union.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  	"go/ast"
     9  	"go/token"
    10  )
    11  
    12  // ----------------------------------------------------------------------------
    13  // API
    14  
    15  // A Union represents a union of terms embedded in an interface.
    16  type Union struct {
    17  	terms []*Term // list of syntactical terms (not a canonicalized termlist)
    18  }
    19  
    20  // NewUnion returns a new Union type with the given terms.
    21  // It is an error to create an empty union; they are syntactically not possible.
    22  func NewUnion(terms []*Term) *Union {
    23  	if len(terms) == 0 {
    24  		panic("empty union")
    25  	}
    26  	return &Union{terms}
    27  }
    28  
    29  func (u *Union) Len() int         { return len(u.terms) }
    30  func (u *Union) Term(i int) *Term { return u.terms[i] }
    31  
    32  func (u *Union) Underlying() Type { return u }
    33  func (u *Union) String() string   { return TypeString(u, nil) }
    34  
    35  // A Term represents a term in a Union.
    36  type Term term
    37  
    38  // NewTerm returns a new union term.
    39  func NewTerm(tilde bool, typ Type) *Term { return &Term{tilde, typ} }
    40  
    41  func (t *Term) Tilde() bool    { return t.tilde }
    42  func (t *Term) Type() Type     { return t.typ }
    43  func (t *Term) String() string { return (*term)(t).String() }
    44  
    45  // ----------------------------------------------------------------------------
    46  // Implementation
    47  
    48  // Avoid excessive type-checking times due to quadratic termlist operations.
    49  const maxTermCount = 100
    50  
    51  // parseUnion parses uexpr as a union of expressions.
    52  // The result is a Union type, or Typ[Invalid] for some errors.
    53  func parseUnion(check *Checker, uexpr ast.Expr) Type {
    54  	blist, tlist := flattenUnion(nil, uexpr)
    55  	assert(len(blist) == len(tlist)-1)
    56  
    57  	var terms []*Term
    58  
    59  	var u Type
    60  	for i, x := range tlist {
    61  		term := parseTilde(check, x)
    62  		if len(tlist) == 1 && !term.tilde {
    63  			// Single type. Ok to return early because all relevant
    64  			// checks have been performed in parseTilde (no need to
    65  			// run through term validity check below).
    66  			return term.typ // typ already recorded through check.typ in parseTilde
    67  		}
    68  		if len(terms) >= maxTermCount {
    69  			if u != Typ[Invalid] {
    70  				check.errorf(x, _InvalidUnion, "cannot handle more than %d union terms (implementation limitation)", maxTermCount)
    71  				u = Typ[Invalid]
    72  			}
    73  		} else {
    74  			terms = append(terms, term)
    75  			u = &Union{terms}
    76  		}
    77  
    78  		if i > 0 {
    79  			check.recordTypeAndValue(blist[i-1], typexpr, u, nil)
    80  		}
    81  	}
    82  
    83  	if u == Typ[Invalid] {
    84  		return u
    85  	}
    86  
    87  	// Check validity of terms.
    88  	// Do this check later because it requires types to be set up.
    89  	// Note: This is a quadratic algorithm, but unions tend to be short.
    90  	check.later(func() {
    91  		for i, t := range terms {
    92  			if t.typ == Typ[Invalid] {
    93  				continue
    94  			}
    95  
    96  			u := under(t.typ)
    97  			f, _ := u.(*Interface)
    98  			if t.tilde {
    99  				if f != nil {
   100  					check.errorf(tlist[i], _InvalidUnion, "invalid use of ~ (%s is an interface)", t.typ)
   101  					continue // don't report another error for t
   102  				}
   103  
   104  				if !Identical(u, t.typ) {
   105  					check.errorf(tlist[i], _InvalidUnion, "invalid use of ~ (underlying type of %s is %s)", t.typ, u)
   106  					continue
   107  				}
   108  			}
   109  
   110  			// Stand-alone embedded interfaces are ok and are handled by the single-type case
   111  			// in the beginning. Embedded interfaces with tilde are excluded above. If we reach
   112  			// here, we must have at least two terms in the syntactic term list (but not necessarily
   113  			// in the term list of the union's type set).
