Warn on dangling comparison operator. (#2779)

Fixes #1685

Co-authored-by: Yong He <yhe@nvidia.com>

4 files changed, 29 insertions, 0 deletions

diff --git a/source/slang/slang-check-stmt.cpp b/source/slang/slang-check-stmt.cpp
index a61f48af6..6efc3e52e 100644
--- a/source/slang/slang-check-stmt.cpp
+++ b/source/slang/slang-check-stmt.cpp
@@ -339,6 +339,16 @@ namespace Slang
     void SemanticsStmtVisitor::visitExpressionStmt(ExpressionStmt *stmt)
     {
         stmt->expression = CheckExpr(stmt->expression);
+        if (auto operatorExpr = as<OperatorExpr>(stmt->expression))
+        {
+            if (auto func = as<VarExpr>(operatorExpr->functionExpr))
+            {
+                if (func->name && func->name->text == "==")
+                {
+                    getSink()->diagnose(operatorExpr, Diagnostics::danglingEqualityExpr);
+                }
+            }
+        }
     }
 
     void SemanticsStmtVisitor::tryInferLoopMaxIterations(ForStmt* stmt)
diff --git a/source/slang/slang-diagnostic-defs.h b/source/slang/slang-diagnostic-defs.h
index fed4944d3..4a9b83c6c 100644
--- a/source/slang/slang-diagnostic-defs.h
+++ b/source/slang/slang-diagnostic-defs.h
@@ -278,6 +278,7 @@ DIAGNOSTIC(30054, Error, targetLabelDoesNotMarkBreakableStmt, "invalid break tar
 DIAGNOSTIC(30055, Error, useOfNonShortCircuitingOperatorInDiffFunc, "non-short-circuiting `?:` operator is not allowed in a differentiable function, use `select` instead.")
 DIAGNOSTIC(30056, Warning, useOfNonShortCircuitingOperator, "non-short-circuiting `?:` operator is deprecated, use 'select' instead.")
 DIAGNOSTIC(30057, Error, assignmentInPredicateExpr, "use an assignment operation as predicate expression is not allowed, wrap the assignment with '()' to clarify the intent.")
+DIAGNOSTIC(30058, Warning, danglingEqualityExpr, "result of '==' not used, did you intend '='?")
 
 DIAGNOSTIC(30043, Error, getStringHashRequiresStringLiteral, "getStringHash parameter can only accept a string literal")
 
diff --git a/tests/diagnostics/dangling-comparison.slang b/tests/diagnostics/dangling-comparison.slang
new file mode 100644
index 000000000..8f0f4b2a5
--- /dev/null
+++ b/tests/diagnostics/dangling-comparison.slang
@@ -0,0 +1,10 @@
+//DIAGNOSTIC_TEST:SIMPLE:
+
+[numthreads(1, 1, 1)]
+void main(
+    uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+    int a = 1;
+    a == 2; // warn
+    (a == 2); // ok.
+}
\ No newline at end of file
diff --git a/tests/diagnostics/dangling-comparison.slang.expected b/tests/diagnostics/dangling-comparison.slang.expected
new file mode 100644
index 000000000..667e2c17a
--- /dev/null
+++ b/tests/diagnostics/dangling-comparison.slang.expected
@@ -0,0 +1,8 @@
+result code = 0
+standard error = {
+tests/diagnostics/dangling-comparison.slang(8): warning 30058: result of '==' not used, did you intend '='?
+    a == 2; // warn
+      ^~
+}
+standard output = {
+}