Shader: can't use varyings as read-only parameters in custom functions

[zhehangd]

Godot version

4.0 Beta9

System information

Issue description

See this example:

shader_type spatial;

varying float foo;

void function2(in float f) {
}

void function1() {
  // This is ok
  float bar = foo;
  function2(bar);
  // This is not ok (bug)
  // error: Varying 'foo' must be assigned in the vertex or fragment function first.
  function2(foo);
}

void vertex()
{
  foo = 1.0;
}

void fragment()
{
  function1();
  // This is ok
  function2(foo);
}

foo is a varying and is assigned in the vertex function.
In fragment, calling function2(foo); is ok as expected.
However, in function1 calling function2(foo); is mistakenly marked as an error.
Instead, assigning foo to bar first then giving bar as the parameter is ok.

Steps to reproduce

Minimal reproduction project

varying_error.zip

[Chaosus]

As a fix, you can place vertex() function above the function1, and it will work. I'm not sure whether it is a bug or just misunderstanding of the limitations of the new varying system.

[Chaosus]

cc @clayjohn