feat(codegen): allow identical EXTERN redeclarations across files

Repeated EXTERN declarations with the same signature share one C symbol,
as C declarations do; conflicting signatures are errors.

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

5 files changed, 27 insertions, 0 deletions

diff --git a/tests/i/multifile-extern/case.json b/tests/i/multifile-extern/case.json
new file mode 100644
index 0000000..032ec6a
--- /dev/null
+++ b/tests/i/multifile-extern/case.json
@@ -0,0 +1,6 @@
+{
+  "sources": [
+    "program.cps",
+    "lib.cps"
+  ]
+}
diff --git a/tests/i/multifile-extern/expected.txt b/tests/i/multifile-extern/expected.txt
new file mode 100644
index 0000000..19d70eb
--- /dev/null
+++ b/tests/i/multifile-extern/expected.txt
@@ -0,0 +1,2 @@
+3
+8
diff --git a/tests/i/multifile-extern/lib.cps b/tests/i/multifile-extern/lib.cps
new file mode 100644
index 0000000..52ccf1f
--- /dev/null
+++ b/tests/i/multifile-extern/lib.cps
@@ -0,0 +1,6 @@
+INCLUDE_OBJ "__libc"
+TYPE NativeInt = C_TYPE("int")
+EXTERN "C" EXTERN_FUNCTION abs(NativeInt) RETURNS C_INT
+FUNCTION Twice(N : C_INT) RETURNS C_INT
+    RETURN abs(N) + abs(N)
+ENDFUNCTION
diff --git a/tests/i/multifile-extern/notes.md b/tests/i/multifile-extern/notes.md
new file mode 100644
index 0000000..f326c82
--- /dev/null
+++ b/tests/i/multifile-extern/notes.md
@@ -0,0 +1,7 @@
+# multifile-extern
+
+Human review: pending. Expected output was independently traced by AI, not captured from cpsc.
+
+Basis: compiler extension; identical EXTERN declarations may repeat across files, as C declarations may.
+
+Trace: abs(-3)=3. Twice(-4) returns abs(-4)+abs(-4)=8.
diff --git a/tests/i/multifile-extern/program.cps b/tests/i/multifile-extern/program.cps
new file mode 100644
index 0000000..7a0cd2d
--- /dev/null
+++ b/tests/i/multifile-extern/program.cps
@@ -0,0 +1,6 @@
+// Both files declare the same C function; aliases of int are interchangeable.
+INCLUDE "lib.cps"
+INCLUDE_OBJ "__libc"
+EXTERN "C" EXTERN_FUNCTION abs(C_INT) RETURNS C_INT
+OUTPUT abs((C_INT)-3)
+OUTPUT Twice((C_INT)-4)