range.go

     1  // Copyright 2025 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 typechecking of range statements.
     6  
     7  package types2
     8  
     9  import (
    10  	"cmd/compile/internal/syntax"
    11  	"go/constant"
    12  	. "internal/types/errors"
    13  )
    14  
    15  // rangeStmt type-checks a range statement of form
    16  //
    17  //	for sKey, sValue = range rangeVar { ... }
    18  //
    19  // where sKey, sValue, sExtra may be nil. isDef indicates whether these
    20  // variables are assigned to only (=) or whether there is a short variable
    21  // declaration (:=). If the latter and there are no variables, an error is
    22  // reported at noNewVarPos.
    23  func (check *Checker) rangeStmt(inner stmtContext, rangeStmt *syntax.ForStmt, noNewVarPos poser, sKey, sValue, sExtra, rangeVar syntax.Expr, isDef bool) {
    24  	// check expression to iterate over
    25  	var x operand
    26  
    27  	// From the spec:
    28  	//   The range expression x is evaluated before beginning the loop,
    29  	//   with one exception: if at most one iteration variable is present
    30  	//   and x or len(x) is constant, the range expression is not evaluated.
    31  	// So we have to be careful not to evaluate the arg in the
    32  	// described situation.
    33  
    34  	check.hasCallOrRecv = false
    35  	check.expr(nil, &x, rangeVar)
    36  
    37  	if isTypes2 && x.mode != invalid && sValue == nil && !check.hasCallOrRecv {
    38  		if t, ok := arrayPtrDeref(under(x.typ)).(*Array); ok {
    39  			for {
    40  				// Put constant info on the thing inside parentheses.
    41  				// That's where (*../noder/writer).expr expects it.
    42  				// See issue 73476.
    43  				p, ok := rangeVar.(*syntax.ParenExpr)
    44  				if !ok {
    45  					break
    46  				}
    47  				rangeVar = p.X
    48  			}
    49  			// Override type of rangeVar to be a constant
    50  			// (and thus side-effects will not be computed
    51  			// by the backend).
    52  			check.record(&operand{
    53  				mode: constant_,
    54  				expr: rangeVar,
    55  				typ:  Typ[Int],
    56  				val:  constant.MakeInt64(t.len),
    57  				id:   x.id,
    58  			})
    59  		}
    60  	}
    61  
    62  	// determine key/value types
    63  	var key, val Type
    64  	if x.mode != invalid {
    65  		k, v, cause, ok := rangeKeyVal(check, x.typ, func(v goVersion) bool {
    66  			return check.allowVersion(v)
    67  		})
    68  		switch {
    69  		case !ok && cause != "":
    70  			check.softErrorf(&x, InvalidRangeExpr, "cannot range over %s: %s", &x, cause)
    71  		case !ok:
    72  			check.softErrorf(&x, InvalidRangeExpr, "cannot range over %s", &x)
    73  		case k == nil && sKey != nil:
    74  			check.softErrorf(sKey, InvalidIterVar, "range over %s permits no iteration variables", &x)
    75  		case v == nil && sValue != nil:
    76  			check.softErrorf(sValue, InvalidIterVar, "range over %s permits only one iteration variable", &x)
    77  		case sExtra != nil:
    78  			check.softErrorf(sExtra, InvalidIterVar, "range clause permits at most two iteration variables")
    79  		}
    80  		key, val = k, v
    81  	}
    82  
    83  	// Open the for-statement block scope now, after the range clause.
    84  	// Iteration variables declared with := need to go in this scope (was go.dev/issue/51437).
