From 2c5479e7891edc2f9aae1f0daaa4555c9b60d22f Mon Sep 17 00:00:00 2001
From: James <101812526+WoodchuckXL@users.noreply.github.com>
Date: Thu, 18 Jun 2026 16:34:11 -0400
Subject: [PATCH] Forced metafunction resolver to check arity on monoarity
 functions

---
 src/classes/metafunction.c         | 12 ++++++++++--
 test/matrix/identity.morpho        |  2 +-
 test/string/err_split_args2.morpho |  3 +++
 test/string/err_split_args3.morpho |  4 ++++
 test/string/err_split_args4.morpho |  5 +++++
 5 files changed, 23 insertions(+), 3 deletions(-)
 create mode 100644 test/string/err_split_args2.morpho
 create mode 100644 test/string/err_split_args3.morpho
 create mode 100644 test/string/err_split_args4.morpho

diff --git a/src/classes/metafunction.c b/src/classes/metafunction.c
index 7f5fa2ba..31099203 100644
--- a/src/classes/metafunction.c
+++ b/src/classes/metafunction.c
@@ -1175,6 +1175,14 @@ static bool mfcompiler_emitresolver(mfcompiler *compiler, int nresolutions, mfco
         exactpath.knownarity = resolutions[0].nparams;
         exactpath.aritychecked = true;
         path = &exactpath;
+
+        // Add a sparse to verify that nargs matches the known arity
+        mfindx deflt;
+        mfcompiler_emitfail(compiler, &deflt);
+        mfcompilersparseentry correctarity;
+        correctarity.value = path->knownarity;
+        ERR_CHECK_RETURN(mfcompiler_emitresolver(compiler, nresolutions, resolutions, path, &correctarity.target));
+        return mfcompiler_emitsparse(compiler, 1, &correctarity, deflt, entry);
     }
 
     /* For exact arity without typed params, emit the lone fixed-arity winner. */
@@ -1298,7 +1306,7 @@ void metafunction_disassemble(objectmetafunction *fn) {
 static bool metafunction_runresolver(objectmetafunction *fn, int nargs, value *args, error *err, value *out) {
     mfinstruction *instructions = fn->resolver.data;
     if (!instructions) return metafunction_resolveslow(fn, nargs, args, err, out);
-    
+
     mfindx pc = fn->entry;
     int reg = nargs; // Single register initialized with nargs
     
@@ -1307,7 +1315,7 @@ static bool metafunction_runresolver(objectmetafunction *fn, int nargs, value *a
             case MFOP_SLOW:
                 return metafunction_resolveslow(fn, nargs, args, err, out);
             case MFOP_RESOLVE: {
-                pc++; *out=fn->fns.data[instructions[pc]];
+                *out=fn->fns.data[instructions[++pc]];
                 return true;
             }
             case MFOP_FAIL:
diff --git a/test/matrix/identity.morpho b/test/matrix/identity.morpho
index ba47e170..be369c82 100644
--- a/test/matrix/identity.morpho
+++ b/test/matrix/identity.morpho
@@ -12,4 +12,4 @@ print a
 // expect: [ 0 0 1 ]
 
 a = IdentityMatrix()
-// expect error 'MtrxIdnttyCns'
+// expect error 'MltplDsptchFld'
diff --git a/test/string/err_split_args2.morpho b/test/string/err_split_args2.morpho
new file mode 100644
index 00000000..443c078d
--- /dev/null
+++ b/test/string/err_split_args2.morpho
@@ -0,0 +1,3 @@
+
+print "Hello world".split(2)
+// expect error 'MltplDsptchFld'
diff --git a/test/string/err_split_args3.morpho b/test/string/err_split_args3.morpho
new file mode 100644
index 00000000..8e0cd01f
--- /dev/null
+++ b/test/string/err_split_args3.morpho
@@ -0,0 +1,4 @@
+// You can not add extra arguments to a function
+
+print "Hello world".split(" ", 2)
+// expect error 'MltplDsptchFld'
diff --git a/test/string/err_split_args4.morpho b/test/string/err_split_args4.morpho
new file mode 100644
index 00000000..a20dc5a9
--- /dev/null
+++ b/test/string/err_split_args4.morpho
@@ -0,0 +1,5 @@
+print "Hello world".split(" ")
+// expect: [ Hello, world ]
+
+print "Hello world".split()
+// expect error 'MltplDsptchFld'