cmd/compile: revert "typecheck types and funcs before consts"

This reverts commit 9ce87a63.

The fix addresses the specific test case, but not the general
problem.

Updates #24755.

Change-Id: I0ba8463b41b099b1ebf49759f88a423b40f70d58
Reviewed-on: https://go-review.googlesource.com/c/145617
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/main.go

@@ -480,7 +480,7 @@ func Main(archInit func(*Arch)) {
 
 	// Process top-level declarations in phases.
 
-	// Phase 1: type, and names and types of funcs.
+	// Phase 1: const, type, and names and types of funcs.
 	//   This will gather all the information about types
 	//   and methods but doesn't depend on any of it.
 	defercheckwidth()
@@ -489,29 +489,16 @@ func Main(archInit func(*Arch)) {
 	timings.Start("fe", "typecheck", "top1")
 	for i := 0; i < len(xtop); i++ {
 		n := xtop[i]
-		if op := n.Op; op != ODCL && op != OAS && op != OAS2 && op != ODCLCONST {
+		if op := n.Op; op != ODCL && op != OAS && op != OAS2 {
 			xtop[i] = typecheck(n, Etop)
 		}
 	}
 
-	// Phase 2: Constant declarations.
-	//   To have named types fully type checked, depends on phase 1.
-
-	// Don't use range--typecheck can add closures to xtop.
-	timings.Start("fe", "typecheck", "top2")
-	for i := 0; i < len(xtop); i++ {
-		n := xtop[i]
-		if op := n.Op; op == ODCLCONST {
-			xtop[i] = typecheck(n, Etop)
-		}
-	}
-
-	// Phase 3: Variable assignments.
+	// Phase 2: Variable assignments.
 	//   To check interface assignments, depends on phase 1.
-	//   To use constants, depends on phase 2.
 
 	// Don't use range--typecheck can add closures to xtop.
-	timings.Start("fe", "typecheck", "top3")
+	timings.Start("fe", "typecheck", "top2")
 	for i := 0; i < len(xtop); i++ {
 		n := xtop[i]
 		if op := n.Op; op == ODCL || op == OAS || op == OAS2 {

test/fixedbugs/issue24755.go

-// errorcheck
-
-// Copyright 2018 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// Tests that all types and functions are type-checked before any constant
-// declaration is. Issue #24755.
-package p
-
-type I interface{ F() }
-type T struct{}
-
-const _ = I(T{}) // ERROR "const initializer I\(T literal\) is not a constant"
-
-func (T) F() {}