GPU: Avoid undocumented/fragile dependency on shader enumerator order

Previously this was using `GPU_SHADER_TEXT` as default value indicating
an "unset" state. This wasn't documented in the definition (and so
D16284 added a new enumerator that broke this). Plus code was assuming
this enumerator would always have the value 0 without specifying this in
the definition either.

In this case it's easy to not rely on the enum value at all, and just
use `std::optional` to add a "unset" state.

Differential Revision: https://developer.blender.org/D16303

2 files changed, 12 insertions, 7 deletions

diff --git a/source/blender/gpu/intern/gpu_immediate.cc b/source/blender/gpu/intern/gpu_immediate.cc
index 3b4accf9cc5..81c0a65bb7c 100644
--- a/source/blender/gpu/intern/gpu_immediate.cc
+++ b/source/blender/gpu/intern/gpu_immediate.cc
@@ -45,7 +45,7 @@ void immBindShader(GPUShader *shader)
   BLI_assert(imm->shader == nullptr);
 
   imm->shader = shader;
-  imm->builtin_shader_bound = GPU_SHADER_TEXT; /* Default value. */
+  imm->builtin_shader_bound = std::nullopt;
 
   if (!imm->vertex_format.packed) {
     VertexFormat_pack(&imm->vertex_format);
@@ -125,9 +125,12 @@ static void wide_line_workaround_start(GPUPrimType prim_type)
     /* No need to change the shader. */
     return;
   }
+  if (!imm->builtin_shader_bound) {
+    return;
+  }
 
   eGPUBuiltinShader polyline_sh;
-  switch (imm->builtin_shader_bound) {
+  switch (*imm->builtin_shader_bound) {
     case GPU_SHADER_3D_CLIPPED_UNIFORM_COLOR:
       polyline_sh = GPU_SHADER_3D_POLYLINE_CLIPPED_UNIFORM_COLOR;
       break;
@@ -180,8 +183,8 @@ static void wide_line_workaround_end()
     }
     immUnbindProgram();
 
-    immBindBuiltinProgram(imm->prev_builtin_shader);
-    imm->prev_builtin_shader = GPU_SHADER_TEXT;
+    immBindBuiltinProgram(*imm->prev_builtin_shader);
+    imm->prev_builtin_shader = std::nullopt;
   }
 }
 
diff --git a/source/blender/gpu/intern/gpu_immediate_private.hh b/source/blender/gpu/intern/gpu_immediate_private.hh
index 74ebbdc7ae3..c4e11e7082b 100644
--- a/source/blender/gpu/intern/gpu_immediate_private.hh
+++ b/source/blender/gpu/intern/gpu_immediate_private.hh
@@ -9,6 +9,8 @@
 
 #pragma once
 
+#include <optional>
+
 #include "GPU_batch.h"
 #include "GPU_primitive.h"
 #include "GPU_shader.h"
@@ -42,9 +44,9 @@ class Immediate {
   /** Wide Line workaround. */
 
   /** Previously bound shader to restore after drawing. */
-  eGPUBuiltinShader prev_builtin_shader = GPU_SHADER_TEXT;
-  /** Builtin shader index. Used to test if the workaround can be done. */
-  eGPUBuiltinShader builtin_shader_bound = GPU_SHADER_TEXT;
+  std::optional<eGPUBuiltinShader> prev_builtin_shader;
+  /** Builtin shader index. Used to test if the line width workaround can be done. */
+  std::optional<eGPUBuiltinShader> builtin_shader_bound;
   /** Uniform color: Kept here to update the wide-line shader just before #immBegin. */
   float uniform_color[4];