[release-branch.go1.23] go/types, types2: use max(fileVersion, go1.21) if fileVersion present

Change the rules for how //go:build "file versions" are applied: instead
of considering whether a file version is an upgrade or downgrade from
the -lang version, always use max(fileVersion, go1.21). This prevents
file versions from downgrading the version below go1.21.  Before Go 1.21
the //go:build version did not have the meaning of setting the file's
langage version.

This fixes an issue that was appearing in GOPATH builds: Go 1.23.0
started providing -lang versions to the compiler in GOPATH mode (among
other places) which it wasn't doing before, and it set -lang to the
toolchain version (1.23). Because the -lang version was greater than
go1.21, language version used to compile the file would be set to the
//go:build file version. //go:build file versions below 1.21 could cause
files that could previously build to stop building.

For example, take a Go file with a //go:build line specifying go1.10.
If that file used a 1.18 feature, that use would compile fine with a Go
1.22 toolchain. But it would produce an error when compiling with the
1.23.0 toolchain because it set the language version to 1.10 and
disallowed the 1.18 feature. This breaks backwards compatibility: when
the build tag was added, it did not have the meaning of restricting the
language version.

For #68658
Fixes #69094

Change-Id: I6cedda81a55bcccffaa3501eef9e2be6541b6ece
Reviewed-on: https://go-review.googlesource.com/c/go/+/607955
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>
(cherry picked from commit aeac0b6cbfb42bc9c9301913a191bb09454d316a)
Reviewed-on: https://go-review.googlesource.com/c/go/+/608935

11 files changed, 174 insertions, 99 deletions

diff --git a/src/cmd/compile/internal/types2/api_test.go b/src/cmd/compile/internal/types2/api_test.go
index 5126ac5111..a6b105ace5 100644
--- a/src/cmd/compile/internal/types2/api_test.go
+++ b/src/cmd/compile/internal/types2/api_test.go
@@ -2898,22 +2898,48 @@ func TestFileVersions(t *testing.T) {
 		fileVersion string
 		wantVersion string
 	}{
-		{"", "", ""},                   // no versions specified
-		{"go1.19", "", "go1.19"},       // module version specified
-		{"", "go1.20", ""},             // file upgrade ignored
-		{"go1.19", "go1.20", "go1.20"}, // file upgrade permitted
-		{"go1.20", "go1.19", "go1.20"}, // file downgrade not permitted
-		{"go1.21", "go1.19", "go1.19"}, // file downgrade permitted (module version is >= go1.21)
+		{"", "", ""},                    // no versions specified
+		{"go1.19", "", "go1.19"},        // module version specified
+		{"", "go1.20", "go1.21"},        // file version specified below minimum of 1.21
+		{"go1", "", "go1"},              // no file version specified
+		{"go1", "goo1.22", "go1"},       // invalid file version specified
+		{"go1", "go1.19", "go1.21"},     // file version specified below minimum of 1.21
+		{"go1", "go1.20", "go1.21"},     // file version specified below minimum of 1.21
+		{"go1", "go1.21", "go1.21"},     // file version specified at 1.21
+		{"go1", "go1.22", "go1.22"},     // file version specified above 1.21
+		{"go1.19", "", "go1.19"},        // no file version specified
+		{"go1.19", "goo1.22", "go1.19"}, // invalid file version specified
+		{"go1.19", "go1.20", "go1.21"},  // file version specified below minimum of 1.21
+		{"go1.19", "go1.21", "go1.21"},  // file version specified at 1.21
+		{"go1.19", "go1.22", "go1.22"},  // file version specified above 1.21
+		{"go1.20", "", "go1.20"},        // no file version specified
+		{"go1.20", "goo1.22", "go1.20"}, // invalid file version specified
+		{"go1.20", "go1.19", "go1.21"},  // file version specified below minimum of 1.21
+		{"go1.20", "go1.20", "go1.21"},  // file version specified below minimum of 1.21
+		{"go1.20", "go1.21", "go1.21"},  // file version specified at 1.21
+		{"go1.20", "go1.22", "go1.22"},  // file version specified above 1.21
+		{"go1.21", "", "go1.21"},        // no file version specified
+		{"go1.21", "goo1.22", "go1.21"}, // invalid file version specified
+		{"go1.21", "go1.19", "go1.21"},  // file version specified below minimum of 1.21
+		{"go1.21", "go1.20", "go1.21"},  // file version specified below minimum of 1.21
+		{"go1.21", "go1.21", "go1.21"},  // file version specified at 1.21
+		{"go1.21", "go1.22", "go1.22"},  // file version specified above 1.21
+		{"go1.22", "", "go1.22"},        // no file version specified
+		{"go1.22", "goo1.22", "go1.22"}, // invalid file version specified
+		{"go1.22", "go1.19", "go1.21"},  // file version specified below minimum of 1.21
+		{"go1.22", "go1.20", "go1.21"},  // file version specified below minimum of 1.21
+		{"go1.22", "go1.21", "go1.21"},  // file version specified at 1.21
+		{"go1.22", "go1.22", "go1.22"},  // file version specified above 1.21
 
