go/types: document that selectors are not recorded in Info.Types

Fixes #11944.

Change-Id: I424ba93725f22fd599e052eb182f9ba2fca8e8bd
Reviewed-on: https://go-review.googlesource.com/32881Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/api.go

@@ -135,7 +135,8 @@ type Config struct {
 // be incomplete.
 type Info struct {
 	// Types maps expressions to their types, and for constant
-	// expressions, their values. Invalid expressions are omitted.
+	// expressions, also their values. Invalid expressions are
+	// omitted.
 	//
 	// For (possibly parenthesized) identifiers denoting built-in
 	// functions, the recorded signatures are call-site specific:
@@ -143,9 +144,13 @@ type Info struct {
 	// an argument-specific signature. Otherwise, the recorded type
 	// is invalid.
 	//
-	// Identifiers on the lhs of declarations (i.e., the identifiers
-	// which are being declared) are collected in the Defs map.
-	// Identifiers denoting packages are collected in the Uses maps.
+	// The Types map does not record the type of every identifier,
+	// only those that appear where an arbitrary expression is
+	// permitted. For instance, the identifier f in a selector
+	// expression x.f is found only in the Selections map, the
+	// identifier z in a variable declaration 'var z int' is found
+	// only in the Defs map, and identifiers denoting packages in
+	// qualified identifiers are collected in the Uses map.
 	Types map[ast.Expr]TypeAndValue
 
 	// Defs maps identifiers to the objects they define (including

src/go/types/check.go

@@ -345,7 +345,6 @@ func (check *Checker) recordImplicit(node ast.Node, obj Object) {
 func (check *Checker) recordSelection(x *ast.SelectorExpr, kind SelectionKind, recv Type, obj Object, index []int, indirect bool) {
 	assert(obj != nil && (recv == nil || len(index) > 0))
 	check.recordUse(x.Sel, obj)
-	// TODO(gri) Should we also call recordTypeAndValue?
 	if m := check.Selections; m != nil {
 		m[x] = &Selection{kind, recv, obj, index, indirect}
 	}