From 0fbc25b399824c934b044a0fbf61df9152b6e48c Mon Sep 17 00:00:00 2001
From: Alex Crichton <alex@alexcrichton.com>
Date: Mon, 1 Jun 2026 16:01:31 -0700
Subject: [PATCH] c: Fix type name collisions with anonymous types

Classify fewer types as "primitive" to help get the names to line up.

Closes #1621
---
 crates/c/src/lib.rs              | 11 +++++++++++
 tests/codegen/issue-1621.wit     | 16 ++++++++++++++++
 tests/runtime/flavorful/runner.c |  2 +-
 tests/runtime/flavorful/test.c   |  2 +-
 4 files changed, 29 insertions(+), 2 deletions(-)
 create mode 100644 tests/codegen/issue-1621.wit

diff --git a/crates/c/src/lib.rs b/crates/c/src/lib.rs
index 96345c210..b44848d51 100644
--- a/crates/c/src/lib.rs
+++ b/crates/c/src/lib.rs
@@ -1068,6 +1068,17 @@ fn is_prim_type(resolve: &Resolve, ty: &Type) -> bool {
 }
 
 fn is_prim_type_id(resolve: &Resolve, id: TypeId) -> bool {
+    // A named type contributes its (interface-local) name to the generated
+    // type name rather than its structure, so even when its underlying
+    // representation is primitive it must not be treated as a world-shareable
+    // "primitive" type. Otherwise two interfaces that each define a
+    // differently-typed but identically-named type (e.g. `field-value` in two
+    // versions of `wasi:http/types`) collapse onto the same anonymous type
+    // name, producing structs with incompatible element pointer types.
+    // See https://github.com/bytecodealliance/wit-bindgen/issues/1621.
+    if resolve.types[id].name.is_some() {
+        return false;
+    }
     match &resolve.types[id].kind {
         TypeDefKind::List(elem) => is_prim_type(resolve, elem),
 
diff --git a/tests/codegen/issue-1621.wit b/tests/codegen/issue-1621.wit
new file mode 100644
index 000000000..05ad6edee
--- /dev/null
+++ b/tests/codegen/issue-1621.wit
@@ -0,0 +1,16 @@
+package my:test;
+
+interface a {
+  type t = list<u8>;
+  f: func() -> list<t>;
+}
+
+interface b {
+  type t = list<u8>;
+  g: func() -> list<t>;
+}
+
+world w {
+  import a;
+  import b;
+}
diff --git a/tests/runtime/flavorful/runner.c b/tests/runtime/flavorful/runner.c
index 57b2017f6..c86eb4c27 100644
--- a/tests/runtime/flavorful/runner.c
+++ b/tests/runtime/flavorful/runner.c
@@ -79,7 +79,7 @@ void exports_runner_run() {
     test_list_typedef3_t b;
     b.ptr = &b_str;
     b.len = 1;
-    runner_tuple2_list_typedef2_list_typedef3_t ret;
+    test_tuple2_list_typedef2_list_typedef3_t ret;
     test_list_typedefs(&a, &b, &ret);
 
     assert(memcmp(ret.f0.ptr, "typedef3", ret.f0.len) == 0);
diff --git a/tests/runtime/flavorful/test.c b/tests/runtime/flavorful/test.c
index 8fb6e6dc6..c200b0fd8 100644
--- a/tests/runtime/flavorful/test.c
+++ b/tests/runtime/flavorful/test.c
@@ -65,7 +65,7 @@ bool exports_test_errno_result(exports_test_my_errno_t *err) {
 void exports_test_list_typedefs(
     exports_test_list_typedef_t *a,
     exports_test_list_typedef3_t *c,
-    test_tuple2_list_typedef2_list_typedef3_t *ret) {
+    exports_test_tuple2_list_typedef2_list_typedef3_t *ret) {
   assert(memcmp(a->ptr, "typedef1", a->len) == 0);
   exports_test_list_typedef_free(a);