go/types: fix internal documentation and strengthen assumptions

Imported (incl. dot-imported) objects are always in file scope,
never in package scope. Fix misleading comment.

Package-scope declared objects must have objMap entry by
construction (of that map). Remove unnecessary check and
corresponding misleading comment.

Found while investigating solutions for @23203.

Change-Id: Idadfdd1576681ae56e11daa27bdcee2c73733c41
Reviewed-on: https://go-review.googlesource.com/110916
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/resolver.go

@@ -479,8 +479,8 @@ func (check *Checker) collectObjects() {
 func (check *Checker) resolveBaseTypeName(name *ast.Ident) *TypeName {
 	var path []*TypeName
 	for {
-		// name must denote an object found in the current package
-		// (it could be explicitly declared or dot-imported)
+		// name must denote an object found in the current package scope
+		// (note that dot-imported objects are not in the package scope!)
 		obj := check.pkg.scope.Lookup(name.Name)
 		if obj == nil {
 			return nil
@@ -496,15 +496,9 @@ func (check *Checker) resolveBaseTypeName(name *ast.Ident) *TypeName {
 			return nil
 		}
 
-		// tname must have been explicitly declared
-		// (dot-imported objects are not in objMap)
-		tdecl := check.objMap[tname]
-		if tdecl == nil {
-			return nil
-		}
-
 		// we're done if tdecl defined tname as a new type
 		// (rather than an alias)
+		tdecl := check.objMap[tname] // must exist for objects in package scope
 		if !tdecl.alias {
 			return tname
 		}