[flang-commits] [flang] [flang] Emit warnings, not errors, for bad subscripts in dead code (PR #174040)

[Andre Kuhlenschmidt via flang-commits]

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@@ -5372,6 +5371,88 @@ bool ExprChecker::Pre(const parser::DataImpliedDo &ido) {
   return false;
 }
 
+// Handle messages from dead code in block IF constructs
+bool ExprChecker::Pre(const parser::IfConstruct &construct) {
+  enum State {
+    NotYetKnownTrue,
+    WasKnownTrue,
+    TrueNow,
+    FalseNow
+  } state{NotYetKnownTrue};
+  auto &messages{exprAnalyzer_.GetContextualMessages()};
+  parser::CharBlock determiner; // source of relevant IF statement
+
+  auto consider{[&](const SomeExpr *expr, parser::CharBlock here) {
+    if (state == TrueNow) {
+      state = WasKnownTrue;
+    } else if (state != WasKnownTrue) {
+      state = NotYetKnownTrue;
+      if (expr) {
+        if (auto known{
+                evaluate::GetScalarConstantValue<evaluate::LogicalResult>(
+                    *expr)}) {
+          state = known->IsTrue() ? TrueNow : FalseNow;
+          determiner = here;
+        }
+      }
+    }
+  }};
+  auto doBlock{[&, this](const parser::Block &block) {
+    if ((state == FalseNow || state == WasKnownTrue) && messages.messages()) {
+      parser::Messages inDeadCode;
+      {
+        auto restorer{messages.SetMessages(inDeadCode)};
+        parser::Walk(block, *this);
+      }
+      for (auto &msg : inDeadCode.messages()) {
+        if (msg.severity() == parser::Severity::ErrorUnlessDeadCode) {
+          msg.set_severity(parser::Severity::Warning);
+          msg.set_usageWarning(common::UsageWarning::BadValueInDeadCode);
+          msg.Attach(determiner,
+              "in code known to be dead due to this compile-time IF expression value"_en_US);
+        }
+      }
+      if (context_.ShouldWarn(common::UsageWarning::BadValueInDeadCode) ||
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akuhlens wrote:

This logic is a little messy in that it will add warnings when there are errors and will omit them when there are no errors, but the `-Wno-bad-value-in-dead-code` flag is passed.

https://github.com/llvm/llvm-project/pull/174040