go/parser: fails to parse an invalid switch statement

[stamblerre]

Repro case:

package main

import "go/ast"

func main() {
	var node ast.Node
	var lit *ast.CompositeLit
	switch n := node.(type) {
	case *ast.CompositeLit:
		lit = n
                lit.
	case *ast.KeyValueExpr:
	}
}

In this case, lit. case *ast.KeyValueExpr: gets parsed into an *ast.BadStmt. This is a problem for tools that need to work on broken code.

[stamblerre]

/cc @ianthehat

[mvdan]

Could be interesting to see what cmd/compile/internal/syntax does here. As far as I've seen, it does a better job with edge cases and errors. Though it doesn't seem to have a BadStmt equivalent right now.