer check.closeScope()
   111  
   112  	if recvPar != nil && len(recvPar.List) > 0 {
   113  		// collect generic receiver type parameters, if any
   114  		// - a receiver type parameter is like any other type parameter, except that it is declared implicitly
   115  		// - the receiver specification acts as local declaration for its type parameters, which may be blank
   116  		_, rname, rparams := check.unpackRecv(recvPar.List[0].Type, true)
   117  		if len(rparams) > 0 {
   118  			tparams := check.declareTypeParams(nil, rparams)
   119  			sig.rparams = bindTParams(tparams)
   120  			// Blank identifiers don't get declared, so naive type-checking of the
   121  			// receiver type expression would fail in Checker.collectParams below,
   122  			// when Checker.ident cannot resolve the _ to a type.
   123  			//
   124  			// Checker.recvTParamMap maps these blank identifiers to their type parameter
   125  			// types, so that they may be resolved in Checker.ident when they fail
   126  			// lookup in the scope.
   127  			for i, p := range rparams {
   128  				if p.Name == "_" {
   129  					if check.recvTParamMap == nil {
   130  						check.recvTParamMap = make(map[*ast.Ident]*TypeParam)
   131  					}
   132  					check.recvTParamMap[p] = tparams[i]
   133  				}
   134  			}
   135  			// determine receiver type to get its type parameters
   136  			// and the respective type parameter bounds
   137  			var recvTParams []*TypeParam
   138  			if rname != nil {
   139  				// recv should be a Named type (otherwise an error is reported elsewhere)
   140  				// Also: Don't report an error via genericType since it will be reported
   141  				//       again when we type-check the signature.
   142  				// TODO(gri) maybe the receiver should be marked as invalid instead?
   143  				if recv, _ := check.genericType(rname, nil).(*Named); recv != nil {
   144  					recvTParams = recv.TypeParams().list()
   145  				}
   146  			}
   147  			// provide type parameter bounds
   148  			if len(tparams) == len(recvTParams) {
   149  				smap := makeRenameMap(recvTParams, tparams)
   150  				for i, tpar := range tparams {
   151  					recvTPar := recvTParams[i]
   152  					check.mono.recordCanon(tpar, recvTPar)
   153  					// recvTPar.bound is (possibly) parameterized in the context of the
   154  					// receiver type declaration. Substitute parameters for the current
   155  					// context.
   156  					tpar.bound = check.subst(tpar.obj.pos, recvTPar.bound, smap, nil, check.context())
   157  				}
   158  			} else if len(tparams) < len(recvTParams) {
   159  				// Reporting an error here is a stop-gap measure to avoid crashes in the
   160  				// compiler when a type parameter/argument cannot be inferred later. It
   161  				// may lead to follow-on errors (see issues #51339, #51343).
   162  				// TODO(gri) find a better solution
   163  				got := measure(len(tparams), "type parameter")
   164  				check.errorf(recvPar, _BadRecv, "got %s, but receiver base type declares %d", got, len(recvTParams))
   165  			}
   166  		}
   167  	}
   168  
   169  	if ftyp.TypeParams != nil {
   170  		check.collectTypeParams(&sig.tparams, ftyp.TypeParams)
   171  		// Always type-check method type parameters but complain that they are not allowed.
   172  		// (A separate check is needed when type-checking interface method signatures because
   173  		// they don't have a receiver specification.)
   174  		if recvPar != nil {
   175  			check.errorf(ftyp.TypeParams, _InvalidMethodTypeParams, "methods cannot have type parameters")
   176  		}
   177  	}
   178  
   179  	// Value (non-type) parameters' scope starts in the function body. Use a temporary scope for their
   180  	// declarations and then squash that scope into the parent scope (and report any redeclarations at
   181  	// that time).
   182  	scope := NewScope(check.scope, token.NoPos, token.NoPos, "function body (temp. scope)")
   183  	recvList, _ := check.collectParams(scope, recvPar, false)
   184  	params, variadic := check.collectParams(scope, ftyp.Params, true)
   185  	results, _ := check.collectParams(scope, ftyp.Results, false)
   186  	scope.squash(func(obj, alt Object) {
   187  		check.errorf(obj, _DuplicateDecl, "%s redeclared in this block", obj.Name())
   188  		check.reportAltDecl(alt)
   189  	})
   190  
   191  	if recvPar != nil {
   192  		// recv parameter list present (may be empty)
   193  		// spec: "The receiver is specified via an extra parameter section preceding the
   194  		// method name. That parameter section must declare a single parameter, the receiver."
   195  		var recv *Var
   196  		switch len(recvList) {
   197  		case 0:
   198  			// error reported by resolver
   199  			recv = NewParam(token.NoPos, nil, "", Typ[Invalid]) // ignore recv below
   200  		default:
   201  			// more than one receiver
   202  			check.error(recvList[len(recvList)-1], _InvalidRecv, "method must have exactly one receiver")
   203  			fallthrough // continue with first receiver
   204  		case 1:
   205  			recv = recvList[0]
   206  		}
   207  		sig.recv = recv
   208  
   209  		// Delay validation of receiver type as it may cause premature expansion
   210  		// of types the receiver type is dependent on (see issues #51232, #51233).
