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