test(extern): check variadic promotions with va_arg

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

5 files changed, 45 insertions, 0 deletions

diff --git a/tests/i/extern-c-variadic/case.json b/tests/i/extern-c-variadic/case.json
new file mode 100644
index 0000000..1ce5970
--- /dev/null
+++ b/tests/i/extern-c-variadic/case.json
@@ -0,0 +1,5 @@
+{
+  "objects": [
+    {"source": "lib.c", "output": "lib.o"}
+  ]
+}
diff --git a/tests/i/extern-c-variadic/expected.txt b/tests/i/extern-c-variadic/expected.txt
new file mode 100644
index 0000000..8920df8
--- /dev/null
+++ b/tests/i/extern-c-variadic/expected.txt
@@ -0,0 +1,2 @@
+65794
+3.750000
diff --git a/tests/i/extern-c-variadic/lib.c b/tests/i/extern-c-variadic/lib.c
new file mode 100644
index 0000000..8f723a5
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+++ b/tests/i/extern-c-variadic/lib.c
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+#include <stdarg.h>
+
+/* Each argument after n arrives promoted, so va_arg reads int or double. */
+int sum_ints(int n, ...) {
+    va_list ap;
+    int total = 0;
+    va_start(ap, n);
+    for (int i = 0; i < n; ++i) total += va_arg(ap, int);
+    va_end(ap);
+    return total;
+}
+
+double sum_doubles(int n, ...) {
+    va_list ap;
+    double total = 0;
+    va_start(ap, n);
+    for (int i = 0; i < n; ++i) total += va_arg(ap, double);
+    va_end(ap);
+    return total;
+}
diff --git a/tests/i/extern-c-variadic/notes.md b/tests/i/extern-c-variadic/notes.md
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+++ b/tests/i/extern-c-variadic/notes.md
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+# extern-c-variadic
+
+Human review: pending. Expected output was independently traced by AI, not captured from cpsc.
+
+Basis: compiler extension (EXTERN "C", INCLUDE_OBJ of a user object); C11 6.5.2.2p6-7 default argument promotions and 7.16.1.1 va_arg.
+
+Trace: the harness compiles `lib.c` with the host C compiler. sum_ints reads 5 arguments with va_arg(ap, int), so each must arrive promoted to int: unsigned char 200 gives 200, short -7 gives -7, unsigned short 65535 gives 65535 (int is wider than short), _Bool TRUE gives 1 and char 'A' gives 65. 200 - 7 + 65535 + 1 + 65 = 65794. sum_doubles reads 2 arguments with va_arg(ap, double): float 1.5 is promoted to double 1.5, and double 2.25 is passed unchanged. 1.5 + 2.25 = 3.75, printed like other C doubles as `3.750000`.
diff --git a/tests/i/extern-c-variadic/program.cps b/tests/i/extern-c-variadic/program.cps
new file mode 100644
index 0000000..2f2ea43
--- /dev/null
+++ b/tests/i/extern-c-variadic/program.cps
@@ -0,0 +1,11 @@
+// Variadic EXTERN arguments arrive with C default promotions applied.
+INCLUDE_OBJ "lib.o"
+TYPE UChar = C_TYPE("unsigned char")
+TYPE CShort = C_TYPE("short")
+TYPE UShort = C_TYPE("unsigned short")
+TYPE CBool = C_TYPE("_Bool")
+TYPE CFloat = C_TYPE("float")
+EXTERN "C" EXTERN_FUNCTION sum_ints(C_INT, ...) RETURNS C_INT
+EXTERN "C" EXTERN_FUNCTION sum_doubles(C_INT, ...) RETURNS C_DOUBLE
+OUTPUT sum_ints((C_INT)5, (UChar)200, (CShort)-7, (UShort)65535, (CBool)TRUE, (C_CHAR)'A')
+OUTPUT sum_doubles((C_INT)2, (CFloat)1.5, (C_DOUBLE)2.25)