If compiling a function parameter creates any generated variables (e.…

…g. `ref`), shift the declarations for those variables into the parent scope; fixes jashkenas#4413

src/nodes.coffee

@@ -2674,7 +2674,14 @@ exports.Code = class Code extends Base
     for param, i in params
       signature.push @makeCode ', ' if i isnt 0
       signature.push @makeCode '...' if haveSplatParam and i is params.length - 1
+      # Compile this parameter, but if any generated variables get created
+      # (e.g. `ref`), shift those into the parent scope since we can’t put a
+      # `var` line inside a function parameter list.
+      scopeVariablesCount = o.scope.variables.length
       signature.push param.compileToFragments(o)...
+      if scopeVariablesCount isnt o.scope.variables.length
+        generatedVariables = o.scope.variables.splice scopeVariablesCount
+        o.scope.parent.variables.push generatedVariables...
     signature.push @makeCode ')'
     # Block comments between `)` and `->`/`=>` get output between `)` and `{`.
     if @funcGlyph?.comments?

test/functions.coffee

@@ -471,3 +471,12 @@ test "#3845/#3446: chain after function glyph", ->
     doThing()
     .then (@result) =>
     .catch handleError
+
+test "#4413: expressions in function parameters that create generated variables have those variables declared correctly", ->
+  'use strict'
+  # We’re in strict mode because we want an error to be thrown if the generated
+  # variable (`ref`) is assigned before being declared.
+  foo = -> null
+  bar = -> 33
+  f = (a = foo() ? bar()) -> a
+  eq f(), 33