[clang] [Clang] Fix crash on subscripting a complete matrix subscript expression (PR #207317)

[via cfe-commits]

https://github.com/Qixi-1 updated https://github.com/llvm/llvm-project/pull/207317

>From d20805f1bf368175d0c46706a558e19db0f56dc2 Mon Sep 17 00:00:00 2001
From: Qixi-1 <tjx2361869673 at gmail.com>
Date: Thu, 11 Jun 2026 15:19:21 +0800
Subject: [PATCH] [Clang] Fix crash on subscripting a complete matrix subscript
 expression

Subscripting a complete MatrixSubscriptExpr (which has scalar type)
caused an assertion failure in ActOnArraySubscriptExpr because the code
unconditionally asserted isIncomplete() on any MatrixSubscriptExpr base.

Fix by guarding the matrix subscript path with an isIncomplete() check,
allowing complete matrix subscript expressions to fall through to the
standard subscript handling, which emits an appropriate diagnostic.

Fixes #203163
---
 clang/lib/Sema/SemaExpr.cpp             | 4 +---
 clang/test/Sema/matrix-type-operators.c | 5 +++++
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index ad6e7183cb3a4..22d7aa60d41ed 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -5078,12 +5078,10 @@ ExprResult Sema::ActOnArraySubscriptExpr(Scope *S, Expr *base,
   // If the base is a MatrixSubscriptExpr, try to create a new
   // MatrixSubscriptExpr.
   auto *matSubscriptE = dyn_cast<MatrixSubscriptExpr>(base);
-  if (matSubscriptE) {
+  if (matSubscriptE && matSubscriptE->isIncomplete()) {
     if (CheckAndReportCommaError(ArgExprs.front()))
       return ExprError();
 
-    assert(matSubscriptE->isIncomplete() &&
-           "base has to be an incomplete matrix subscript");
     return CreateBuiltinMatrixSubscriptExpr(matSubscriptE->getBase(),
                                             matSubscriptE->getRowIdx(),
                                             ArgExprs.front(), rbLoc);
diff --git a/clang/test/Sema/matrix-type-operators.c b/clang/test/Sema/matrix-type-operators.c
index c83685fc7c640..b31802608d003 100644
--- a/clang/test/Sema/matrix-type-operators.c
+++ b/clang/test/Sema/matrix-type-operators.c
@@ -228,3 +228,8 @@ float *address_of_element(sx5x10_t *a) {
   return &(*a)[0][1];
   // expected-error at -1 {{address of matrix element requested}}
 }
+
+void test_triple_subscript(sx5x10_t a) {
+  float v13 = a[0][0][0];
+  // expected-error at -1 {{subscripted value is not an array, pointer, or vector}}
+}
\ No newline at end of file