configure: Check version number validity

Here we verify the previously informal invariant that stable release
version numbers must have three components, preventing costly failed
releases.

Specifically, the check fails in the following scenarios:

 * `version=9.13` while `RELEASE=YES` since this would imply a
   release made from an unstable branch
 * `version=9.13.0` since unstable versions should only have two
   components
 * `version=9.12` since this has the wrong number of version components
   for a stable branch

Fixes #25390.

m4/fp_setup_project_version.m4

@@ -2,6 +2,9 @@
 # ---------------------
 AC_DEFUN([FP_SETUP_PROJECT_VERSION],
 [
+    # number of version number components
+    NumVersionComponents="$(( $(echo "$PACKAGE_VERSION" | tr -cd . | wc -c) + 1 ))"
+
     if test "$RELEASE" = "NO"; then
         AC_MSG_CHECKING([for GHC version date])
         if test -f VERSION_DATE; then
@@ -62,6 +65,22 @@ AC_DEFUN([FP_SETUP_PROJECT_VERSION],
     VERSION_MINOR=`echo $VERSION_TMP | sed 's/^\(@<:@^.@:>@*\)\(\.\{0,1\}\(.*\)\)$/\1'/`
     ProjectPatchLevel=`echo $VERSION_TMP | sed 's/^\(@<:@^.@:>@*\)\(\.\{0,1\}\(.*\)\)$/\3'/`
 
+    # Verify that the version number has three components if a release version
+    # (that is, even minor version number).
+    AC_MSG_CHECKING([package version validity])
+    StableRelease="$(( ($VERSION_MINOR & 1) == 0))"
+    if test "$StableRelease" = "1" -a "$NumVersionComponents" != "3"; then
+        AC_MSG_ERROR([Stable (even) version numbers must have three components])
+    elif test "$StableRelease" = "0" -a "$NumVersionComponents" != "2"; then
+        AC_MSG_ERROR([Unstable (odd) version numbers must have two components])
+    elif test "$RELEASE" = "YES" -a "$StableRelease" = "0"; then
+        AC_MSG_ERROR([RELEASE=YES despite having an unstable odd minor version number])
+    elif test "$StableRelease" = "1"; then
+        AC_MSG_RESULT([okay stable branch version])
+    else
+        AC_MSG_RESULT([okay unstable branch version])
+    fi
+
     # Calculate project version as an integer, using 2 digits for minor version
     case $VERSION_MINOR in
       ?) ProjectVersionInt=${VERSION_MAJOR}0${VERSION_MINOR} ;;