 		// versions containing release numbers
 		// (file versions containing release numbers are considered invalid)
 		{"go1.19.0", "", "go1.19.0"},         // no file version specified
-		{"go1.20", "go1.20.1", "go1.20"},     // file upgrade ignored
-		{"go1.20.1", "go1.20", "go1.20.1"},   // file upgrade ignored
-		{"go1.20.1", "go1.21", "go1.21"},     // file upgrade permitted
-		{"go1.20.1", "go1.19", "go1.20.1"},   // file downgrade not permitted
-		{"go1.21.1", "go1.19.1", "go1.21.1"}, // file downgrade not permitted (invalid file version)
-		{"go1.21.1", "go1.19", "go1.19"},     // file downgrade permitted (module version is >= go1.21)
+		{"go1.20.1", "go1.19.1", "go1.20.1"}, // invalid file version
+		{"go1.20.1", "go1.21.1", "go1.20.1"}, // invalid file version
+		{"go1.21.1", "go1.19.1", "go1.21.1"}, // invalid file version
+		{"go1.21.1", "go1.21.1", "go1.21.1"}, // invalid file version
+		{"go1.22.1", "go1.19.1", "go1.22.1"}, // invalid file version
+		{"go1.22.1", "go1.21.1", "go1.22.1"}, // invalid file version
 	} {
 		var src string
 		if test.fileVersion != "" {
diff --git a/src/cmd/compile/internal/types2/check.go b/src/cmd/compile/internal/types2/check.go
index 91ad474e9d..ada421ba93 100644
--- a/src/cmd/compile/internal/types2/check.go
+++ b/src/cmd/compile/internal/types2/check.go
@@ -327,7 +327,6 @@ func (check *Checker) initFiles(files []*syntax.File) {
 		check.errorf(files[0], TooNew, "package requires newer Go version %v (application built with %v)",
 			check.version, go_current)
 	}
-	downgradeOk := check.version.cmp(go1_21) >= 0
 
 	// determine Go version for each file
 	for _, file := range check.files {
@@ -336,33 +335,18 @@ func (check *Checker) initFiles(files []*syntax.File) {
 		// unlike file versions which are Go language versions only, if valid.)
 		v := check.conf.GoVersion
 
-		fileVersion := asGoVersion(file.GoVersion)
-		if fileVersion.isValid() {
-			// use the file version, if applicable
-			// (file versions are either the empty string or of the form go1.dd)
-			if pkgVersionOk {
-				cmp := fileVersion.cmp(check.version)
-				// Go 1.21 introduced the feature of setting the go.mod
-				// go line to an early version of Go and allowing //go:build lines
-				// to “upgrade” (cmp > 0) the Go version in a given file.
-				// We can do that backwards compatibly.
-				//
-				// Go 1.21 also introduced the feature of allowing //go:build lines
-				// to “downgrade” (cmp < 0) the Go version in a given file.
-				// That can't be done compatibly in general, since before the
-				// build lines were ignored and code got the module's Go version.
-				// To work around this, downgrades are only allowed when the
-				// module's Go version is Go 1.21 or later.
-				//
-				// If there is no valid check.version, then we don't really know what
-				// Go version to apply.
-				// Legacy tools may do this, and they historically have accepted everything.
-				// Preserve that behavior by ignoring //go:build constraints entirely in that
-				// case (!pkgVersionOk).
-				if cmp > 0 || cmp < 0 && downgradeOk {
-					v = file.GoVersion
-				}
-			}
+		// If the file specifies a version, use max(fileVersion, go1.21).
+		if fileVersion := asGoVersion(file.GoVersion); fileVersion.isValid() {
+			// Go 1.21 introduced the feature of allowing //go:build lines
+			// to sometimes set the Go version in a given file. Versions Go 1.21 and later
+			// can be set backwards compatibly as that was the first version
+			// files with go1.21 or later build tags could be built with.
+			//
+			// Set the version to max(fileVersion, go1.21): That will allow a
+			// downgrade to a version before go1.22, where the for loop semantics
+			// change was made, while being backwards compatible with versions of
+			// go before the new //go:build semantics were introduced.
+			v = string(versionMax(fileVersion, go1_21))
 