   114  			if f != nil {
   115  				tset := f.typeSet()
   116  				switch {
   117  				case tset.NumMethods() != 0:
   118  					check.errorf(tlist[i], _InvalidUnion, "cannot use %s in union (%s contains methods)", t, t)
   119  				case t.typ == universeComparable.Type():
   120  					check.error(tlist[i], _InvalidUnion, "cannot use comparable in union")
   121  				case tset.comparable:
   122  					check.errorf(tlist[i], _InvalidUnion, "cannot use %s in union (%s embeds comparable)", t, t)
   123  				}
   124  				continue // terms with interface types are not subject to the no-overlap rule
   125  			}
   126  
   127  			// Report overlapping (non-disjoint) terms such as
   128  			// a|a, a|~a, ~a|~a, and ~a|A (where under(A) == a).
   129  			if j := overlappingTerm(terms[:i], t); j >= 0 {
   130  				check.softErrorf(tlist[i], _InvalidUnion, "overlapping terms %s and %s", t, terms[j])
   131  			}
   132  		}
   133  	}).describef(uexpr, "check term validity %s", uexpr)
   134  
   135  	return u
   136  }
   137  
   138  func parseTilde(check *Checker, tx ast.Expr) *Term {
   139  	x := tx
   140  	var tilde bool
   141  	if op, _ := x.(*ast.UnaryExpr); op != nil && op.Op == token.TILDE {
   142  		x = op.X
   143  		tilde = true
   144  	}
   145  	typ := check.typ(x)
   146  	// Embedding stand-alone type parameters is not permitted (issue #47127).
   147  	// We don't need this restriction anymore if we make the underlying type of a type
   148  	// parameter its constraint interface: if we embed a lone type parameter, we will
   149  	// simply use its underlying type (like we do for other named, embedded interfaces),
   150  	// and since the underlying type is an interface the embedding is well defined.
   151  	if isTypeParam(typ) {
   152  		if tilde {
   153  			check.errorf(x, _MisplacedTypeParam, "type in term %s cannot be a type parameter", tx)
   154  		} else {
   155  			check.error(x, _MisplacedTypeParam, "term cannot be a type parameter")
   156  		}
   157  		typ = Typ[Invalid]
   158  	}
   159  	term := NewTerm(tilde, typ)
   160  	if tilde {
   161  		check.recordTypeAndValue(tx, typexpr, &Union{[]*Term{term}}, nil)
   162  	}
   163  	return term
   164  }
   165  
   166  // overlappingTerm reports the index of the term x in terms which is
   167  // overlapping (not disjoint) from y. The result is < 0 if there is no
   168  // such term. The type of term y must not be an interface, and terms
   169  // with an interface type are ignored in the terms list.
   170  func overlappingTerm(terms []*Term, y *Term) int {
   171  	assert(!IsInterface(y.typ))
   172  	for i, x := range terms {
   173  		if IsInterface(x.typ) {
   174  			continue
   175  		}
   176  		// disjoint requires non-nil, non-top arguments,
   177  		// and non-interface types as term types.
   178  		if debug {
   179  			if x == nil || x.typ == nil || y == nil || y.typ == nil {
   180  				panic("empty or top union term")
   181  			}
   182  		}
   183  		if !(*term)(x).disjoint((*term)(y)) {
   184  			return i
   185  		}
   186  	}
   187  	return -1
   188  }
   189  
   190  // flattenUnion walks a union type expression of the form A | B | C | ...,
   191  // extracting both the binary exprs (blist) and leaf types (tlist).
   192  func flattenUnion(list []ast.Expr, x ast.Expr) (blist, tlist []ast.Expr) {
   193  	if o, _ := x.(*ast.BinaryExpr); o != nil && o.Op == token.OR {
   194  		blist, tlist = flattenUnion(list, o.X)
   195  		blist = append(blist, o)
   196  		x = o.Y
   197  	}
   198  	return blist, append(tlist, x)
   199  }
   200
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