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Source file src/go/types/lookup.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  // This file implements various field and method lookup functions.
     6  
     7  package types
     8  
     9  import "go/token"
    10  
    11  // LookupFieldOrMethod looks up a field or method with given package and name
    12  // in T and returns the corresponding *Var or *Func, an index sequence, and a
    13  // bool indicating if there were any pointer indirections on the path to the
    14  // field or method. If addressable is set, T is the type of an addressable
    15  // variable (only matters for method lookups).
    16  //
    17  // The last index entry is the field or method index in the (possibly embedded)
    18  // type where the entry was found, either:
    19  //
    20  //	1) the list of declared methods of a named type; or
    21  //	2) the list of all methods (method set) of an interface type; or
    22  //	3) the list of fields of a struct type.
    23  //
    24  // The earlier index entries are the indices of the embedded struct fields
    25  // traversed to get to the found entry, starting at depth 0.
    26  //
    27  // If no entry is found, a nil object is returned. In this case, the returned
    28  // index and indirect values have the following meaning:
    29  //
    30  //	- If index != nil, the index sequence points to an ambiguous entry
    31  //	(the same name appeared more than once at the same embedding level).
    32  //
    33  //	- If indirect is set, a method with a pointer receiver type was found
    34  //      but there was no pointer on the path from the actual receiver type to
    35  //	the method's formal receiver base type, nor was the receiver addressable.
    36  //
    37  func LookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (obj Object, index []int, indirect bool) {
    38  	return (*Checker)(nil).lookupFieldOrMethod(T, addressable, pkg, name)
    39  }
    40  
    41  // Internal use of Checker.lookupFieldOrMethod: If the obj result is a method
    42  // associated with a concrete (non-interface) type, the method's signature
    43  // may not be fully set up. Call Checker.objDecl(obj, nil) before accessing
    44  // the method's type.
    45  // TODO(gri) Now that we provide the *Checker, we can probably remove this
    46  // caveat by calling Checker.objDecl from lookupFieldOrMethod. Investigate.
    47  
    48  // lookupFieldOrMethod is like the external version but completes interfaces
    49  // as necessary.
    50  func (check *Checker) lookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (obj Object, index []int, indirect bool) {
    51  	// Methods cannot be associated to a named pointer type
    52  	// (spec: "The type denoted by T is called the receiver base type;
    53  	// it must not be a pointer or interface type and it must be declared
    54  	// in the same package as the method.").
    55  	// Thus, if we have a named pointer type, proceed with the underlying
    56  	// pointer type but discard the result if it is a method since we would
    57  	// not have found it for T (see also issue 8590).
    58  	if t := asNamed(T); t != nil {
    59  		if p, _ := t.underlying.(*Pointer); p != nil {
    60  			obj, index, indirect = check.rawLookupFieldOrMethod(p, false, pkg, name)
    61  			if _, ok := obj.(*Func); ok {
    62  				return nil, nil, false
    63  			}
    64  			return
    65  		}
    66  	}
    67  
    68  	return check.rawLookupFieldOrMethod(T, addressable, pkg, name)
    69  }
    70  
    71  // TODO(gri) The named type consolidation and seen maps below must be
    72  //           indexed by unique keys for a given type. Verify that named
    73  //           types always have only one representation (even when imported
    74  //           indirectly via different packages.)
    75  
    76  // rawLookupFieldOrMethod should only be called by lookupFieldOrMethod and missingMethod.
    77  func (check *Checker) rawLookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (obj Object, index []int, indirect bool) {
    78  	// WARNING: The code in this function is extremely subtle - do not modify casually!
    79  	//          This function and NewMethodSet should be kept in sync.
    80  
    81  	if name == "_" {
    82  		return // blank fields/methods are never found
    83  	}
    84  
    85  	typ, isPtr := deref(T)
    86  
    87  	// *typ where typ is an interface has no methods.
    88  	// Be cautious: typ may be nil (issue 39634, crash #3).
