go.tools/go/types: const decls with invalid init expr must not panic

Make sure const objects always have a valid (possibly unknown) value.

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/106530044

go/types/assignments.go

@@ -9,8 +9,6 @@ package types
 import (
 	"go/ast"
 	"go/token"
-
-	"code.google.com/p/go.tools/go/exact"
 )
 
 // assignment reports whether x can be assigned to a variable of type T,
@@ -94,7 +92,6 @@ func (check *Checker) initConst(lhs *Const, x *operand) {
 		if x.mode != invalid {
 			check.errorf(x.pos(), "cannot define constant %s (type %s) as %s", lhs.Name(), lhs.typ, x)
 		}
-		lhs.val = exact.MakeUnknown()
 		return
 	}
 

go/types/decl.go

@@ -99,6 +99,9 @@ func (check *Checker) constDecl(obj *Const, typ, init ast.Expr) {
 	check.iota = obj.val
 	defer func() { check.iota = nil }()
 
+	// provide valid constant value under all circumstances
+	obj.val = exact.MakeUnknown()
+
 	// determine type, if any
 	if typ != nil {
 		t := check.typ(typ)

go/types/testdata/constdecl.src

@@ -82,4 +82,13 @@ func _() {
 	)
 }
 
-// TODO(gri) move extra tests from testdata/const0.src into here
+// Test case for constant with invalid initialization.
+// Caused panic because the constant value was not set up (gri - 7/8/2014).
+func _() {
+	const (
+	    x string = missing /* ERROR "undeclared name" */
+	    y = x + ""
+	)
+}
+
+// TODO(gri) move extra tests from testdata/const0.src into here