go.tools/go/types: don't crash dealing with unknown constants

Also: Better tracing for top-level declarations.

Fixes golang/go#8518.

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/132310043

go/types/api_test.go

@@ -647,7 +647,7 @@ func TestSelection(t *testing.T) {
 		conf.Packages[path] = pkg
 	}
 
-	libSrc := `
+	const libSrc = `
 package lib
 type T float64
 const C T = 3
@@ -655,7 +655,7 @@ var V T
 func F() {}
 func (T) M() {}
 `
-	mainSrc := `
+	const mainSrc = `
 package main
 import "lib"
 
@@ -791,3 +791,39 @@ func main() {
 		}
 	}
 }
+
+func TestIssue8518(t *testing.T) {
+	fset := token.NewFileSet()
+	conf := Config{
+		Packages: make(map[string]*Package),
+		Error:    func(err error) { t.Log(err) }, // don't exit after first error
+		Import: func(imports map[string]*Package, path string) (*Package, error) {
+			return imports[path], nil
+		},
+	}
+	makePkg := func(path, src string) {
+		f, err := parser.ParseFile(fset, path, src, 0)
+		if err != nil {
+			t.Fatal(err)
+		}
+		pkg, _ := conf.Check(path, fset, []*ast.File{f}, nil) // errors logged via conf.Error
+		conf.Packages[path] = pkg
+	}
+
+	const libSrc = `
+package a 
+import "missing"
+const C1 = foo
+const C2 = missing.C
+`
+
+	const mainSrc = `
+package main
+import "a"
+var _ = a.C1
+var _ = a.C2
+`
+
+	makePkg("a", libSrc)
+	makePkg("main", mainSrc) // don't crash when type-checking this package
+}

go/types/check.go

@@ -253,7 +253,7 @@ func (check *Checker) recordTypeAndValue(x ast.Expr, mode operandMode, typ Type,
 	assert(typ != nil)
 	if mode == constant {
 		assert(val != nil)
-		assert(isConstType(typ))
+		assert(typ == Typ[Invalid] || isConstType(typ))
 	}
 	if m := check.Types; m != nil {
 		m[x] = TypeAndValue{mode, typ, val}

go/types/decl.go

@@ -45,7 +45,12 @@ func (check *Checker) objDecl(obj Object, def *Named, path []*TypeName) {
 	}
 
 	if trace {
-		check.trace(obj.Pos(), "-- resolving %s", obj.Name())
+		check.trace(obj.Pos(), "-- declaring %s", obj.Name())
+		check.indent++
+		defer func() {
+			check.indent--
+			check.trace(obj.Pos(), "=> %s", obj)
+		}()
 	}
 
 	d := check.objMap[obj]