 			// Report a specific error for each tagged file that's too new.
 			// (Normally the build system will have filtered files by version,
@@ -377,6 +361,13 @@ func (check *Checker) initFiles(files []*syntax.File) {
 	}
 }
 
+func versionMax(a, b goVersion) goVersion {
+	if a.cmp(b) > 0 {
+		return a
+	}
+	return b
+}
+
 // A bailout panic is used for early termination.
 type bailout struct{}
 
diff --git a/src/go/types/api_test.go b/src/go/types/api_test.go
index beed94f355..a7aa648802 100644
--- a/src/go/types/api_test.go
+++ b/src/go/types/api_test.go
@@ -2904,22 +2904,48 @@ func TestFileVersions(t *testing.T) {
 		fileVersion string
 		wantVersion string
 	}{
-		{"", "", ""},                   // no versions specified
-		{"go1.19", "", "go1.19"},       // module version specified
-		{"", "go1.20", ""},             // file upgrade ignored
-		{"go1.19", "go1.20", "go1.20"}, // file upgrade permitted
-		{"go1.20", "go1.19", "go1.20"}, // file downgrade not permitted
-		{"go1.21", "go1.19", "go1.19"}, // file downgrade permitted (module version is >= go1.21)
+		{"", "", ""},                    // no versions specified
+		{"go1.19", "", "go1.19"},        // module version specified
+		{"", "go1.20", "go1.21"},        // file version specified below minimum of 1.21
+		{"go1", "", "go1"},              // no file version specified
+		{"go1", "goo1.22", "go1"},       // invalid file version specified
+		{"go1", "go1.19", "go1.21"},     // file version specified below minimum of 1.21
+		{"go1", "go1.20", "go1.21"},     // file version specified below minimum of 1.21
+		{"go1", "go1.21", "go1.21"},     // file version specified at 1.21
+		{"go1", "go1.22", "go1.22"},     // file version specified above 1.21
+		{"go1.19", "", "go1.19"},        // no file version specified
+		{"go1.19", "goo1.22", "go1.19"}, // invalid file version specified
+		{"go1.19", "go1.20", "go1.21"},  // file version specified below minimum of 1.21
+		{"go1.19", "go1.21", "go1.21"},  // file version specified at 1.21
+		{"go1.19", "go1.22", "go1.22"},  // file version specified above 1.21
+		{"go1.20", "", "go1.20"},        // no file version specified
+		{"go1.20", "goo1.22", "go1.20"}, // invalid file version specified
+		{"go1.20", "go1.19", "go1.21"},  // file version specified below minimum of 1.21
+		{"go1.20", "go1.20", "go1.21"},  // file version specified below minimum of 1.21
+		{"go1.20", "go1.21", "go1.21"},  // file version specified at 1.21
+		{"go1.20", "go1.22", "go1.22"},  // file version specified above 1.21
+		{"go1.21", "", "go1.21"},        // no file version specified
+		{"go1.21", "goo1.22", "go1.21"}, // invalid file version specified
+		{"go1.21", "go1.19", "go1.21"},  // file version specified below minimum of 1.21
+		{"go1.21", "go1.20", "go1.21"},  // file version specified below minimum of 1.21
+		{"go1.21", "go1.21", "go1.21"},  // file version specified at 1.21
+		{"go1.21", "go1.22", "go1.22"},  // file version specified above 1.21
+		{"go1.22", "", "go1.22"},        // no file version specified
+		{"go1.22", "goo1.22", "go1.22"}, // invalid file version specified
+		{"go1.22", "go1.19", "go1.21"},  // file version specified below minimum of 1.21
+		{"go1.22", "go1.20", "go1.21"},  // file version specified below minimum of 1.21
+		{"go1.22", "go1.21", "go1.21"},  // file version specified at 1.21
+		{"go1.22", "go1.22", "go1.22"},  // file version specified above 1.21
 
