[dev.typealias] cmd/compile: a few better comments related to alias types

For #18130.

Change-Id: I50bded3af0db673fc92b20c41a86b9cae614acd9
Reviewed-on: https://go-review.googlesource.com/35191
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/syntax.go

@@ -284,7 +284,7 @@ type Param struct {
 	//
 	// TODO: Should Func pragmas also be stored on the Name?
 	Pragma Pragma
-	Alias  bool // node is alias for Ntype
+	Alias  bool // node is alias for Ntype (only used when type-checking ODCLTYPE)
 }
 
 // Func holds Node fields used only with function-like nodes.

src/cmd/compile/internal/gc/universe.go

@@ -398,6 +398,14 @@ func lexinit1() {
 	// errortype.Orig = makeErrorInterface()
 	s.Def = typenod(errortype)
 
+	// We create separate byte and rune types for better error messages
+	// rather than just creating type alias *Sym's for the uint8 and
+	// int32 types. Hence, (bytetype|runtype).Sym.isAlias() is false.
+	// TODO(gri) Should we get rid of this special case (at the cost
+	// of less informative error messages involving bytes and runes)?
+	// (Alternatively, we could introduce an OTALIAS node representing
+	// type aliases, albeit at the cost of having to deal with it everywhere).
+
 	// byte alias
 	s = Pkglookup("byte", builtinpkg)
 	bytetype = typ(TUINT8)