x/tools/cmd/eg: matches should be limited to value expressions

[mdempsky]

$ cat a.go
package a
func before(x *int) int { return *x }
func after(x *int) int { return 42 }

$ cat b.go
package b
func x(p **int) {}

$ eg -t a.go b.go
=== b.go (1 matches)
package b

func x(p 42)    {}

The problem appears to be that in the type expression **int, the subexpression *int has type *int, so eg is deciding that it's a candidate match for the original expression. However, *int is a type expression, so it doesn't make sense to replace it with a value expression like 42.

I think eg needs to make use of go/types.TypeAndValue's IsValue method.

/cc @alandonovan

[mdempsky]

The problem is that eg uses go/types.(*Info).TypeOf (in (*Transformer).matchWildcard) to find the type of a candidate expression to match against, but TypeOf doesn't distinguish type vs value expressions.

Easiest fix seems to be to duplicate TypeOf into eg and augment it with the extra type vs value logic. I'm curious if this functionality is useful to other go/types consumers though.

[gopherbot]

CL https://golang.org/cl/40772 mentions this issue.