go/types: process each segment of delayed actions in FIFO order

The stack of delayed actions is grown by pushing a new action
on top with Checker.later. Checker.processDelayed processes
all actions above a top watermark and then resets the stack
to top.

Until now, pushed actions above the watermark were processed
in LIFO order. This change processes them in FIFO order, which
seems more natural (if an action A was delayed before an action
B, A should be processed before B for that stack segment).

(With this change, Checker.later could be used instead of
Checker.atEnd to postpone interface method type comparison
and then the specific example in issue #33656 does type-check.
However, in general we want interface method type comparisons
to run after all interfaces are completed. With Checker.later
we may still end up mixing interface completions and interface
method type comparisons in ways leading to other errors for
sufficiently convoluted code.)

Also, move Checker.processDelayed from resolver.go to check.go.

Change-Id: Id31254605e6944c490eab410553fff907630cc64
Reviewed-on: https://go-review.googlesource.com/c/go/+/191458Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/go/types/check.go

@@ -269,6 +269,21 @@ func (check *Checker) checkFiles(files []*ast.File) (err error) {
 	return
 }
 
+// processDelayed processes all delayed actions pushed after top.
+func (check *Checker) processDelayed(top int) {
+	// If each delayed action pushes a new action, the
+	// stack will continue to grow during this loop.
+	// However, it is only processing functions (which
+	// are processed in a delayed fashion) that may
+	// add more actions (such as nested functions), so
+	// this is a sufficiently bounded process.
+	for i := top; i < len(check.delayed); i++ {
+		check.delayed[i]() // may append to check.delayed
+	}
+	assert(top <= len(check.delayed)) // stack must not have shrunk
+	check.delayed = check.delayed[:top]
+}
+
 func (check *Checker) processFinals() {
 	n := len(check.finals)
 	for _, f := range check.finals {

src/go/types/resolver.go

@@ -607,16 +607,6 @@ func (a inSourceOrder) Len() int           { return len(a) }
 func (a inSourceOrder) Less(i, j int) bool { return a[i].order() < a[j].order() }
 func (a inSourceOrder) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
 
-// processDelayed processes all delayed actions pushed after top.
-func (check *Checker) processDelayed(top int) {
-	for len(check.delayed) > top {
-		i := len(check.delayed) - 1
-		f := check.delayed[i]
-		check.delayed = check.delayed[:i]
-		f() // may append to check.delayed
-	}
-}
-
 // unusedImports checks for unused imports.
 func (check *Checker) unusedImports() {
 	// if function bodies are not checked, packages' uses are likely missing - don't check