go.tools/go/types: reduce spurious errors after missing identifiers

Fixes golang/go#8799.

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/143560043

go/types/assignments.go

@@ -206,14 +206,17 @@ func (check *Checker) assignVar(lhs ast.Expr, x *operand) Type {
 func (check *Checker) initVars(lhs []*Var, rhs []ast.Expr, returnPos token.Pos) {
 	l := len(lhs)
 	get, r, commaOk := unpack(func(x *operand, i int) { check.expr(x, rhs[i]) }, len(rhs), l == 2 && !returnPos.IsValid())
-	if l != r {
+	if get == nil || l != r {
 		// invalidate lhs and use rhs
 		for _, obj := range lhs {
 			if obj.typ == nil {
 				obj.typ = Typ[Invalid]
 			}
 		}
-		check.use(rhs...)
+		if get == nil {
+			return // error reported by unpack
+		}
+		check.useGetter(get, r)
 		if returnPos.IsValid() {
 			check.errorf(returnPos, "wrong number of return values (want %d, got %d)", l, r)
 			return
@@ -242,9 +245,12 @@ func (check *Checker) initVars(lhs []*Var, rhs []ast.Expr, returnPos token.Pos)
 func (check *Checker) assignVars(lhs, rhs []ast.Expr) {
 	l := len(lhs)
 	get, r, commaOk := unpack(func(x *operand, i int) { check.expr(x, rhs[i]) }, len(rhs), l == 2)
+	if get == nil {
+		return // error reported by unpack
+	}
 	if l != r {
+		check.useGetter(get, r)
 		check.errorf(rhs[0].Pos(), "assignment count mismatch (%d vs %d)", l, r)
-		check.use(rhs...)
 		return
 	}
 

go/types/builtins.go

@@ -46,6 +46,10 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 	default:
 		// make argument getter
 		arg, nargs, _ = unpack(func(x *operand, i int) { check.expr(x, call.Args[i]) }, nargs, false)
+		if arg == nil {
+			x.mode = invalid
+			return
+		}
 		// evaluate first argument, if present
 		if nargs > 0 {
 			arg(x, 0)
@@ -372,6 +376,7 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 		if T == Typ[Invalid] {
 			return
 		}
+
 		var min int // minimum number of arguments
 		switch T.Underlying().(type) {
 		case *Slice:
@@ -483,10 +488,12 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 			check.use(arg0)
 			return
 		}
+
 		check.expr(x, selx.X)
 		if x.mode == invalid {
 			return
 		}
+
 		base := derefStructPtr(x.typ)
 		sel := selx.Sel.Name
 		obj, index, indirect := LookupFieldOrMethod(base, false, check.pkg, sel)

go/types/call.go

@@ -62,6 +62,12 @@ func (check *Checker) call(x *operand, e *ast.CallExpr) exprKind {
 		}
 
 		arg, n, _ := unpack(func(x *operand, i int) { check.expr(x, e.Args[i]) }, len(e.Args), false)
+		if arg == nil {
+			x.mode = invalid
+			x.expr = e
+			return statement
+		}
+
 		check.arguments(x, e, sig, arg, n)
 
 		// determine result
@@ -92,27 +98,39 @@ func (check *Checker) use(arg ...ast.Expr) {
 	}
 }
 
+// useGetter is like use, but takes a getter instead of a list of expressions.
+// It should be called instead of use if a getter is present to avoid repeated
+// evaluation of the first argument (since the getter was likely obtained via
+// unpack, which may have evaluated the first argument already).
+func (check *Checker) useGetter(get getter, n int) {
+	var x operand
+	for i := 0; i < n; i++ {
+		get(&x, i)
+	}
+}
+
 // A getter sets x as the i'th operand, where 0 <= i < n and n is the total
 // number of operands (context-specific, and maintained elsewhere). A getter
 // type-checks the i'th operand; the details of the actual check are getter-
 // specific.
 type getter func(x *operand, i int)
 
