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

## Motivation  
`-Wunreachable-code` missed—or in rare cases crashed on—tautological
comparisons such as

```cpp
x != 0 || x != 1.0   // always true
x == 0 && x == 1.0   // always false
```

when the *same* variable appears on both sides but one operand goes
through a floating‑rank promotion that is target‑dependent. On back‑ends
with **native half‑precision** (`_Float16` / `__fp16`) such as
AArch64 `+fullfp16`, no promotion occurs, so the cast stacks between the
two operands differ and the existing heuristic bails out.

## Technical description 
* **Extends `Expr::isSameComparisonOperand()`** – the helper now ignores
parentheses **and value‑preserving implicit casts**
(`CK_LValueToRValue`, floating‑rank `CK_FloatingCast`) before comparing
the underlying operands. This prevents floating‑rank promotions and
lvalue‑to‑rvalue conversions from blocking the unreachable‑code
diagnostic on targets with native FP16.
*No change needed in `CheckIncorrectLogicOperator`; it simply benefits
from the improved helper.*
* **Regression test** – `warn-unreachable_crash.cpp` updated to cover
both the promoted case (x86‑64) and the native‑half case
(AArch64 `+fullfp16`).
* **Docs** – release‑note bullet added under *Bug Fixes in This
Version*.

@ziqingluo-90 @yronglin Could you please review promptly? (feel free to
also merge it on my behalf) Thanks!

Fixes llvm#149967

Co-authored-by: Zeeshan Siddiqui <[email protected]>

Author: 2 people

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
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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.

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;
 }