catch integer literal loop sources

... before they get to JavaScript, where a 0[i$] expression results in a
very unhelpful SyntaxError.

Author: rhendric

lib/ast.js

@@ -2350,6 +2350,9 @@ class exports.For extends While
                 then "#idx #{ '<>'char-at pvar < 0 }#eq #tvar"
                 else "#pvar < 0 ? #idx >#eq #tvar : #idx <#eq #tvar"
         else
+            if @source instanceof Literal and @source.value is /^[0-9]+$/
+                # There are lots of invalid loop sources, but integer literals specifically result in JavaScript compilation errors
+                @carp "invalid loop source #{@source.value}"
             @item = Var o.scope.temporary \x if @ref
             if @item or @object and @own or @let
                 [svar, srcPart] = @source.compile-loop-reference o, \ref, not @object

src/ast.ls

@@ -686,3 +686,7 @@ eq 1, i
 o = { [k, -> v] for let k, v of {a: 1, b: 2} }
 eq 1 o.a!
 eq 2 o.b!
+
+# Integer literals as loop sources generate illegal JavaScript and must be
+# caught by us
+compileThrows 'invalid loop source 42' 1 '[0 for x in 42]'