build: use build context ReleaseTags slice that goes to go1.12 only

flimzy · web-flow · commit fce0ec30dd00 · 2020-02-17T09:24:28.000-05:00
We want GopherJS 1.12-3 to have predictable behavior when building packages regardless of the version of Go it was built with; it should not be matching files with "// +build go1.13" constraints some times but not other times. Previously, ReleaseTags of the default build context were inherited from the Go version used to build GopherJS, which may be newer than Go 1.12. This change makes it have ReleaseTags that go up to go1.12 always. That corresponds to the major version of Go and its standard library that is supported by GopherJS 1.12-3. Fixes #969. GitHub-Pull-Request: #970
diff --git a/build/build.go b/build/build.go
@@ -67,7 +67,7 @@ func NewBuildContext(installSuffix string, buildTags []string) *build.Context {
 			"netgo",  // See https://godoc.org/net#hdr-Name_Resolution.
 			"purego", // See https://golang.org/issues/23172.
 		),
-		ReleaseTags: build.Default.ReleaseTags,
+		ReleaseTags: build.Default.ReleaseTags[:compiler.GoVersion],
 		CgoEnabled:  true, // detect `import "C"` to throw proper error
 
 		IsDir: func(path string) bool {
diff --git a/compiler/version_check.go b/compiler/version_check.go
@@ -1,3 +1,5 @@
+// +build go1.12
+
 package compiler
 
 import (
@@ -10,6 +12,9 @@ import (
 // Version is the GopherJS compiler version string.
 const Version = "1.12-3"
 
+// GoVersion is the current Go 1.x version that GopherJS is compatible with.
+const GoVersion = 12
+
 // CheckGoVersion checks the version of the Go distribution
 // at goroot, and reports an error if it's not compatible
 // with this version of the GopherJS compiler.
