go.tools/go/types: check map key type when it is fully known

Fixes golang/go#6667.

R=adonovan
CC=golang-dev
https://golang.org/cl/17360043

go/types/check.go

@@ -38,6 +38,7 @@ type checker struct {
 	conversions map[*ast.CallExpr]bool // set of type-checked conversions (to distinguish from calls)
 	untyped     map[ast.Expr]exprInfo  // map of expressions without final type
 	lhsVarsList [][]*Var               // type switch lhs variable sets, for 'declared but not used' errors
+	delayed     []func()               // delayed checks that require fully setup types
 
 	firstErr error // first error encountered
 	Info           // collected type info
@@ -66,6 +67,10 @@ func newChecker(conf *Config, fset *token.FileSet, pkg *Package) *checker {
 	}
 }
 
+func (check *checker) delay(f func()) {
+	check.delayed = append(check.delayed, f)
+}
+
 func (check *checker) recordTypeAndValue(x ast.Expr, typ Type, val exact.Value) {
 	assert(x != nil && typ != nil)
 	if m := check.Types; m != nil {
@@ -196,6 +201,12 @@ func (conf *Config) check(pkgPath string, fset *token.FileSet, files []*ast.File
 
 	check.resolveFiles(files[:i])
 
+	// perform delayed checks
+	for _, f := range check.delayed {
+		f()
+	}
+	check.delayed = nil // not needed anymore
+
 	// remaining untyped expressions must indeed be untyped
 	if debug {
 		for x, info := range check.untyped {

go/types/predicates.go

@@ -79,7 +79,9 @@ func isInterface(typ Type) bool {
 func isComparable(typ Type) bool {
 	switch t := typ.Underlying().(type) {
 	case *Basic:
-		return t.kind != Invalid && t.kind != UntypedNil
+		// assume invalid types to be comparable
+		// to avoid follow-up errors
+		return t.kind != UntypedNil
 	case *Pointer, *Interface, *Chan:
 		return true
 	case *Struct:

go/types/testdata/cycles.src

@@ -108,4 +108,12 @@ func _() {
 		// channels
 		c0 chan c0
 	)
-}
+}
+
+// test cases for issue 6667
+
+type A [10]map[A /* ERROR invalid map key */ ]bool
+
+type S struct {
+	m map[S /* ERROR invalid map key */ ]bool
+}

go/types/typexpr.go

@@ -314,11 +314,14 @@ func (check *checker) typInternal(e ast.Expr, def *Named, cycleOk bool) Type {
 		// spec: "The comparison operators == and != must be fully defined
 		// for operands of the key type; thus the key type must not be a
 		// function, map, or slice."
-		// TODO(gri) if the key type is not fully defined yet, this test will be incorrect
-		if !isComparable(typ.key) {
-			check.errorf(e.Key.Pos(), "invalid map key type %s", typ.key)
-			// ok to continue
-		}
+		//
+		// Delay this check because it requires fully setup types;
+		// it is safe to continue in any case (was issue 6667).
+		check.delay(func() {
+			if !isComparable(typ.key) {
+				check.errorf(e.Key.Pos(), "invalid map key type %s", typ.key)
+			}
+		})
 
 		return typ