    89  	if typ == nil || isPtr && IsInterface(typ) {
    90  		return
    91  	}
    92  
    93  	// Start with typ as single entry at shallowest depth.
    94  	current := []embeddedType{{typ, nil, isPtr, false}}
    95  
    96  	// Named types that we have seen already, allocated lazily.
    97  	// Used to avoid endless searches in case of recursive types.
    98  	// Since only Named types can be used for recursive types, we
    99  	// only need to track those.
   100  	// (If we ever allow type aliases to construct recursive types,
   101  	// we must use type identity rather than pointer equality for
   102  	// the map key comparison, as we do in consolidateMultiples.)
   103  	var seen map[*Named]bool
   104  
   105  	// search current depth
   106  	for len(current) > 0 {
   107  		var next []embeddedType // embedded types found at current depth
   108  
   109  		// look for (pkg, name) in all types at current depth
   110  		var tpar *_TypeParam // set if obj receiver is a type parameter
   111  		for _, e := range current {
   112  			typ := e.typ
   113  
   114  			// If we have a named type, we may have associated methods.
   115  			// Look for those first.
   116  			if named := asNamed(typ); named != nil {
   117  				if seen[named] {
   118  					// We have seen this type before, at a more shallow depth
   119  					// (note that multiples of this type at the current depth
   120  					// were consolidated before). The type at that depth shadows
   121  					// this same type at the current depth, so we can ignore
   122  					// this one.
   123  					continue
   124  				}
   125  				if seen == nil {
   126  					seen = make(map[*Named]bool)
   127  				}
   128  				seen[named] = true
   129  
   130  				// look for a matching attached method
   131  				if i, m := lookupMethod(named.methods, pkg, name); m != nil {
   132  					// potential match
   133  					// caution: method may not have a proper signature yet
   134  					index = concat(e.index, i)
   135  					if obj != nil || e.multiples {
   136  						return nil, index, false // collision
   137  					}
   138  					obj = m
   139  					indirect = e.indirect
   140  					continue // we can't have a matching field or interface method
   141  				}
   142  
   143  				// continue with underlying type, but only if it's not a type parameter
   144  				// TODO(gri) is this what we want to do for type parameters? (spec question)
   145  				// TODO(#45639) the error message produced as a result of skipping an
   146  				//              underlying type parameter should be improved.
   147  				typ = named.under()
   148  				if asTypeParam(typ) != nil {
   149  					continue
   150  				}
   151  			}
   152  
   153  			tpar = nil
   154  			switch t := typ.(type) {
   155  			case *Struct:
   156  				// look for a matching field and collect embedded types
   157  				for i, f := range t.fields {
   158  					if f.sameId(pkg, name) {
   159  						assert(f.typ != nil)
   160  						index = concat(e.index, i)
   161  						if obj != nil || e.multiples {
   162  							return nil, index, false // collision
   163  						}
   164  						obj = f
   165  						indirect = e.indirect
   166  						continue // we can't have a matching interface method
   167  					}
   168  					// Collect embedded struct fields for searching the next
   169  					// lower depth, but only if we have not seen a match yet
   170  					// (if we have a match it is either the desired field or
   171  					// we have a name collision on the same depth; in either
   172  					// case we don't need to look further).
   173  					// Embedded fields are always of the form T or *T where
   174  					// T is a type name. If e.typ appeared multiple times at
   175  					// this depth, f.typ appears multiple times at the next
   176  					// depth.
   177  					if obj == nil && f.embedded {
   178  						typ, isPtr := deref(f.typ)
   179  						// TODO(gri) optimization: ignore types that can't
   180  						// have fields or methods (only Named, Struct, and
   181  						// Interface types need to be considered).