    85  	check.openScope(rangeStmt, "range")
    86  	defer check.closeScope()
    87  
    88  	// check assignment to/declaration of iteration variables
    89  	// (irregular assignment, cannot easily map to existing assignment checks)
    90  
    91  	// lhs expressions and initialization value (rhs) types
    92  	lhs := [2]syntax.Expr{sKey, sValue} // sKey, sValue may be nil
    93  	rhs := [2]Type{key, val}            // key, val may be nil
    94  
    95  	rangeOverInt := isInteger(x.typ)
    96  
    97  	if isDef {
    98  		// short variable declaration
    99  		var vars []*Var
   100  		for i, lhs := range lhs {
   101  			if lhs == nil {
   102  				continue
   103  			}
   104  
   105  			// determine lhs variable
   106  			var obj *Var
   107  			if ident, _ := lhs.(*syntax.Name); ident != nil {
   108  				// declare new variable
   109  				name := ident.Value
   110  				obj = newVar(LocalVar, ident.Pos(), check.pkg, name, nil)
   111  				check.recordDef(ident, obj)
   112  				// _ variables don't count as new variables
   113  				if name != "_" {
   114  					vars = append(vars, obj)
   115  				}
   116  			} else {
   117  				check.errorf(lhs, InvalidSyntaxTree, "cannot declare %s", lhs)
   118  				obj = newVar(LocalVar, lhs.Pos(), check.pkg, "_", nil) // dummy variable
   119  			}
   120  			assert(obj.typ == nil)
   121  
   122  			// initialize lhs iteration variable, if any
   123  			typ := rhs[i]
   124  			if typ == nil || typ == Typ[Invalid] {
   125  				// typ == Typ[Invalid] can happen if allowVersion fails.
   126  				obj.typ = Typ[Invalid]
   127  				check.usedVars[obj] = true // don't complain about unused variable
   128  				continue
   129  			}
   130  
   131  			if rangeOverInt {
   132  				assert(i == 0) // at most one iteration variable (rhs[1] == nil or Typ[Invalid] for rangeOverInt)
   133  				check.initVar(obj, &x, "range clause")
   134  			} else {
   135  				var y operand
   136  				y.mode = value
   137  				y.expr = lhs // we don't have a better rhs expression to use here
   138  				y.typ = typ
   139  				check.initVar(obj, &y, "assignment") // error is on variable, use "assignment" not "range clause"
   140  			}
   141  			assert(obj.typ != nil)
   142  		}
   143  
   144  		// declare variables
   145  		if len(vars) > 0 {
   146  			scopePos := rangeStmt.Body.Pos()
   147  			for _, obj := range vars {
   148  				check.declare(check.scope, nil /* recordDef already called */, obj, scopePos)
   149  			}
   150  		} else {
   151  			check.error(noNewVarPos, NoNewVar, "no new variables on left side of :=")
   152  		}
   153  	} else if sKey != nil /* lhs[0] != nil */ {
   154  		// ordinary assignment
   155  		for i, lhs := range lhs {
   156  			if lhs == nil {
   157  				continue
   158  			}
   159  
   160  			// assign to lhs iteration variable, if any
   161  			typ := rhs[i]
   162  			if typ == nil || typ == Typ[Invalid] {
   163  				continue
   164  			}
   165  
   166  			if rangeOverInt {
   167  				assert(i == 0) // at most one iteration variable (rhs[1] == nil or Typ[Invalid] for rangeOverInt)
   168  				check.assignVar(lhs, nil, &x, "range clause")
   169  				// If the assignment succeeded, if x was untyped before, it now
   170  				// has a type inferred via the assignment. It must be an integer.
   171  				// (go.dev/issues/67027)
   172  				if x.mode != invalid && !isInteger(x.typ) {
   173  					check.softErrorf(lhs, InvalidRangeExpr, "cannot use iteration variable of type %s", x.typ)
   174  				}
   175  			} else {
   176  				var y operand
   177  				y.mode = value
   178  				y.expr = lhs // we don't have a better rhs expression to use here
   179  				y.typ = typ
   180  				check.assignVar(lhs, nil, &y, "assignment") // error is on variable, use "assignment" not "range clause"
   181  			}
   182  		}
   183  	} else if rangeOverInt {
   184  		// If we don't have any iteration variables, we still need to
   185  		// check that a (possibly untyped) integer range expression x
   186  		// is valid.
   187  		// We do this by checking the assignment _ = x. This ensures
   188  		// that an untyped x can be converted to a value of its default
   189  		// type (rune or int).
