cmd/gc: fix computation of equality class of types.

A struct with a single field was considered as equivalent to the
field type, which is incorrect is the field is blank.

Fields with padding could make the compiler think some
types are comparable when they are not.

Fixes #5698.

R=rsc, golang-dev, daniel.morsing, bradfitz, gri, r
CC=golang-dev
https://golang.org/cl/10271046

1 files changed, 7 insertions, 0 deletions

diff --git a/test/blank1.go b/test/blank1.go
index 4edb2db702..f46a50051b 100644
--- a/test/blank1.go
+++ b/test/blank1.go
@@ -13,9 +13,16 @@ var t struct {
 	_ int
 }
 
+type T struct {
+      _ []int
+}
+
 func main() {
 	_()	// ERROR "cannot use _ as value"
 	x := _+1	// ERROR "cannot use _ as value"
 	_ = x
 	_ = t._ // ERROR "cannot refer to blank field"
+
+      var v1, v2 T
+      _ = v1 == v2 // ERROR "cannot be compared|non-comparable"
 }