Build v3 C tests under clang too; drop GNU VA_ARGS extension

ci.yml now runs the v3 C suites under three Linux toolchains:
- v3-c-tests        (default cc / GCC, -Wall -Wextra)
- v3-c-tests-clang  (stock Ubuntu clang)
- v3-c-tests-clang21 (clang 21 from apt.llvm.org, matching local dev)

The validator helper macros TG3__IDX_BAD / TG3__CHECK_REQ /
TG3__CHECK_OPT used the GNU `, ##__VA_ARGS__` extension, which clang
flags under -Wpedantic. Every call site already passes at least one
variadic argument, so plain __VA_ARGS__ (C11) suffices.

Local verification: clang-21 -Wall -Wextra builds tester_v3_c and
tester_v3_c_v1port with zero warnings introduced by the v3 changes
(only the pre-existing tg3__json_set_value_copy unused-function
notice remains, unrelated to this work). All suites pass under gcc
13.3 and clang 21.1 alike, plus the 134-model cross-version
verifier.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/ci.yml

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c65f976..99a3ef3 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -332,7 +332,7 @@
   # v3 C runtime: internal security regression tests + ported v1 unit tests.
   v3-c-tests:
     runs-on: ubuntu-latest
-    name: v3 C tests (Linux x64)
+    name: v3 C tests (Linux x64, GCC)
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -340,13 +340,13 @@
       - name: Build tester_v3_c
         run: |
           cd tests
-          cc -I../ -std=c11 -g -O0 -DTINYGLTF3_ENABLE_FS \
+          cc -I../ -std=c11 -g -O0 -Wall -Wextra -DTINYGLTF3_ENABLE_FS \
              -o tester_v3_c tester_v3_c.c ../tiny_gltf_v3.c
 
       - name: Build tester_v3_c_v1port
         run: |
           cd tests
-          cc -I../ -std=c11 -g -O0 -DTINYGLTF3_ENABLE_FS \
+          cc -I../ -std=c11 -g -O0 -Wall -Wextra -DTINYGLTF3_ENABLE_FS \
              -o tester_v3_c_v1port tester_v3_c_v1port.c ../tiny_gltf_v3.c
 
       - name: Run tester_v3_c (security regressions)
@@ -359,6 +359,77 @@
           cd tests
           ./tester_v3_c_v1port
 
+  # v3 C runtime under stock Ubuntu clang.
+  v3-c-tests-clang:
+    runs-on: ubuntu-latest
+    name: v3 C tests (Linux x64, Clang)
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install clang
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y clang
+
+      - name: Build tester_v3_c
+        run: |
+          cd tests
+          clang -I../ -std=c11 -g -O0 -Wall -Wextra -DTINYGLTF3_ENABLE_FS \
+                -o tester_v3_c tester_v3_c.c ../tiny_gltf_v3.c
+
+      - name: Build tester_v3_c_v1port
+        run: |
+          cd tests
+          clang -I../ -std=c11 -g -O0 -Wall -Wextra -DTINYGLTF3_ENABLE_FS \
+                -o tester_v3_c_v1port tester_v3_c_v1port.c ../tiny_gltf_v3.c
+
+      - name: Run tester_v3_c
+        run: |
+          cd tests
+          ./tester_v3_c
+
+      - name: Run tester_v3_c_v1port
+        run: |
+          cd tests
+          ./tester_v3_c_v1port
+
+  # v3 C runtime under bleeding-edge clang 21 (matches local dev environment).
+  v3-c-tests-clang21:
+    runs-on: ubuntu-24.04
+    name: v3 C tests (Linux x64, Clang 21)
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install clang 21
+        run: |
+          wget https://apt.llvm.org/llvm.sh
+          chmod +x llvm.sh
+          sudo ./llvm.sh 21
+
+      - name: Build tester_v3_c
+        run: |
+          cd tests
+          clang-21 -I../ -std=c11 -g -O0 -Wall -Wextra -DTINYGLTF3_ENABLE_FS \
+                   -o tester_v3_c tester_v3_c.c ../tiny_gltf_v3.c
+
+      - name: Build tester_v3_c_v1port
+        run: |
+          cd tests
+          clang-21 -I../ -std=c11 -g -O0 -Wall -Wextra -DTINYGLTF3_ENABLE_FS \
+                   -o tester_v3_c_v1port tester_v3_c_v1port.c ../tiny_gltf_v3.c
+
+      - name: Run tester_v3_c
+        run: |
+          cd tests
+          ./tester_v3_c
+
+      - name: Run tester_v3_c_v1port
+        run: |
+          cd tests
+          ./tester_v3_c_v1port
+
   # v3 C runtime under ASan + UBSan for memory-safety + UB checks.
   v3-c-tests-sanitizers:
     runs-on: ubuntu-latest

tiny_gltf_v3.c

diff --git a/tiny_gltf_v3.c b/tiny_gltf_v3.c
index 1d35b49..1aa4bc7 100644
--- a/tiny_gltf_v3.c
+++ b/tiny_gltf_v3.c
@@ -1617,22 +1617,24 @@
 static int tg3__validate_indices(tg3__parse_ctx *ctx, const tg3_model *m) {
     uint32_t i, j;
     int errs = 0;
-    /* Helper macros — push with format and bump error counter. */
+    /* Helper macros — push with format and bump error counter. Every call
+     * site passes at least one variadic arg, so plain __VA_ARGS__ (C11) is
+     * sufficient and avoids the GNU `, ##__VA_ARGS__` extension. */
     #define TG3__IDX_BAD(path_str, fmt, ...) do { \
         if (errs < TG3__IDX_ERR_CAP) { \
             tg3__error_pushf(ctx->errors, ctx->arena, TG3_SEVERITY_ERROR, \
-                             TG3_ERR_INVALID_INDEX, (path_str), fmt, ##__VA_ARGS__); \
+                             TG3_ERR_INVALID_INDEX, (path_str), fmt, __VA_ARGS__); \
         } \
         ++errs; \
     } while (0)
     #define TG3__CHECK_REQ(idx, max, path_str, fmt, ...) do { \
         if ((idx) < 0 || (uint32_t)(idx) >= (max)) { \
-            TG3__IDX_BAD(path_str, fmt, ##__VA_ARGS__); \
+            TG3__IDX_BAD(path_str, fmt, __VA_ARGS__); \
         } \
     } while (0)
     #define TG3__CHECK_OPT(idx, max, path_str, fmt, ...) do { \
         if ((idx) != -1 && ((idx) < 0 || (uint32_t)(idx) >= (max))) { \
-            TG3__IDX_BAD(path_str, fmt, ##__VA_ARGS__); \
+            TG3__IDX_BAD(path_str, fmt, __VA_ARGS__); \
         } \
     } while (0)