From 0ed917cdc21082aa6d80a0d38d6e930245c4fd17 Mon Sep 17 00:00:00 2001
From: Christoffer Lerno <christoffer@aegik.com>
Date: Mon, 23 Feb 2026 19:15:23 +0100
Subject: [PATCH] Improve error when trying to use an extern const as a compile
 time constant. #2969

---
 releasenotes.md                                |  1 +
 src/compiler/sema_decls.c                      | 18 ++++++++++++++++--
 src/compiler/sema_expr.c                       | 10 +++++++++-
 .../compile_time/extern_is_not_const.c3        | 16 ++++++++++++++++
 4 files changed, 42 insertions(+), 3 deletions(-)
 create mode 100644 test/test_suite/compile_time/extern_is_not_const.c3

diff --git a/releasenotes.md b/releasenotes.md
index cfb679486..b60a0532f 100644
--- a/releasenotes.md
+++ b/releasenotes.md
@@ -20,6 +20,7 @@
 - Constdef declarations introduced.
 - Properly support `@deprecated` as contract.
 - Support deprecating enum values.
+- Improve error when trying to use an extern const as a compile time constant. #2969
 
 ### Stdlib changes
 - Summarize sort macros as generic function wrappers to reduce the amount of generated code. #2831
diff --git a/src/compiler/sema_decls.c b/src/compiler/sema_decls.c
index 7769a20dd..2783745b6 100644
--- a/src/compiler/sema_decls.c
+++ b/src/compiler/sema_decls.c
@@ -4707,6 +4707,20 @@ static bool sema_analyse_variable_type(SemaContext *context, Type *type, SourceS
 	UNREACHABLE
 }
 
+INLINE void sema_error_not_constant(SemaContext *context, Decl *target, Expr *init)
+{
+	if (init->expr_kind == EXPR_IDENTIFIER)
+	{
+		Decl *decl = init->ident_expr;
+		if (decl->decl_kind == DECL_VAR && decl->var.kind == VARDECL_CONST && decl->is_extern)
+		{
+			SEMA_ERROR(init, "%s must be assigned a compile-time constant. 'extern' constants are resolved at link time and therefore are not compile-time values.", target->name);
+			return;
+		}
+	}
+	SEMA_ERROR(init, "Expected a compile time constant value assigned to %s.", target->name);
+
+}
 /**
  * Analyse $foo and $Foo variables.
  */
@@ -4792,7 +4806,7 @@ bool sema_analyse_var_decl_ct(SemaContext *context, Decl *decl, bool *check_defi
 				if (!expr_is_runtime_const(init))
 				{
 					if (check_defined) goto FAIL_CHECK;
-					SEMA_ERROR(init, "Expected a constant expression assigned to %s.", decl->name);
+					sema_error_not_constant(context, decl, init);
 					goto FAIL;
 				}
 				break;
@@ -4811,7 +4825,7 @@ bool sema_analyse_var_decl_ct(SemaContext *context, Decl *decl, bool *check_defi
 				if (!expr_is_runtime_const(init))
 				{
 					if (check_defined) goto FAIL_CHECK;
-					SEMA_ERROR(init, "Expected a constant expression assigned to %s.", decl->name);
+					sema_error_not_constant(context, decl, init);
 					goto FAIL;
 				}
 				// Update the type.
diff --git a/src/compiler/sema_expr.c b/src/compiler/sema_expr.c
index 644f7e5d5..b93a768de 100644
--- a/src/compiler/sema_expr.c
+++ b/src/compiler/sema_expr.c
@@ -1679,7 +1679,15 @@ static bool sema_analyse_parameter(SemaContext *context, Expr *arg, Decl *param,
 				{
 					RETURN_SEMA_FUNC_ERROR(definition, arg, "This method is only valid on a compile time constant value.");
 				}
-				RETURN_SEMA_FUNC_ERROR(definition, arg, "A compile time parameter must always be a constant, did you mistake it for a normal parameter?");
+				if (arg->expr_kind == EXPR_IDENTIFIER)
+				{
+					Decl *decl = arg->ident_expr;
+					if (decl->decl_kind == DECL_VAR && decl->var.kind == VARDECL_CONST && decl->is_extern)
+					{
+						RETURN_SEMA_FUNC_ERROR(definition, arg, "A compile-time parameter requires a value known at compile time. Did you mistake if for a regular parameter? 'extern' constants are not compile-time constants.");
+					}
+				}
+				RETURN_SEMA_FUNC_ERROR(definition, arg, "A compile time parameter must always be a compile-time constant value, did you mistake it for a regular parameter?");
 			}
 			break;
 		case VARDECL_PARAM_CT_TYPE:
diff --git a/test/test_suite/compile_time/extern_is_not_const.c3 b/test/test_suite/compile_time/extern_is_not_const.c3
new file mode 100644
index 000000000..0f6682b74
--- /dev/null
+++ b/test/test_suite/compile_time/extern_is_not_const.c3
@@ -0,0 +1,16 @@
+module test;
+import std;
+
+extern const void* FOO @cname("malloc");
+
+macro foo($x) {}
+
+fn void test()
+{
+	var $x = FOO; // #error: $x must be assigned a compile-time constant. 'extern' constants are resolved
+}
+
+fn void test2()
+{
+	foo(FOO); // #error: $x must be assigned a compile-time constant. 'extern' constants are resolved
+}