typeparam.go

     1  // Copyright 2011 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 types2
     6  
     7  import "sync/atomic"
     8  
     9  // Note: This is a uint32 rather than a uint64 because the
    10  // respective 64 bit atomic instructions are not available
    11  // on all platforms.
    12  var lastID atomic.Uint32
    13  
    14  // nextID returns a value increasing monotonically by 1 with
    15  // each call, starting with 1. It may be called concurrently.
    16  func nextID() uint64 { return uint64(lastID.Add(1)) }
    17  
    18  // A TypeParam represents the type of a type parameter in a generic declaration.
    19  //
    20  // A TypeParam has a name; use the [TypeParam.Obj] method to access
    21  // its [TypeName] object.
    22  type TypeParam struct {
    23  	check *Checker  // for lazy type bound completion
    24  	id    uint64    // unique id, for debugging only
    25  	obj   *TypeName // corresponding type name
    26  	index int       // type parameter index in source order, starting at 0
    27  	bound Type      // any type, but underlying is eventually *Interface for correct programs (see TypeParam.iface)
    28  }
    29  
    30  // NewTypeParam returns a new TypeParam. Type parameters may be set on a Named
    31  // type by calling SetTypeParams. Setting a type parameter on more than one type
    32  // will result in a panic.
    33  //
    34  // The constraint argument can be nil, and set later via SetConstraint. If the
    35  // constraint is non-nil, it must be fully defined.
    36  func NewTypeParam(obj *TypeName, constraint Type) *TypeParam {
    37  	return (*Checker)(nil).newTypeParam(obj, constraint)
    38  }
    39  
    40  // check may be nil
    41  func (check *Checker) newTypeParam(obj *TypeName, constraint Type) *TypeParam {
    42  	// Always increment lastID, even if it is not used.
    43  	id := nextID()
    44  	if check != nil {
    45  		check.nextID++
    46  		id = check.nextID
    47  	}
    48  	typ := &TypeParam{check: check, id: id, obj: obj, index: -1, bound: constraint}
    49  	if obj.typ == nil {
    50  		obj.typ = typ
    51  	}
    52  	// iface may mutate typ.bound, so we must ensure that iface() is called
    53  	// at least once before the resulting TypeParam escapes.
    54  	if check != nil {
    55  		check.needsCleanup(typ)
    56  	} else if constraint != nil {
    57  		typ.iface()
    58  	}
    59  	return typ
    60  }
    61  
    62  // Obj returns the type name for the type parameter t.
    63  func (t *TypeParam) Obj() *TypeName { return t.obj }
    64  
    65  // Index returns the index of the type param within its param list, or -1 if
    66  // the type parameter has not yet been bound to a type.
    67  func (t *TypeParam) Index() int {
    68  	return t.index
    69  }
    70  
    71  // Constraint returns the type constraint specified for t.
    72  func (t *TypeParam) Constraint() Type {
    73  	return t.bound
    74  }
    75  
    76  // SetConstraint sets the type constraint for t.
    77  //
    78  // It must be called by users of NewTypeParam after the bound's underlying is
    79  // fully defined, and before using the type parameter in any way other than to
    80  // form other types. Once SetConstraint returns the receiver, t is safe for
    81  // concurrent use.
    82  func (t *TypeParam) SetConstraint(bound Type) {
    83  	if bound == nil {
    84  		panic("nil constraint")
    85  	}
    86  	t.bound = bound
    87  	// iface may mutate t.bound (if bound is not an interface), so ensure that
    88  	// this is done before returning.
    89  	t.iface()
    90  }
    91  
    92  // Underlying returns the [underlying type] of the type parameter t, which is
    93  // the underlying type of its constraint. This type is always an interface.
    94  //
    95  // [underlying type]: https://go.dev/ref/spec#Underlying_types.
    96  func (t *TypeParam) Underlying() Type {
    97  	return t.iface()
    98  }
    99  
   100  func (t *TypeParam) String() string { return TypeString(t, nil) }
   101  
   102  // ----------------------------------------------------------------------------
   103  // Implementation
   104  
   105  func (t *TypeParam) cleanup() {
   106  	t.iface()
   107  	t.check = nil
   108  }
   109  
   110  // iface returns the constraint interface of t.
   111  func (t *TypeParam) iface() *Interface {
   112  	bound := t.bound
   113  
   114  	// determine constraint interface
   115  	var ityp *Interface
   116  	switch u := under(bound).(type) {
   117  	case *Basic:
   118  		if !isValid(u) {
   119  			// error is reported elsewhere
   120  			return &emptyInterface
   121  		}
   122  	case *Interface:
   123  		if isTypeParam(bound) {
   124  			// error is reported in Checker.collectTypeParams
   125  			return &emptyInterface
   126  		}
   127  		ityp = u
   128  	}
   129  
   130  	// If we don't have an interface, wrap constraint into an implicit interface.
   131  	if ityp == nil {
   132  		ityp = NewInterfaceType(nil, []Type{bound})
   133  		ityp.implicit = true
   134  		t.bound = ityp // update t.bound for next time (optimization)
   135  	}
   136  
   137  	// compute type set if necessary
   138  	if ityp.tset == nil {
   139  		// pos is used for tracing output; start with the type parameter position.
   140  		pos := t.obj.pos
   141  		// use the (original or possibly instantiated) type bound position if we have one
   142  		if n := asNamed(bound); n != nil {
   143  			pos = n.obj.pos
   144  		}
   145  		computeInterfaceTypeSet(t.check, pos, ityp)
   146  	}
   147  
   148  	return ityp
   149  }
   150  
   151  // is calls f with the specific type terms of t's constraint and reports whether
   152  // all calls to f returned true. If there are no specific terms, is
   153  // returns the result of f(nil).
   154  func (t *TypeParam) is(f func(*term) bool) bool {
   155  	return t.iface().typeSet().is(f)
   156  }
   157  
   158  // typeset is an iterator over the (type/underlying type) pairs of the
   159  // specific type terms of t's constraint.
   160  // If there are no specific terms, typeset calls yield with (nil, nil).
   161  // In any case, typeset is guaranteed to call yield at least once.
   162  func (t *TypeParam) typeset(yield func(t, u Type) bool) {
   163  	t.iface().typeSet().typeset(yield)
   164  }
   165
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