From 5da7e5a1a8fb12a9a131434e45af1ebfbb44f4d2 Mon Sep 17 00:00:00 2001
From: Alan Donovan <adonovan@google.com>
Date: Mon, 8 Jul 2013 18:02:50 -0400
Subject: [PATCH] go.tools/ssa: give consistent positions for
 CanonicalPos(Literal).

(There are only seven ast.Expr types that can be constant.)

R=gri
CC=golang-dev
https://golang.org/cl/10766044
---
 ssa/source.go | 19 ++++++++++---------
 ssa/ssa.go    |  3 ++-
 2 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/ssa/source.go b/ssa/source.go
index 7f09b92d..09daf616 100644
--- a/ssa/source.go
+++ b/ssa/source.go
@@ -169,21 +169,24 @@ func CanonicalPos(n ast.Node) token.Pos {
 		return CanonicalPos(n.X)
 
 	case *ast.CallExpr:
-		// f(x):    *Call, *Go, *Defer.
-		// T(x):    *ChangeType, *Convert, *MakeInterface, *ChangeInterface.
+		// f(x):    *Call, *Go, *Defer, *Literal (e.g. len)
+		// T(x):    *ChangeType, *Convert, *MakeInterface, *ChangeInterface, *Literal.
 		// make():  *MakeMap, *MakeChan, *MakeSlice.
 		// new():   *Alloc.
 		// panic(): *Panic.
 		return n.Lparen
 
+	case *ast.BasicLit:
+		return n.ValuePos // *Literal
+
 	case *ast.Ident:
-		return n.NamePos // *Parameter, *Alloc, *Capture
+		return n.NamePos // *Parameter, *Alloc, *Capture, *Literal
 
 	case *ast.TypeAssertExpr:
 		return n.Lparen // *ChangeInterface or *TypeAssertExpr
 
 	case *ast.SelectorExpr:
-		return n.Sel.NamePos // *MakeClosure, *Field or *FieldAddr
+		return n.Sel.NamePos // *MakeClosure, *Field, *FieldAddr, *Literal
 
 	case *ast.FuncLit:
 		return n.Type.Func // *Function or *MakeClosure
@@ -192,10 +195,10 @@ func CanonicalPos(n ast.Node) token.Pos {
 		return n.Lbrace // *Alloc or *Slice
 
 	case *ast.BinaryExpr:
-		return n.OpPos // *Phi or *BinOp
+		return n.OpPos // *Phi, *BinOp or *Literal
 
 	case *ast.UnaryExpr:
-		return n.OpPos // *Phi or *UnOp
+		return n.OpPos // *Phi, *UnOp, or *Literal
 
 	case *ast.IndexExpr:
 		return n.Lbrack // *Index or *IndexAddr
@@ -222,7 +225,5 @@ func CanonicalPos(n ast.Node) token.Pos {
 		return n.Colon // *MapUpdate
 	}
 
-	// Almost anything can be a constant expression (*Literal).
-	// TODO(adonovan): actually that's not so; audit and restrict.
-	return n.Pos()
+	return token.NoPos
 }
diff --git a/ssa/ssa.go b/ssa/ssa.go
index 37693363..0ffd166f 100644
--- a/ssa/ssa.go
+++ b/ssa/ssa.go
@@ -378,7 +378,8 @@ type Parameter struct {
 // constants.
 //
 // Pos() returns the canonical position (see CanonicalPos) of the
-// originating constant expression, if explicit in the source.
+// originating constant expression (ast.Ident or ast.BasicLit, or
+// ast.{Call,Selector,Unary,Binary}Expr), if explicit in the source.
 //
 // Example printed form:
 // 	42:int