   182  						next = append(next, embeddedType{typ, concat(e.index, i), e.indirect || isPtr, e.multiples})
   183  					}
   184  				}
   185  
   186  			case *Interface:
   187  				// look for a matching method
   188  				// TODO(gri) t.allMethods is sorted - use binary search
   189  				check.completeInterface(token.NoPos, t)
   190  				if i, m := lookupMethod(t.allMethods, pkg, name); m != nil {
   191  					assert(m.typ != nil)
   192  					index = concat(e.index, i)
   193  					if obj != nil || e.multiples {
   194  						return nil, index, false // collision
   195  					}
   196  					obj = m
   197  					indirect = e.indirect
   198  				}
   199  
   200  			case *_TypeParam:
   201  				// only consider explicit methods in the type parameter bound, not
   202  				// methods that may be common to all types in the type list.
   203  				if i, m := lookupMethod(t.Bound().allMethods, pkg, name); m != nil {
   204  					assert(m.typ != nil)
   205  					index = concat(e.index, i)
   206  					if obj != nil || e.multiples {
   207  						return nil, index, false // collision
   208  					}
   209  					tpar = t
   210  					obj = m
   211  					indirect = e.indirect
   212  				}
   213  			}
   214  		}
   215  
   216  		if obj != nil {
   217  			// found a potential match
   218  			// spec: "A method call x.m() is valid if the method set of (the type of) x
   219  			//        contains m and the argument list can be assigned to the parameter
   220  			//        list of m. If x is addressable and &x's method set contains m, x.m()
   221  			//        is shorthand for (&x).m()".
   222  			if f, _ := obj.(*Func); f != nil {
   223  				// determine if method has a pointer receiver
   224  				hasPtrRecv := tpar == nil && ptrRecv(f)
   225  				if hasPtrRecv && !indirect && !addressable {
   226  					return nil, nil, true // pointer/addressable receiver required
   227  				}
   228  			}
   229  			return
   230  		}
   231  
   232  		current = check.consolidateMultiples(next)
   233  	}
   234  
   235  	return nil, nil, false // not found
   236  }
   237  
   238  // embeddedType represents an embedded type
   239  type embeddedType struct {
   240  	typ       Type
   241  	index     []int // embedded field indices, starting with index at depth 0
   242  	indirect  bool  // if set, there was a pointer indirection on the path to this field
   243  	multiples bool  // if set, typ appears multiple times at this depth
   244  }
   245  
   246  // consolidateMultiples collects multiple list entries with the same type
   247  // into a single entry marked as containing multiples. The result is the
   248  // consolidated list.
   249  func (check *Checker) consolidateMultiples(list []embeddedType) []embeddedType {
   250  	if len(list) <= 1 {
   251  		return list // at most one entry - nothing to do
   252  	}
   253  
   254  	n := 0                     // number of entries w/ unique type
   255  	prev := make(map[Type]int) // index at which type was previously seen
   256  	for _, e := range list {
   257  		if i, found := check.lookupType(prev, e.typ); found {
   258  			list[i].multiples = true
   259  			// ignore this entry
   260  		} else {
   261  			prev[e.typ] = n
   262  			list[n] = e
   263  			n++
   264  		}
   265  	}
   266  	return list[:n]
   267  }
   268  
   269  func (check *Checker) lookupType(m map[Type]int, typ Type) (int, bool) {
   270  	// fast path: maybe the types are equal
   271  	if i, found := m[typ]; found {
   272  		return i, true
   273  	}
   274  
   275  	for t, i := range m {
   276  		if check.identical(t, typ) {
   277  			return i, true
   278  		}
   279  	}
   280  
   281  	return 0, false
   282  }
   283  
   284  // MissingMethod returns (nil, false) if V implements T, otherwise it
   285  // returns a missing method required by T and whether it is missing or
   286  // just has the wrong type.
   287  //
   288  // For non-interface types V, or if static is set, V implements T if all
   289  // methods of T are present in V. Otherwise (V is an interface and static
   290  // is not set), MissingMethod only checks that methods of T which are also
   291  // present in V have matching types (e.g., for a type assertion x.(T) where
   292  // x is of interface type V).