-// unpack takes a getter get and a number of operands n. If n == 1 and the
-// first operand is a function call, or a comma,ok expression and allowCommaOk
-// is set, the result is a new getter and operand count providing access to the
-// function results, or comma,ok values, respectively. The third result value
-// reports if it is indeed the comma,ok case. In all other cases, the incoming
-// getter and operand count are returned unchanged, and the third result value
-// is false.
+// unpack takes a getter get and a number of operands n. If n == 1, unpack
+// calls the incoming getter for the first operand. If that operand is
+// invalid, unpack returns (nil, 0, false). Otherwise, if that operand is a
+// function call, or a comma-ok expression and allowCommaOk is set, the result
+// is a new getter and operand count providing access to the function results,
+// or comma-ok values, respectively. The third result value reports if it
+// is indeed the comma-ok case. In all other cases, the incoming getter and
+// operand count are returned unchanged, and the third result value is false.
 //
-// In other words, if there's exactly one operand that - after type-checking by
-// calling get - stands for multiple operands, the resulting getter provides access
-// to those operands instead.
+// In other words, if there's exactly one operand that - after type-checking
+// by calling get - stands for multiple operands, the resulting getter provides
+// access to those operands instead.
 //
-// Note that unpack may call get(..., 0); but if the result getter is called
-// at most once for a given operand index i (including i == 0), that operand
-// is guaranteed to cause only one call of the incoming getter with that i.
+// If the returned getter is called at most once for a given operand index i
+// (including i == 0), that operand is guaranteed to cause only one call of
+// the incoming getter with that i.
 //
 func unpack(get getter, n int, allowCommaOk bool) (getter, int, bool) {
 	if n == 1 {
@@ -120,12 +138,7 @@ func unpack(get getter, n int, allowCommaOk bool) (getter, int, bool) {
 		var x0 operand
 		get(&x0, 0)
 		if x0.mode == invalid {
-			return func(x *operand, i int) {
-				if i != 0 {
-					unreachable()
-				}
-				x.mode = invalid
-			}, 1, false
+			return nil, 0, false
 		}
 
 		if t, ok := x0.typ.(*Tuple); ok {

go/types/testdata/expr0.src

@@ -116,10 +116,10 @@ var (
 	s2 = -s0 /* ERROR "not defined" */
 	s3 = !s0 /* ERROR "not defined" */
 	s4 = ^s0 /* ERROR "not defined" */
-	s5 = *s4 /* ERROR "cannot indirect" */
+	s5 = *s4
 	s6 = &s4
 	s7 = *s6
-	s8 = <-s7  /* ERROR "cannot receive" */
+	s8 = <-s7
 
 	// channel
 	ch chan int

go/types/testdata/expr3.src

@@ -424,7 +424,7 @@ func _calls() {
 	f2(3.14) /* ERROR "too few arguments" */
 	f2(3.14, "foo")
 	f2(x /* ERROR "cannot pass" */ , "foo")
-	f2(g0 /* ERROR "used as value" */ ()) /* ERROR "too few arguments" */
+	f2(g0 /* ERROR "used as value" */ ())
 	f2(g1 /* ERROR "cannot pass" */ ()) /* ERROR "too few arguments" */
 	f2(g2())
 

go/types/testdata/issues.src

@@ -22,3 +22,16 @@ func issue8066() {
 		_ = float32(340282356779733661637539395458142568448 /* ERROR cannot convert */ )
 	)
 }
+
+// Check that a missing identifier doesn't lead to a spurious error cascade.
+func issue8799a() {
+	x, ok := missing /* ERROR undeclared */ ()
+	_ = !ok
+	_ = x
+}
+
+func issue8799b(x int, ok bool) {
+	x, ok = missing /* ERROR undeclared */ ()
+	_ = !ok
+	_ = x
+}

go/types/typexpr.go

@@ -88,6 +88,9 @@ func (check *Checker) ident(x *operand, e *ast.Ident, def *Named, path []*TypeNa
 	case *Var:
 		obj.used = true
 		check.addDeclDep(obj)
+		if typ == Typ[Invalid] {
+			return
+		}
 		x.mode = variable
 
 	case *Func: