[dev.typeparams] cmd/compile/internal/types2: no "declared but not used" errors for invalid var decls

Matches compiler behavior.

Change-Id: I87ca46fb7269fbac61ffbf8ed48902156b06f6e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/274615
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/assignments.go

@@ -112,6 +112,7 @@ func (check *Checker) initVar(lhs *Var, x *operand, context string) Type {
 		if lhs.typ == nil {
 			lhs.typ = Typ[Invalid]
 		}
+		lhs.used = true // avoid follow-on "declared but not used" errors
 		return nil
 	}
 

src/cmd/compile/internal/types2/decl.go

@@ -457,6 +457,20 @@ func (check *Checker) constDecl(obj *Const, typ, init syntax.Expr) {
 func (check *Checker) varDecl(obj *Var, lhs []*Var, typ, init syntax.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.varType(typ)

src/cmd/compile/internal/types2/testdata/vardecl.src

@@ -155,7 +155,18 @@ func _() {
 	}
 }
 
-// Invalid (unused) expressions must not lead to spurious "declared but not used errors"
+// 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, x, x          /* ERROR x */ /* ERROR x */ /* ERROR x */
+	var j, k, l float32 = x, x, x  /* ERROR x */ /* ERROR 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
 	var x, y int