[dev.typeparams] go/types: strip annotations from errors

Strip annotations from errors before emitting them. This is a partial
merge from dev.go2go: the Error.Full field is omitted for now, and
stripAnnotations is integrated with the updated error handling from
master.

Change-Id: Ia24d66b691a10d90b258b0b688d50c6b176bd629
Reviewed-on: https://go-review.googlesource.com/c/go/+/284253
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Griesemer <gri@golang.org>
Trust: Robert Findley <rfindley@google.com>

src/go/types/errors.go

@@ -89,7 +89,9 @@ func (check *Checker) err(err error) {
 		return
 	}
 
-	if check.errpos != nil && isInternal {
+	if isInternal {
+		e.Msg = stripAnnotations(e.Msg)
+		if check.errpos != nil {
 			// If we have an internal error and the errpos override is set, use it to
 			// augment our error positioning.
 			// TODO(rFindley) we may also want to augment the error message and refer
@@ -98,6 +100,7 @@ func (check *Checker) err(err error) {
 			e.Pos = span.pos
 			e.go116start = span.start
 			e.go116end = span.end
+		}
 		err = e
 	}
 
@@ -225,3 +228,18 @@ func spanOf(at positioner) posSpan {
 		return posSpan{pos, pos, pos}
 	}
 }
+
+// stripAnnotations removes internal (type) annotations from s.
+func stripAnnotations(s string) string {
+	var b strings.Builder
+	for _, r := range s {
+		// strip #'s and subscript digits
+		if r != instanceMarker && !('₀' <= r && r < '₀'+10) { // '₀' == U+2080
+			b.WriteRune(r)
+		}
+	}
+	if b.Len() < len(s) {
+		return b.String()
+	}
+	return s
+}