[clang] [clang-repl] Suppress [[nodiscard]] warnings for REPL printed express (PR #178661)

[via cfe-commits]

https://github.com/bala-bhargav updated https://github.com/llvm/llvm-project/pull/178661

>From 6f5493409ed22734fbb0b3710a46ff31d281b1a5 Mon Sep 17 00:00:00 2001
From: bhargav <penugondabalabharghav at gmail.com>
Date: Thu, 12 Feb 2026 15:38:32 +0530
Subject: [PATCH] [clang-repl] Suppress [[nodiscard]] warnings for REPL printed
 expressions

In clang-repl, expressions typed without a semicolon have their values
printed by the value printing mechanism. Since the result is used (for
printing), we should not emit [[nodiscard]] warnings for these
expressions.

This suppresses warn_unused_result during parsing (since we don't know
the semicolon status yet), then after parsing, re-enables it and
diagnoses only for TopLevelStmtDecl expressions where the semicolon IS
present (value is being discarded). This ensures [[nodiscard]] warnings
still fire for genuinely discarded results like getValue();

All changes are confined to Interpreter.cpp, following the reviewer's
guidance.

Fixes #178595
---
 clang/lib/Interpreter/Interpreter.cpp | 21 +++++++++++++++++++++
 clang/test/Interpreter/nodiscard.cpp  | 25 +++++++++++++++++++++++++
 2 files changed, 46 insertions(+)
 create mode 100644 clang/test/Interpreter/nodiscard.cpp

diff --git a/clang/lib/Interpreter/Interpreter.cpp b/clang/lib/Interpreter/Interpreter.cpp
index 9c94cfa5ee381..8c8199e184a4e 100644
--- a/clang/lib/Interpreter/Interpreter.cpp
+++ b/clang/lib/Interpreter/Interpreter.cpp
@@ -40,6 +40,7 @@
 #include "clang/Options/OptionUtils.h"
 #include "clang/Options/Options.h"
 #include "clang/Sema/Lookup.h"
+#include "clang/Sema/Sema.h"
 #include "clang/Serialization/ObjectFilePCHContainerReader.h"
 #include "llvm/ExecutionEngine/JITSymbol.h"
 #include "llvm/ExecutionEngine/Orc/EPCDynamicLibrarySearchGenerator.h"
@@ -469,11 +470,31 @@ Interpreter::Parse(llvm::StringRef Code) {
   // printing could cause it.
   getCompilerInstance()->getDiagnostics().setSeverity(
       clang::diag::warn_unused_expr, diag::Severity::Ignored, SourceLocation());
+  // Suppress [[nodiscard]] warnings during parsing since we don't know yet
+  // if the expression has a missing semicolon (value printed) or not.
+  // If the value is printed, it's considered "used" so no warning is needed.
+  getCompilerInstance()->getDiagnostics().setSeverity(
+      clang::diag::warn_unused_result, diag::Severity::Ignored,
+      SourceLocation());
 
   llvm::Expected<TranslationUnitDecl *> TuOrErr = IncrParser->Parse(Code);
   if (!TuOrErr)
     return TuOrErr.takeError();
 
+  // After parsing, re-enable [[nodiscard]] warnings and diagnose for
+  // top-level expressions where the semicolon IS present (value discarded).
+  // Expressions without semicolons have their values printed, so they are
+  // considered "used" and should not trigger [[nodiscard]] warnings.
+  getCompilerInstance()->getDiagnostics().setSeverity(
+      clang::diag::warn_unused_result, diag::Severity::Warning,
+      SourceLocation());
+  for (Decl *D : (*TuOrErr)->decls()) {
+    if (auto *TLSD = llvm::dyn_cast<TopLevelStmtDecl>(D))
+      if (!TLSD->isSemiMissing())
+        getCompilerInstance()->getSema().DiagnoseUnusedExprResult(
+            TLSD->getStmt(), diag::warn_unused_result);
+  }
+
   PartialTranslationUnit &LastPTU = IncrParser->RegisterPTU(*TuOrErr);
 
   return LastPTU;
diff --git a/clang/test/Interpreter/nodiscard.cpp b/clang/test/Interpreter/nodiscard.cpp
new file mode 100644
index 0000000000000..154e8ae39f618
--- /dev/null
+++ b/clang/test/Interpreter/nodiscard.cpp
@@ -0,0 +1,25 @@
+// REQUIRES: host-supports-jit
+// RUN: cat %s | clang-repl 2>&1 | FileCheck %s
+
+// Test that [[nodiscard]] warnings are suppressed for REPL top-level
+// expressions that will have their values printed (no semicolon),
+// but are still emitted when the value is actually discarded (with semicolon).
+
+extern "C" int printf(const char*,...);
+
+[[nodiscard]] int getValue() { return 42; }
+
+// Negative test: Warning when value is discarded (with semicolon)
+getValue();
+// CHECK: warning: ignoring return value of function declared with 'nodiscard' attribute
+
+// Positive test: No warning when expression value is printed (no semicolon)
+getValue()
+// CHECK: (int) 42
+
+// Verify assignment doesn't warn
+int x = getValue();
+printf("x = %d\n", x);
+// CHECK: x = 42
+
+%quit