   293  //
   294  func MissingMethod(V Type, T *Interface, static bool) (method *Func, wrongType bool) {
   295  	m, typ := (*Checker)(nil).missingMethod(V, T, static)
   296  	return m, typ != nil
   297  }
   298  
   299  // missingMethod is like MissingMethod but accepts a *Checker as
   300  // receiver and an addressable flag.
   301  // The receiver may be nil if missingMethod is invoked through
   302  // an exported API call (such as MissingMethod), i.e., when all
   303  // methods have been type-checked.
   304  // If the type has the correctly named method, but with the wrong
   305  // signature, the existing method is returned as well.
   306  // To improve error messages, also report the wrong signature
   307  // when the method exists on *V instead of V.
   308  func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method, wrongType *Func) {
   309  	check.completeInterface(token.NoPos, T)
   310  
   311  	// fast path for common case
   312  	if T.Empty() {
   313  		return
   314  	}
   315  
   316  	if ityp := asInterface(V); ityp != nil {
   317  		check.completeInterface(token.NoPos, ityp)
   318  		// TODO(gri) allMethods is sorted - can do this more efficiently
   319  		for _, m := range T.allMethods {
   320  			_, f := lookupMethod(ityp.allMethods, m.pkg, m.name)
   321  
   322  			if f == nil {
   323  				// if m is the magic method == we're ok (interfaces are comparable)
   324  				if m.name == "==" || !static {
   325  					continue
   326  				}
   327  				return m, f
   328  			}
   329  
   330  			ftyp := f.typ.(*Signature)
   331  			mtyp := m.typ.(*Signature)
   332  			if len(ftyp.tparams) != len(mtyp.tparams) {
   333  				return m, f
   334  			}
   335  
   336  			// If the methods have type parameters we don't care whether they
   337  			// are the same or not, as long as they match up. Use unification
   338  			// to see if they can be made to match.
   339  			// TODO(gri) is this always correct? what about type bounds?
   340  			// (Alternative is to rename/subst type parameters and compare.)
   341  			u := newUnifier(check, true)
   342  			u.x.init(ftyp.tparams)
   343  			if !u.unify(ftyp, mtyp) {
   344  				return m, f
   345  			}
   346  		}
   347  
   348  		return
   349  	}
   350  
   351  	// A concrete type implements T if it implements all methods of T.
   352  	Vd, _ := deref(V)
   353  	Vn := asNamed(Vd)
   354  	for _, m := range T.allMethods {
   355  		// TODO(gri) should this be calling lookupFieldOrMethod instead (and why not)?
   356  		obj, _, _ := check.rawLookupFieldOrMethod(V, false, m.pkg, m.name)
   357  
   358  		// Check if *V implements this method of T.
   359  		if obj == nil {
   360  			ptr := NewPointer(V)
   361  			obj, _, _ = check.rawLookupFieldOrMethod(ptr, false, m.pkg, m.name)
   362  			if obj != nil {
   363  				return m, obj.(*Func)
   364  			}
   365  		}
   366  
   367  		// we must have a method (not a field of matching function type)
   368  		f, _ := obj.(*Func)
   369  		if f == nil {
   370  			// if m is the magic method == and V is comparable, we're ok
   371  			if m.name == "==" && Comparable(V) {
   372  				continue
   373  			}
   374  			return m, nil
   375  		}
   376  
   377  		// methods may not have a fully set up signature yet
   378  		if check != nil {
   379  			check.objDecl(f, nil)
   380  		}
   381  
   382  		// both methods must have the same number of type parameters
   383  		ftyp := f.typ.(*Signature)
   384  		mtyp := m.typ.(*Signature)
   385  		if len(ftyp.tparams) != len(mtyp.tparams) {
   386  			return m, f
   387  		}
   388  
   389  		// If V is a (instantiated) generic type, its methods are still
   390  		// parameterized using the original (declaration) receiver type
   391  		// parameters (subst simply copies the existing method list, it
   392  		// does not instantiate the methods).
   393  		// In order to compare the signatures, substitute the receiver
   394  		// type parameters of ftyp with V's instantiation type arguments.
