From 229fdd6193b8832be1730533b0b45db5ff282c7e Mon Sep 17 00:00:00 2001
From: Christoffer Lerno <christoffer@aegik.com>
Date: Tue, 5 Aug 2025 02:55:32 +0200
Subject: [PATCH] Detect recursive creation of generics #2366.

---
 releasenotes.md                              |  1 +
 src/compiler/compiler_internal.h             |  2 ++
 src/compiler/sema_types.c                    |  6 ++++++
 test/test_suite/generic/recursive_generic.c3 | 14 ++++++++++++++
 4 files changed, 23 insertions(+)
 create mode 100644 test/test_suite/generic/recursive_generic.c3

diff --git a/releasenotes.md b/releasenotes.md
index 33b0a51de..750fcdcbd 100644
--- a/releasenotes.md
+++ b/releasenotes.md
@@ -14,6 +14,7 @@
 - Reduce allocated Vmem for the compiler on 32 bit machines.
 - Bug causing a compiler error when parsing a broken lambda inside of an expression.
 - Fixed: regression in comments for `@deprecated` and `@pure`.
+- Detect recursive creation of generics #2366.
 
 ### Stdlib changes
 - Add `==` to `Pair`, `Triple` and TzDateTime. Add print to `Pair` and `Triple`.
diff --git a/src/compiler/compiler_internal.h b/src/compiler/compiler_internal.h
index b6946b9b2..273efa1c6 100644
--- a/src/compiler/compiler_internal.h
+++ b/src/compiler/compiler_internal.h
@@ -48,6 +48,7 @@ typedef uint16_t FileId;
 #define MAX_BITSTRUCT 0x1000
 #define MAX_MEMBERS ((StructIndex)1) << 15
 #define MAX_ALIGNMENT ((ArrayIndex)(((uint64_t)2) << 28))
+#define MAX_GENERIC_DEPTH 32
 #define MAX_PRIORITY 0xFFFF
 #define MAX_TYPE_SIZE UINT32_MAX
 #define MAX_GLOBAL_DECL_STACK (65536)
@@ -1931,6 +1932,7 @@ typedef struct
 	Linking linking;
 	GlobalContext context;
 	const char *obj_output;
+	int generic_depth;
 } CompilerState;
 
 extern CompilerState compiler;
diff --git a/src/compiler/sema_types.c b/src/compiler/sema_types.c
index a2e9f1f1d..8054d7071 100644
--- a/src/compiler/sema_types.c
+++ b/src/compiler/sema_types.c
@@ -450,8 +450,14 @@ INLINE bool sema_resolve_generic_type(SemaContext *context, TypeInfo *type_info)
 	ASSERT_SPAN(inner, inner->resolve_status == RESOLVE_NOT_DONE);
 
 	bool was_recursive = false;
+	if (compiler.generic_depth >= MAX_GENERIC_DEPTH)
+	{
+		RETURN_SEMA_ERROR(type_info, "Generic resolution of this type has become deeply nested, it was aborted after reaching %d recursions.", compiler.generic_depth);
+	}
+	compiler.generic_depth++;
 	Decl *type = sema_analyse_parameterized_identifier(context, inner->unresolved.path, inner->unresolved.name,
 	                                                   inner->span, type_info->generic.params, &was_recursive);
+	compiler.generic_depth--;
 	if (!decl_ok(type)) return false;
 	if (!sema_analyse_decl(context, type)) return false;
 	type_info->type = type->type;
diff --git a/test/test_suite/generic/recursive_generic.c3 b/test/test_suite/generic/recursive_generic.c3
new file mode 100644
index 000000000..0cdf3475a
--- /dev/null
+++ b/test/test_suite/generic/recursive_generic.c3
@@ -0,0 +1,14 @@
+module test;
+import test1;
+
+fn int main() => 0;
+
+Aa {int} a;
+
+module test1 {Type};
+import test2;
+alias Cc = Aa {Bb {Type}}; // #error: Generic resolution of this type has become deeply nested
+struct Aa {Type a;}
+
+module test2 {Type};
+struct Bb {Type b;}
\ No newline at end of file