[clang] [Sema] Fix -Wunreachable-code false negative when operands differ only by implicit casts (PR #149972)

[M. Zeeshan Siddiqui via cfe-commits]

https://github.com/codemzs updated https://github.com/llvm/llvm-project/pull/149972

>From c97467c8adefbf494f673d88a7e9806f7ac74827 Mon Sep 17 00:00:00 2001
From: Zeeshan Siddiqui <mzs at ntdev.microsoft.com>
Date: Tue, 22 Jul 2025 06:37:54 +0000
Subject: [PATCH] [Sema] Make incorrect-logic operator check cast-agnostic

The check for tautological comparisons (e.g. `x != 0 || x != 1`)
failed on targets that support native scalar half precision because the
operands differed only by an implicit `FloatingCast`.

Teach CheckIncorrectLogicOperator to retry the operand-equality test
after stripping implicit casts and parentheses.  This preserves the
original, cast-sensitive fast-path and fixes Issue#149967.

Updates existing test exercising both `__fp16` (promoted) and `_Float16`
(native with +fullfp16) on x86-64 and AArch64.
---
 clang/docs/ReleaseNotes.rst                |  5 +++
 clang/lib/AST/Expr.cpp                     | 14 ++++++--
 clang/test/Sema/warn-unreachable_crash.cpp | 41 +++++++++++++++-------
 3 files changed, 45 insertions(+), 15 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 9d9a0008e0001..4a2edae7509de 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -134,6 +134,11 @@ Bug Fixes in This Version
 -------------------------
 - Fix a crash when marco name is empty in ``#pragma push_macro("")`` or
   ``#pragma pop_macro("")``. (#GH149762).
+- `-Wunreachable-code`` now diagnoses tautological or contradictory
+  comparisons such as ``x != 0 || x != 1.0`` and ``x == 0 && x == 1.0`` on
+  targets that treat ``_Float16``/``__fp16`` as native scalar types. Previously
+  the warning was silently lost because the operands differed only by an implicit
+  cast chain. (#GH149967).
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/AST/Expr.cpp b/clang/lib/AST/Expr.cpp
index d85655b1596f4..cd9672d32a013 100644
--- a/clang/lib/AST/Expr.cpp
+++ b/clang/lib/AST/Expr.cpp
@@ -4233,8 +4233,15 @@ bool Expr::isSameComparisonOperand(const Expr* E1, const Expr* E2) {
       // template parameters.
       const auto *DRE1 = cast<DeclRefExpr>(E1);
       const auto *DRE2 = cast<DeclRefExpr>(E2);
-      return DRE1->isPRValue() && DRE2->isPRValue() &&
-             DRE1->getDecl() == DRE2->getDecl();
+
+      if (DRE1->getDecl() != DRE2->getDecl())
+        return false;
+
+      if ((DRE1->isPRValue() && DRE2->isPRValue()) ||
+          (DRE1->isLValue() && DRE2->isLValue()))
+        return true;
+
+      return false;
     }
     case ImplicitCastExprClass: {
       // Peel off implicit casts.
@@ -4244,7 +4251,8 @@ bool Expr::isSameComparisonOperand(const Expr* E1, const Expr* E2) {
         if (!ICE1 || !ICE2)
           return false;
         if (ICE1->getCastKind() != ICE2->getCastKind())
-          return false;
+          return isSameComparisonOperand(ICE1->IgnoreParenImpCasts(),
+                                         ICE2->IgnoreParenImpCasts());
         E1 = ICE1->getSubExpr()->IgnoreParens();
         E2 = ICE2->getSubExpr()->IgnoreParens();
         // The final cast must be one of these types.
diff --git a/clang/test/Sema/warn-unreachable_crash.cpp b/clang/test/Sema/warn-unreachable_crash.cpp
index 628abcc83f810..1955c2c2547f5 100644
--- a/clang/test/Sema/warn-unreachable_crash.cpp
+++ b/clang/test/Sema/warn-unreachable_crash.cpp
@@ -1,16 +1,33 @@
-// RUN: %clang_cc1 -verify -Wunreachable-code %s
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -verify -Wunreachable-code %s
+// RUN: %clang_cc1 -triple aarch64-unknown-linux-gnu -target-feature +fullfp16 -verify -Wunreachable-code %s
+// REQUIRES: aarch64-registered-target
 
-// Previously this test will crash
-static void test(__fp16& x) {
-  if (x != 0 || x != 1.0) { // expected-note{{}} no-crash
-      x = 0.9;
-    } else
-      x = 0.8; // expected-warning{{code will never be executed}}
+// =======  __fp16 version  =======
+static void test_fp16(__fp16 &x) {
+  if (x != 0 || x != 1.0) {           // expected-note {{}} no-crash
+    x = 0.9;
+  } else
+    x = 0.8;                          // expected-warning{{code will never be executed}}
 }
 
-static void test2(__fp16& x) {
-  if (x != 1 && x == 1.0) { // expected-note{{}} no-crash
-      x = 0.9; // expected-warning{{code will never be executed}}
-    } else
-      x = 0.8;
+static void test_fp16_b(__fp16 &x) {
+  if (x != 1 && x == 1.0) {           // expected-note {{}} no-crash
+    x = 0.9;                          // expected-warning{{code will never be executed}}
+  } else
+    x = 0.8;
+}
+
+// =======  _Float16 version  =======
+static void test_f16(_Float16 &x) {
+  if (x != 0 || x != 1.0) {           // expected-note {{}} no-crash
+    x = 0.9;
+  } else
+    x = 0.8;                          // expected-warning{{code will never be executed}}
+}
+
+static void test_f16_b(_Float16 &x) {
+  if (x != 1 && x == 1.0) {           // expected-note {{}} no-crash
+    x = 0.9;                          // expected-warning{{code will never be executed}}
+  } else
+    x = 0.8;
 }