go.tools/go/types: record correct type for parenthesized x.(T) in comma-ok expressions

Fixes golang/go#6796.

R=adonovan
CC=golang-dev
https://golang.org/cl/30070044

go/types/api_test.go

@@ -62,6 +62,26 @@ func TestCommaOkTypes(t *testing.T) {
 			`<-c`,
 			`(string, bool)`,
 		},
+		{`package issue6796_a; var x interface{}; var _, _ = (x.(int))`,
+			`x.(int)`,
+			`(int, bool)`,
+		},
+		{`package issue6796_b; var c chan string; var _, _ = (<-c)`,
+			`(<-c)`,
+			`(string, bool)`,
+		},
+		{`package issue6796_c; var c chan string; var _, _ = (<-c)`,
+			`<-c`,
+			`(string, bool)`,
+		},
+		{`package issue6796_d; var c chan string; var _, _ = ((<-c))`,
+			`(<-c)`,
+			`(string, bool)`,
+		},
+		{`package issue6796_e; func f(c chan string) { _, _ = ((<-c)) }`,
+			`(<-c)`,
+			`(string, bool)`,
+		},
 	}
 
 	for _, test := range tests {

go/types/check.go

@@ -124,12 +124,20 @@ func (check *checker) recordBuiltinType(f ast.Expr, sig *Signature) {
 func (check *checker) recordCommaOkTypes(x ast.Expr, t1, t2 Type) {
 	assert(x != nil && isTyped(t1) && isTyped(t2) && isBoolean(t2))
 	if m := check.Types; m != nil {
-		assert(m[x] != nil) // should have been recorded already
+		for {
-		pos := x.Pos()
+			assert(m[x] != nil) // should have been recorded already
-		m[x] = NewTuple(
+			pos := x.Pos()
-			NewVar(pos, check.pkg, "", t1),
+			m[x] = NewTuple(
-			NewVar(pos, check.pkg, "", t2),
+				NewVar(pos, check.pkg, "", t1),
-		)
+				NewVar(pos, check.pkg, "", t2),
+			)
+			// if x is a parenthesized expression (p.X), update p.X
+			p, _ := x.(*ast.ParenExpr)
+			if p == nil {
+				break
+			}
+			x = p.X
+		}
 	}
 }