 		// versions containing release numbers
 		// (file versions containing release numbers are considered invalid)
 		{"go1.19.0", "", "go1.19.0"},         // no file version specified
-		{"go1.20", "go1.20.1", "go1.20"},     // file upgrade ignored
-		{"go1.20.1", "go1.20", "go1.20.1"},   // file upgrade ignored
-		{"go1.20.1", "go1.21", "go1.21"},     // file upgrade permitted
-		{"go1.20.1", "go1.19", "go1.20.1"},   // file downgrade not permitted
-		{"go1.21.1", "go1.19.1", "go1.21.1"}, // file downgrade not permitted (invalid file version)
-		{"go1.21.1", "go1.19", "go1.19"},     // file downgrade permitted (module version is >= go1.21)
+		{"go1.20.1", "go1.19.1", "go1.20.1"}, // invalid file version
+		{"go1.20.1", "go1.21.1", "go1.20.1"}, // invalid file version
+		{"go1.21.1", "go1.19.1", "go1.21.1"}, // invalid file version
+		{"go1.21.1", "go1.21.1", "go1.21.1"}, // invalid file version
+		{"go1.22.1", "go1.19.1", "go1.22.1"}, // invalid file version
+		{"go1.22.1", "go1.21.1", "go1.22.1"}, // invalid file version
 	} {
 		var src string
 		if test.fileVersion != "" {
diff --git a/src/go/types/check.go b/src/go/types/check.go
index 1a5a41a3bb..8a72909496 100644
--- a/src/go/types/check.go
+++ b/src/go/types/check.go
@@ -349,7 +349,6 @@ func (check *Checker) initFiles(files []*ast.File) {
 		check.errorf(files[0], TooNew, "package requires newer Go version %v (application built with %v)",
 			check.version, go_current)
 	}
-	downgradeOk := check.version.cmp(go1_21) >= 0
 
 	// determine Go version for each file
 	for _, file := range check.files {
@@ -358,33 +357,19 @@ func (check *Checker) initFiles(files []*ast.File) {
 		// unlike file versions which are Go language versions only, if valid.)
 		v := check.conf.GoVersion
 
-		fileVersion := asGoVersion(file.GoVersion)
-		if fileVersion.isValid() {
-			// use the file version, if applicable
-			// (file versions are either the empty string or of the form go1.dd)
-			if pkgVersionOk {
-				cmp := fileVersion.cmp(check.version)
-				// Go 1.21 introduced the feature of setting the go.mod
-				// go line to an early version of Go and allowing //go:build lines
-				// to “upgrade” (cmp > 0) the Go version in a given file.
-				// We can do that backwards compatibly.
-				//
-				// Go 1.21 also introduced the feature of allowing //go:build lines
-				// to “downgrade” (cmp < 0) the Go version in a given file.
-				// That can't be done compatibly in general, since before the
-				// build lines were ignored and code got the module's Go version.
-				// To work around this, downgrades are only allowed when the
-				// module's Go version is Go 1.21 or later.
-				//
-				// If there is no valid check.version, then we don't really know what
-				// Go version to apply.
-				// Legacy tools may do this, and they historically have accepted everything.
-				// Preserve that behavior by ignoring //go:build constraints entirely in that
-				// case (!pkgVersionOk).
-				if cmp > 0 || cmp < 0 && downgradeOk {
-					v = file.GoVersion
-				}
-			}
+		// If the file specifies a version, use max(fileVersion, go1.21).
+		if fileVersion := asGoVersion(file.GoVersion); fileVersion.isValid() {
+			// Go 1.21 introduced the feature of setting the go.mod
+			// go line to an early version of Go and allowing //go:build lines
+			// to set the Go version in a given file. Versions Go 1.21 and later
+			// can be set backwards compatibly as that was the first version
+			// files with go1.21 or later build tags could be built with.
+			//
+			// Set the version to max(fileVersion, go1.21): That will allow a
+			// downgrade to a version before go1.22, where the for loop semantics
+			// change was made, while being backwards compatible with versions of
+			// go before the new //go:build semantics were introduced.
+			v = string(versionMax(fileVersion, go1_21))
 
 			// Report a specific error for each tagged file that's too new.
 			// (Normally the build system will have filtered files by version,
@@ -399,6 +384,13 @@ func (check *Checker) initFiles(files []*ast.File) {
 	}
 }
 
