go.tools/go/types: avoid spurious "declared but not used" errors in case of other errors

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/76810044

go/types/assignments.go

@@ -210,12 +210,13 @@ 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 {
-		// invalidate lhs
+		// invalidate lhs and use rhs
 		for _, obj := range lhs {
 			if obj.typ == nil {
 				obj.typ = Typ[Invalid]
 			}
 		}
+		check.use(rhs...)
 		if returnPos.IsValid() {
 			check.errorf(returnPos, "wrong number of return values (want %d, got %d)", l, r)
 			return
@@ -246,6 +247,7 @@ func (check *checker) assignVars(lhs, rhs []ast.Expr) {
 	get, r, commaOk := unpack(func(x *operand, i int) { check.expr(x, rhs[i]) }, len(rhs), l == 2)
 	if l != r {
 		check.errorf(rhs[0].Pos(), "assignment count mismatch (%d vs %d)", l, r)
+		check.use(rhs...)
 		return
 	}
 

go/types/builtins.go

@@ -23,7 +23,7 @@ func (check *checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 	bin := predeclaredFuncs[id]
 	if call.Ellipsis.IsValid() && id != _Append {
 		check.invalidOp(call.Ellipsis, "invalid use of ... with built-in %s", bin.name)
-		check.use(call.Args)
+		check.use(call.Args...)
 		return
 	}
 
@@ -481,7 +481,7 @@ func (check *checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 		selx, _ := unparen(arg0).(*ast.SelectorExpr)
 		if selx == nil {
 			check.invalidArg(arg0.Pos(), "%s is not a selector expression", arg0)
-			check.rawExpr(x, arg0, nil) // evaluate to avoid spurious "declared but not used" errors
+			check.use(arg0)
 			return
 		}
 		check.expr(x, selx.X)

go/types/call.go

@@ -16,7 +16,7 @@ func (check *checker) call(x *operand, e *ast.CallExpr) exprKind {
 
 	switch x.mode {
 	case invalid:
-		check.use(e.Args)
+		check.use(e.Args...)
 		x.mode = invalid
 		x.expr = e
 		return statement
@@ -82,12 +82,12 @@ func (check *checker) call(x *operand, e *ast.CallExpr) exprKind {
 	}
 }
 
-// use type-checks each list element.
+// use type-checks each argument.
-// Useful to make sure a list of expressions is evaluated
+// Useful to make sure expressions are evaluated
 // (and variables are "used") in the presence of other errors.
-func (check *checker) use(list []ast.Expr) {
+func (check *checker) use(arg ...ast.Expr) {
 	var x operand
-	for _, e := range list {
+	for _, e := range arg {
 		check.rawExpr(&x, e, nil)
 	}
 }

go/types/resolver.go

@@ -57,9 +57,11 @@ func (check *checker) arityMatch(s, init *ast.ValueSpec) {
 			// init exprs from s
 			n := s.Values[l]
 			check.errorf(n.Pos(), "extra init expr %s", n)
+			// TODO(gri) avoid declared but not used error here
 		} else {
 			// init exprs "inherited"
 			check.errorf(s.Pos(), "extra init expr at %s", init.Pos())
+			// TODO(gri) avoid declared but not used error here
 		}
 	case l > r && (init != nil || r != 1):
 		n := s.Names[r]

go/types/stmt.go

@@ -341,6 +341,7 @@ func (check *checker) stmt(ctxt stmtContext, s ast.Stmt) {
 			}
 		} else if len(s.Results) > 0 {
 			check.error(s.Results[0].Pos(), "no result values expected")
+			check.use(s.Results...)
 		}
 
 	case *ast.BranchStmt:

go/types/testdata/vardecl.src

@@ -5,6 +5,7 @@
 package vardecl
 
 // Prerequisites.
+import "math"
 func f() {}
 func g() (x, y int) { return }
 var m map[string]int
@@ -147,6 +148,26 @@ func (r T) _(a, b, c int) (u, v, w int) {
 	return
 }
 
+// Invalid (unused) expressions must not lead to spurious "declared but not used errors"
+func _() {
+	var a, b, c int
+	var x, y int
+	x, y = a /* ERROR assignment count mismatch */ , b, c
+	_ = x
+	_ = y
+}
+
+func _() {
+	var x int
+	return x /* ERROR no result values expected */
+	return math /* ERROR no result values expected */ .Sin(0)
+}
+
+func _() int {
+	var x, y int
+	return /* ERROR wrong number of return values */ x, y
+}
+
 // Short variable declarations must declare at least one new non-blank variable.
 func _() {
 	_ := /* ERROR no new variables */ 0