[flang] Fix crash in error recovery

[klausler]

EQUIVALENCE set processing assumes that any arrays have explicit shape and constant lower bounds. When an erroneous program violates those assumptions, the compiler crashes. Fix.

Fixes #92636.

[llvmbot]

@llvm/pr-subscribers-flang-semantics

Author: Peter Klausler (klausler)

Changes

EQUIVALENCE set processing assumes that any arrays have explicit shape and constant lower bounds. When an erroneous program violates those assumptions, the compiler crashes. Fix.

Fixes #92636.

---

Full diff: https://github.com/llvm/llvm-project/pull/92786.diff

4 Files Affected:

• (modified) flang/include/flang/Semantics/symbol.h (+1)
• (modified) flang/lib/Semantics/resolve-names-utils.cpp (+27-17)
• (modified) flang/lib/Semantics/resolve-names.cpp (+1-1)
• (modified) flang/lib/Semantics/symbol.cpp (+10-2)

diff --git a/flang/include/flang/Semantics/symbol.h b/flang/include/flang/Semantics/symbol.h
index 50f7b68d80cb1..f130036d949d7 100644
--- a/flang/include/flang/Semantics/symbol.h
+++ b/flang/include/flang/Semantics/symbol.h
@@ -815,6 +815,7 @@ class Symbol {
   void SetIsExplicitBindName(bool);
   bool IsFuncResult() const;
   bool IsObjectArray() const;
+  const ArraySpec *GetShape() const;
   bool IsSubprogram() const;
   bool IsFromModFile() const;
   bool HasExplicitInterface() const {
diff --git a/flang/lib/Semantics/resolve-names-utils.cpp b/flang/lib/Semantics/resolve-names-utils.cpp
index 3ca460b8e46ac..e27a54361749f 100644
--- a/flang/lib/Semantics/resolve-names-utils.cpp
+++ b/flang/lib/Semantics/resolve-names-utils.cpp
@@ -376,25 +376,35 @@ static void PropagateSaveAttr(const EquivalenceSet &src, EquivalenceSet &dst) {
 
 void EquivalenceSets::AddToSet(const parser::Designator &designator) {
   if (CheckDesignator(designator)) {
-    Symbol &symbol{*currObject_.symbol};
-    if (!currSet_.empty()) {
-      // check this symbol against first of set for compatibility
-      Symbol &first{currSet_.front().symbol};
-      CheckCanEquivalence(designator.source, first, symbol) &&
-          CheckCanEquivalence(designator.source, symbol, first);
-    }
-    auto subscripts{currObject_.subscripts};
-    if (subscripts.empty() && symbol.IsObjectArray()) {
-      // record a whole array as its first element
-      for (const ShapeSpec &spec : symbol.get<ObjectEntityDetails>().shape()) {
-        auto &lbound{spec.lbound().GetExplicit().value()};
-        subscripts.push_back(evaluate::ToInt64(lbound).value());
+    if (Symbol * symbol{currObject_.symbol}) {
+      if (!currSet_.empty()) {
+        // check this symbol against first of set for compatibility
+        Symbol &first{currSet_.front().symbol};
+        CheckCanEquivalence(designator.source, first, *symbol) &&
+            CheckCanEquivalence(designator.source, *symbol, first);
+      }
+      auto subscripts{currObject_.subscripts};
+      if (subscripts.empty()) {
+        if (const ArraySpec * shape{symbol->GetShape()};
+            shape && shape->IsExplicitShape()) {
+          // record a whole array as its first element
+          for (const ShapeSpec &spec : *shape) {
+            if (auto lbound{spec.lbound().GetExplicit()}) {
+              if (auto lbValue{evaluate::ToInt64(*lbound)}) {
+                subscripts.push_back(*lbValue);
+                continue;
+              }
+            }
+            subscripts.clear(); // error recovery
+            break;
+          }
+        }
       }
+      auto substringStart{currObject_.substringStart};
+      currSet_.emplace_back(
+          *symbol, subscripts, substringStart, designator.source);
+      PropagateSaveAttr(currSet_.back(), currSet_);
     }
-    auto substringStart{currObject_.substringStart};
-    currSet_.emplace_back(
-        symbol, subscripts, substringStart, designator.source);
-    PropagateSaveAttr(currSet_.back(), currSet_);
   }
   currObject_ = {};
 }
diff --git a/flang/lib/Semantics/resolve-names.cpp b/flang/lib/Semantics/resolve-names.cpp
index a46c0f378d5d0..2622fa1e50747 100644
--- a/flang/lib/Semantics/resolve-names.cpp
+++ b/flang/lib/Semantics/resolve-names.cpp
@@ -6227,7 +6227,7 @@ void DeclarationVisitor::CheckEquivalenceSets() {
     }
     for (const parser::EquivalenceObject &object : *set) {
       const auto &designator{object.v.value()};
-      // The designator was not resolved when it was encountered so do it now.
+      // The designator was not resolved when it was encountered, so do it now.
       // AnalyzeExpr causes array sections to be changed to substrings as needed
       Walk(designator);
       if (AnalyzeExpr(context(), designator)) {
diff --git a/flang/lib/Semantics/symbol.cpp b/flang/lib/Semantics/symbol.cpp
index 381905b89fb2d..3eb120fd962fa 100644
--- a/flang/lib/Semantics/symbol.cpp
+++ b/flang/lib/Semantics/symbol.cpp
@@ -385,9 +385,17 @@ bool Symbol::IsFuncResult() const {
       details_);
 }
 
+const ArraySpec *Symbol::GetShape() const {
+  if (const auto *details{std::get_if<ObjectEntityDetails>(&details_)}) {
+    return &details->shape();
+  } else {
+    return nullptr;
+  }
+}
+
 bool Symbol::IsObjectArray() const {
-  const auto *details{std::get_if<ObjectEntityDetails>(&details_)};
-  return details && details->IsArray();
+  const ArraySpec *shape{GetShape()};
+  return shape && !shape->empty();
 }
 
 bool Symbol::IsSubprogram() const {