   190  		check.assignment(&x, nil, "range clause")
   191  	}
   192  
   193  	check.stmt(inner, rangeStmt.Body)
   194  }
   195  
   196  // rangeKeyVal returns the key and value type produced by a range clause
   197  // over an expression of type orig.
   198  // If allowVersion != nil, it is used to check the required language version.
   199  // If the range clause is not permitted, rangeKeyVal returns ok = false.
   200  // When ok = false, rangeKeyVal may also return a reason in cause.
   201  // The check parameter is only used in case of an error; it may be nil.
   202  func rangeKeyVal(check *Checker, orig Type, allowVersion func(goVersion) bool) (key, val Type, cause string, ok bool) {
   203  	bad := func(cause string) (Type, Type, string, bool) {
   204  		return Typ[Invalid], Typ[Invalid], cause, false
   205  	}
   206  
   207  	rtyp, err := commonUnder(orig, func(t, u Type) *typeError {
   208  		// A channel must permit receive operations.
   209  		if ch, _ := u.(*Chan); ch != nil && ch.dir == SendOnly {
   210  			return typeErrorf("receive from send-only channel %s", t)
   211  		}
   212  		return nil
   213  	})
   214  	if rtyp == nil {
   215  		return bad(err.format(check))
   216  	}
   217  
   218  	switch typ := arrayPtrDeref(rtyp).(type) {
   219  	case *Basic:
   220  		if isString(typ) {
   221  			return Typ[Int], universeRune, "", true // use 'rune' name
   222  		}
   223  		if isInteger(typ) {
   224  			if allowVersion != nil && !allowVersion(go1_22) {
   225  				return bad("requires go1.22 or later")
   226  			}
   227  			return orig, nil, "", true
   228  		}
   229  	case *Array:
   230  		return Typ[Int], typ.elem, "", true
   231  	case *Slice:
   232  		return Typ[Int], typ.elem, "", true
   233  	case *Map:
   234  		return typ.key, typ.elem, "", true
   235  	case *Chan:
   236  		assert(typ.dir != SendOnly)
   237  		return typ.elem, nil, "", true
   238  	case *Signature:
   239  		if allowVersion != nil && !allowVersion(go1_23) {
   240  			return bad("requires go1.23 or later")
   241  		}
   242  		// check iterator arity
   243  		switch {
   244  		case typ.Params().Len() != 1:
   245  			return bad("func must be func(yield func(...) bool): wrong argument count")
   246  		case typ.Results().Len() != 0:
   247  			return bad("func must be func(yield func(...) bool): unexpected results")
   248  		}
   249  		assert(typ.Recv() == nil)
   250  		// check iterator argument type
   251  		u, err := commonUnder(typ.Params().At(0).Type(), nil)
   252  		cb, _ := u.(*Signature)
   253  		switch {
   254  		case cb == nil:
   255  			if err != nil {
   256  				return bad(check.sprintf("func must be func(yield func(...) bool): in yield type, %s", err.format(check)))
   257  			} else {
   258  				return bad("func must be func(yield func(...) bool): argument is not func")
   259  			}
   260  		case cb.Params().Len() > 2:
   261  			return bad("func must be func(yield func(...) bool): yield func has too many parameters")
   262  		case cb.Results().Len() != 1 || !Identical(cb.Results().At(0).Type(), universeBool):
   263  			// see go.dev/issues/71131, go.dev/issues/71164
   264  			if cb.Results().Len() == 1 && isBoolean(cb.Results().At(0).Type()) {
   265  				return bad("func must be func(yield func(...) bool): yield func returns user-defined boolean, not bool")
   266  			} else {
   267  				return bad("func must be func(yield func(...) bool): yield func does not return bool")
   268  			}
   269  		}
   270  		assert(cb.Recv() == nil)
   271  		// determine key and value types, if any
   272  		if cb.Params().Len() >= 1 {
   273  			key = cb.Params().At(0).Type()
   274  		}
   275  		if cb.Params().Len() >= 2 {
   276  			val = cb.Params().At(1).Type()
   277  		}
   278  		return key, val, "", true
   279  	}
   280  	return
   281  }
   282
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