Merge pull request #6103 from Vexu/extern

Disallow extern variables with initializers.

test/compile_errors.zig

@@ -2,6 +2,12 @@ const tests = @import("tests.zig");
 const std = @import("std");
 
 pub fn addCases(cases: *tests.CompileErrorContext) void {
+    cases.addTest("reject extern variables with initializers",
+        \\extern var foo: int = 2;
+    , &[_][]const u8{
+        "tmp.zig:1:1: error: extern variables have no initializers",
+    });
+
     cases.addTest("duplicate/unused labels",
         \\comptime {
         \\    blk: { blk: while (false) {} }