   211  		check.later(func() {
   212  			// spec: "The receiver type must be of the form T or *T where T is a type name."
   213  			rtyp, _ := deref(recv.typ)
   214  			if rtyp == Typ[Invalid] {
   215  				return // error was reported before
   216  			}
   217  			// spec: "The type denoted by T is called the receiver base type; it must not
   218  			// be a pointer or interface type and it must be declared in the same package
   219  			// as the method."
   220  			switch T := rtyp.(type) {
   221  			case *Named:
   222  				// The receiver type may be an instantiated type referred to
   223  				// by an alias (which cannot have receiver parameters for now).
   224  				if T.TypeArgs() != nil && sig.RecvTypeParams() == nil {
   225  					check.errorf(recv, _InvalidRecv, "cannot define new methods on instantiated type %s", rtyp)
   226  					break
   227  				}
   228  				if T.obj.pkg != check.pkg {
   229  					check.errorf(recv, _InvalidRecv, "cannot define new methods on non-local type %s", rtyp)
   230  					break
   231  				}
   232  				var cause string
   233  				switch u := T.under().(type) {
   234  				case *Basic:
   235  					// unsafe.Pointer is treated like a regular pointer
   236  					if u.kind == UnsafePointer {
   237  						cause = "unsafe.Pointer"
   238  					}
   239  				case *Pointer, *Interface:
   240  					cause = "pointer or interface type"
   241  				case *TypeParam:
   242  					// The underlying type of a receiver base type cannot be a
   243  					// type parameter: "type T[P any] P" is not a valid declaration.
   244  					unreachable()
   245  				}
   246  				if cause != "" {
   247  					check.errorf(recv, _InvalidRecv, "invalid receiver type %s (%s)", rtyp, cause)
   248  				}
   249  			case *Basic:
   250  				check.errorf(recv, _InvalidRecv, "cannot define new methods on non-local type %s", rtyp)
   251  			default:
   252  				check.errorf(recv, _InvalidRecv, "invalid receiver type %s", recv.typ)
   253  			}
   254  		}).describef(recv, "validate receiver %s", recv)
   255  	}
   256  
   257  	sig.params = NewTuple(params...)
   258  	sig.results = NewTuple(results...)
   259  	sig.variadic = variadic
   260  }
   261  
   262  // collectParams declares the parameters of list in scope and returns the corresponding
   263  // variable list.
   264  func (check *Checker) collectParams(scope *Scope, list *ast.FieldList, variadicOk bool) (params []*Var, variadic bool) {
   265  	if list == nil {
   266  		return
   267  	}
   268  
   269  	var named, anonymous bool
   270  	for i, field := range list.List {
   271  		ftype := field.Type
   272  		if t, _ := ftype.(*ast.Ellipsis); t != nil {
   273  			ftype = t.Elt
   274  			if variadicOk && i == len(list.List)-1 && len(field.Names) <= 1 {
   275  				variadic = true
   276  			} else {
   277  				check.softErrorf(t, _MisplacedDotDotDot, "can only use ... with final parameter in list")
   278  				// ignore ... and continue
   279  			}
   280  		}
   281  		typ := check.varType(ftype)
   282  		// The parser ensures that f.Tag is nil and we don't
   283  		// care if a constructed AST contains a non-nil tag.
   284  		if len(field.Names) > 0 {
   285  			// named parameter
   286  			for _, name := range field.Names {
   287  				if name.Name == "" {
   288  					check.invalidAST(name, "anonymous parameter")
   289  					// ok to continue
   290  				}
   291  				par := NewParam(name.Pos(), check.pkg, name.Name, typ)
   292  				check.declare(scope, name, par, scope.pos)
   293  				params = append(params, par)
   294  			}
   295  			named = true
   296  		} else {
   297  			// anonymous parameter
   298  			par := NewParam(ftype.Pos(), check.pkg, "", typ)
   299  			check.recordImplicit(field, par)
   300  			params = append(params, par)
   301  			anonymous = true
   302  		}
   303  	}
   304  
   305  	if named && anonymous {
   306  		check.invalidAST(list, "list contains both named and anonymous parameters")
   307  		// ok to continue
   308  	}
   309  
   310  	// For a variadic function, change the last parameter's type from T to []T.
   311  	// Since we type-checked T rather than ...T, we also need to retro-actively
   312  	// record the type for ...T.
   313  	if variadic {
   314  		last := params[len(params)-1]
   315  		last.typ = &Slice{elem: last.typ}
   316  		check.recordTypeAndValue(list.List[len(list.List)-1].Type, typexpr, last.typ, nil)
   317  	}
   318  
   319  	return
   320  }
   321
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