   395  		// This lazily instantiates the signature of method f.
   396  		if Vn != nil && len(Vn.tparams) > 0 {
   397  			// Be careful: The number of type arguments may not match
   398  			// the number of receiver parameters. If so, an error was
   399  			// reported earlier but the length discrepancy is still
   400  			// here. Exit early in this case to prevent an assertion
   401  			// failure in makeSubstMap.
   402  			// TODO(gri) Can we avoid this check by fixing the lengths?
   403  			if len(ftyp.rparams) != len(Vn.targs) {
   404  				return
   405  			}
   406  			ftyp = check.subst(token.NoPos, ftyp, makeSubstMap(ftyp.rparams, Vn.targs)).(*Signature)
   407  		}
   408  
   409  		// If the methods have type parameters we don't care whether they
   410  		// are the same or not, as long as they match up. Use unification
   411  		// to see if they can be made to match.
   412  		// TODO(gri) is this always correct? what about type bounds?
   413  		// (Alternative is to rename/subst type parameters and compare.)
   414  		u := newUnifier(check, true)
   415  		u.x.init(ftyp.tparams)
   416  		if !u.unify(ftyp, mtyp) {
   417  			return m, f
   418  		}
   419  	}
   420  
   421  	return
   422  }
   423  
   424  // assertableTo reports whether a value of type V can be asserted to have type T.
   425  // It returns (nil, false) as affirmative answer. Otherwise it returns a missing
   426  // method required by V and whether it is missing or just has the wrong type.
   427  // The receiver may be nil if assertableTo is invoked through an exported API call
   428  // (such as AssertableTo), i.e., when all methods have been type-checked.
   429  // If the global constant forceStrict is set, assertions that are known to fail
   430  // are not permitted.
   431  func (check *Checker) assertableTo(V *Interface, T Type) (method, wrongType *Func) {
   432  	// no static check is required if T is an interface
   433  	// spec: "If T is an interface type, x.(T) asserts that the
   434  	//        dynamic type of x implements the interface T."
   435  	if asInterface(T) != nil && !forceStrict {
   436  		return
   437  	}
   438  	return check.missingMethod(T, V, false)
   439  }
   440  
   441  // deref dereferences typ if it is a *Pointer and returns its base and true.
   442  // Otherwise it returns (typ, false).
   443  func deref(typ Type) (Type, bool) {
   444  	if p, _ := typ.(*Pointer); p != nil {
   445  		return p.base, true
   446  	}
   447  	return typ, false
   448  }
   449  
   450  // derefStructPtr dereferences typ if it is a (named or unnamed) pointer to a
   451  // (named or unnamed) struct and returns its base. Otherwise it returns typ.
   452  func derefStructPtr(typ Type) Type {
   453  	if p := asPointer(typ); p != nil {
   454  		if asStruct(p.base) != nil {
   455  			return p.base
   456  		}
   457  	}
   458  	return typ
   459  }
   460  
   461  // concat returns the result of concatenating list and i.
   462  // The result does not share its underlying array with list.
   463  func concat(list []int, i int) []int {
   464  	var t []int
   465  	t = append(t, list...)
   466  	return append(t, i)
   467  }
   468  
   469  // fieldIndex returns the index for the field with matching package and name, or a value < 0.
   470  func fieldIndex(fields []*Var, pkg *Package, name string) int {
   471  	if name != "_" {
   472  		for i, f := range fields {
   473  			if f.sameId(pkg, name) {
   474  				return i
   475  			}
   476  		}
   477  	}
   478  	return -1
   479  }
   480  
   481  // lookupMethod returns the index of and method with matching package and name, or (-1, nil).
   482  func lookupMethod(methods []*Func, pkg *Package, name string) (int, *Func) {
   483  	if name != "_" {
   484  		for i, m := range methods {
   485  			if m.sameId(pkg, name) {
   486  				return i, m
   487  			}
   488  		}
   489  	}
   490  	return -1, nil
   491  }
   492  

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