[dev.regabi] go/types: no "declared but not used" errors for invalid var decls

This is a port of CL 274615, adapted to go/types. The only change was in
the positioning of expected errors in vardecl.src: in go/types they are
positioned on the identifier.

Change-Id: Iab03265a7c4287749373e4380c6db6a95f262f30
Reviewed-on: https://go-review.googlesource.com/c/go/+/289712
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/assignments.go

@@ -120,6 +120,7 @@ func (check *Checker) initVar(lhs *Var, x *operand, context string) Type {
 		if lhs.typ == nil {
 			lhs.typ = Typ[Invalid]
 		}
+		lhs.used = true
 		return nil
 	}
 

src/go/types/decl.go

@@ -504,6 +504,20 @@ func (check *Checker) constDecl(obj *Const, typ, init ast.Expr, inherited bool)
 func (check *Checker) varDecl(obj *Var, lhs []*Var, typ, init ast.Expr) {
 	assert(obj.typ == nil)
 
+	// If we have undefined variable types due to errors,
+	// mark variables as used to avoid follow-on errors.
+	// Matches compiler behavior.
+	defer func() {
+		if obj.typ == Typ[Invalid] {
+			obj.used = true
+		}
+		for _, lhs := range lhs {
+			if lhs.typ == Typ[Invalid] {
+				lhs.used = true
+			}
+		}
+	}()
+
 	// determine type, if any
 	if typ != nil {
 		obj.typ = check.typ(typ)

src/go/types/testdata/vardecl.src

@@ -158,6 +158,18 @@ func _() {
 	}
 }
 
+
+// Invalid variable declarations must not lead to "declared but not used errors".
+func _() {
+	var a x        // ERROR undeclared name: x
+	var b = x      // ERROR undeclared name: x
+	var c int = x  // ERROR undeclared name: x
+	var d, e, f x  /* ERROR x */ /* ERROR x */ /* ERROR x */
+	var g, h, i = x /* ERROR x */, x /* ERROR x */, x /* ERROR x */
+	var j, k, l float32 = x /* ERROR x */, x /* ERROR x */, x /* ERROR x */
+	// but no "declared but not used" errors
+}
+
 // Invalid (unused) expressions must not lead to spurious "declared but not used errors"
 func _() {
 	var a, b, c int
@@ -203,4 +215,4 @@ func _() {
 	_, _, _ = x, y, z
 }
 
-// TODO(gri) consolidate other var decl checks in this file
+// TODO(gri) consolidate other var decl checks in this file