+func versionMax(a, b goVersion) goVersion {
+	if a.cmp(b) < 0 {
+		return b
+	}
+	return a
+}
+
 // A bailout panic is used for early termination.
 type bailout struct{}
 
diff --git a/src/internal/types/testdata/check/go1_20_19.go b/src/internal/types/testdata/check/go1_20_19.go
index 08365a7cfb..e040d396c7 100644
--- a/src/internal/types/testdata/check/go1_20_19.go
+++ b/src/internal/types/testdata/check/go1_20_19.go
@@ -14,4 +14,4 @@ type Slice []byte
 type Array [8]byte
 
 var s Slice
-var p = (Array)(s /* ok because Go 1.20 ignored the //go:build go1.19 */)
+var p = (Array)(s /* ok because file versions below go1.21 set the langage version to go1.21 */)
diff --git a/src/internal/types/testdata/check/go1_21_19.go b/src/internal/types/testdata/check/go1_21_19.go
index 2acd25865d..5866033eaf 100644
--- a/src/internal/types/testdata/check/go1_21_19.go
+++ b/src/internal/types/testdata/check/go1_21_19.go
@@ -14,4 +14,4 @@ type Slice []byte
 type Array [8]byte
 
 var s Slice
-var p = (Array)(s /* ERROR "requires go1.20 or later" */)
+var p = (Array)(s /* ok because file versions below go1.21 set the langage version to go1.21 */)
diff --git a/src/internal/types/testdata/check/go1_21_22.go b/src/internal/types/testdata/check/go1_21_22.go
new file mode 100644
index 0000000000..3939b7b1d8
--- /dev/null
+++ b/src/internal/types/testdata/check/go1_21_22.go
@@ -0,0 +1,16 @@
+// -lang=go1.21
+
+// 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.
+
+// Check Go language version-specific errors.
+
+//go:build go1.22
+
+package p
+
+func f() {
+	for _ = range /* ok because of upgrade to 1.22 */ 10 {
+	}
+}
diff --git a/src/internal/types/testdata/check/go1_22_21.go b/src/internal/types/testdata/check/go1_22_21.go
new file mode 100644
index 0000000000..f910ecb59c
--- /dev/null
+++ b/src/internal/types/testdata/check/go1_22_21.go
@@ -0,0 +1,16 @@
+// -lang=go1.22
+
+// Copyright 2024 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.
+
+// Check Go language version-specific errors.
+
+//go:build go1.21
+
+package p
+
+func f() {
+	for _ = range 10 /* ERROR "requires go1.22 or later" */ {
+	}
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue66285.go b/src/internal/types/testdata/fixedbugs/issue66285.go
index 9811fec3f3..4af76f05da 100644
--- a/src/internal/types/testdata/fixedbugs/issue66285.go
+++ b/src/internal/types/testdata/fixedbugs/issue66285.go
@@ -1,14 +1,9 @@
-// -lang=go1.21
+// -lang=go1.13
 
 // Copyright 2024 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.
 
-// Note: Downgrading to go1.13 requires at least go1.21,
-//       hence the need for -lang=go1.21 at the top.
-
-//go:build go1.13
-
 package p
 
 import "io"
diff --git a/test/fixedbugs/issue63489a.go b/test/fixedbugs/issue63489a.go
index b88120f2c0..2b46814f95 100644
--- a/test/fixedbugs/issue63489a.go
+++ b/test/fixedbugs/issue63489a.go
@@ -1,16 +1,20 @@
-// errorcheck -lang=go1.21
+// errorcheck -lang=go1.22
 
 // Copyright 2023 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.
 
-//go:build go1.4
+// This file has been changed from its original version as
+// //go:build file versions below 1.21 set the language version to 1.21.
+// The original tested a -lang version of 1.21 with a file version of
+// go1.4 while this new version tests a -lang version of go1.22
+// with a file version of go1.21.
 
-package p
-
-const c = 0o123 // ERROR "file declares //go:build go1.4"
+//go:build go1.21
 
-// ERROR "file declares //go:build go1.4"
+package p
 
-//line issue63489a.go:13:1
-const d = 0o124
+func f() {
+	for _ = range 10 { // ERROR "file declares //go:build go1.21"
+	}
+}
diff --git a/test/fixedbugs/issue63489b.go b/test/fixedbugs/issue63489b.go
index 2ad590dfc3..fd897dea97 100644
--- a/test/fixedbugs/issue63489b.go
+++ b/test/fixedbugs/issue63489b.go
@@ -1,11 +1,20 @@
-// errorcheck -lang=go1.4
+// errorcheck -lang=go1.21
 
 // Copyright 2023 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.
 
-//go:build go1.4
+// This file has been changed from its original version as
+// //go:build file versions below 1.21 set the language version to 1.21.
+// The original tested a -lang version of 1.4 with a file version of
+// go1.4 while this new version tests a -lang version of go1.1
+// with a file version of go1.21.
+
+//go:build go1.21
 
 package p
 
-const c = 0o123 // ERROR "file declares //go:build go1.4"
+func f() {
+	for _ = range 10 { // ERROR "file declares //go:build go1.21"
+	}
+}