go/types: revert "no 'declared but not used' errors for invalid var decls"

This reverts commit CL 289712 (afd67f3). It breaks x/tools tests, and
those tests highlight that perhaps I didn't think through the
repercussions of this change as much as I should have.

Fixes #44316

Change-Id: I5db39b4e2a3714131aa22423abfe0f34a0376192
Reviewed-on: https://go-review.googlesource.com/c/go/+/292751
Reviewed-by: Matthew Dempsky <[email protected]>
Trust: Robert Findley <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: findleyr

src/go/types/assignments.go

@@ -120,7 +120,6 @@ 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

@@ -504,20 +504,6 @@ 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

@@ -158,18 +158,6 @@ 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
@@ -215,4 +203,4 @@ func _() {
 	_, _, _ = x, y, z
 }
 
-// TODO(gri) consolidate other var decl checks in this file
+// TODO(gri) consolidate other var decl checks in this file