cmd/compile/internal/types2: use correct value of iota

Fixes #52438.

Change-Id: I5cbf8c448dba037e9e0c5fe8f209401d6bf7d43f
Reviewed-on: https://go-review.googlesource.com/c/go/+/401134
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

src/cmd/compile/internal/types2/decl.go

@@ -735,7 +735,7 @@ func (check *Checker) declStmt(list []syntax.Decl) {
 			top := len(check.delayed)
 
 			// iota is the index of the current constDecl within the group
-			if first < 0 || list[index-1].(*syntax.ConstDecl).Group != s.Group {
+			if first < 0 || s.Group == nil || list[index-1].(*syntax.ConstDecl).Group != s.Group {
 				first = index
 				last = nil
 			}

src/cmd/compile/internal/types2/resolver.go

@@ -340,7 +340,7 @@ func (check *Checker) collectObjects() {
 
 			case *syntax.ConstDecl:
 				// iota is the index of the current constDecl within the group
-				if first < 0 || file.DeclList[index-1].(*syntax.ConstDecl).Group != s.Group {
+				if first < 0 || s.Group == nil || file.DeclList[index-1].(*syntax.ConstDecl).Group != s.Group {
 					first = index
 					last = nil
 				}

src/cmd/compile/internal/types2/testdata/check/const0.go

@@ -349,6 +349,25 @@ const _ = unsafe.Sizeof(func() {
 	assert(iota == 0)
 })
 
+// issue #52438
+const i1 = iota
+const i2 = iota
+const i3 = iota
+
+func _() {
+	assert(i1 == 0)
+	assert(i2 == 0)
+	assert(i3 == 0)
+
+	const i4 = iota
+	const i5 = iota
+	const i6 = iota
+
+	assert(i4 == 0)
+	assert(i5 == 0)
+	assert(i6 == 0)
+}
+
 // untyped constants must not get arbitrarily large
 const prec = 512 // internal maximum precision for integers
 const maxInt = (1<<(prec/2) - 1) * (1<<(prec/2) + 1) // == 1<<prec - 1

src/go/types/testdata/check/const0.go

@@ -349,6 +349,25 @@ const _ = unsafe.Sizeof(func() {
 	assert(iota == 0)
 })
 
+// issue #52438
+const i1 = iota
+const i2 = iota
+const i3 = iota
+
+func _() {
+	assert(i1 == 0)
+	assert(i2 == 0)
+	assert(i3 == 0)
+
+	const i4 = iota
+	const i5 = iota
+	const i6 = iota
+
+	assert(i4 == 0)
+	assert(i5 == 0)
+	assert(i6 == 0)
+}
+
 // untyped constants must not get arbitrarily large
 const prec = 512 // internal maximum precision for integers
 const maxInt = (1<<(prec/2) - 1) * (1<<(prec/2) + 1) // == 1<<prec - 1

test/fixedbugs/issue52438.go

@@ -0,0 +1,39 @@
+// run
+
+// Copyright 2022 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.
+
+package main
+
+const c1 = iota
+const c2 = iota
+
+const c3 = 0 + iota<<8
+const c4 = 1 + iota<<8
+
+func main() {
+	if c1 != 0 {
+		panic(c1)
+	}
+	if c2 != 0 {
+		panic(c2)
+	}
+
+	if c3 != 0 {
+		panic(c3)
+	}
+	if c4 != 1 {
+		panic(c4)
+	}
+
+	const c5 = iota
+	const c6 = iota
+
+	if c5 != 0 {
+		panic(c5)
+	}
+	if c6 != 0 {
+		panic(c6)
+	}
+}