[flang] Fix crash in error recovery (bad binding)

flang/include/flang/Evaluate/characteristics.h

@@ -386,7 +386,7 @@ struct Procedure {
   bool HasExplicitInterface() const {
     return !attrs.test(Attr::ImplicitInterface);
   }
-  int FindPassIndex(std::optional<parser::CharBlock>) const;
+  std::optional<int> FindPassIndex(std::optional<parser::CharBlock>) const;
   bool CanBeCalledViaImplicitInterface(std::string *whyNot = nullptr) const;
   bool CanOverride(const Procedure &, std::optional<int> passIndex) const;
   bool IsCompatibleWith(const Procedure &, bool ignoreImplicitVsExplicit,

flang/lib/Evaluate/characteristics.cpp

@@ -1333,16 +1333,21 @@ bool Procedure::IsCompatibleWith(const Procedure &actual,
   return false;
 }
 
-int Procedure::FindPassIndex(std::optional<parser::CharBlock> name) const {
+std::optional<int> Procedure::FindPassIndex(
+    std::optional<parser::CharBlock> name) const {
   int argCount{static_cast<int>(dummyArguments.size())};
-  int index{0};
   if (name) {
-    while (index < argCount && *name != dummyArguments[index].name.c_str()) {
-      ++index;
+    for (int index{0}; index < argCount; ++index) {
+      if (*name == dummyArguments[index].name.c_str()) {
+        return index;
+      }
     }
+    return std::nullopt;
+  } else if (argCount > 0) {
+    return 0;
+  } else {
+    return std::nullopt;
   }
-  CHECK(index < argCount);
-  return index;
 }
 
 bool Procedure::CanOverride(

flang/lib/Semantics/check-declarations.cpp

@@ -2430,16 +2430,18 @@ void CheckHelper::CheckProcBinding(
                   "A NOPASS type-bound procedure and its override must have identical interfaces"_err_en_US);
             }
           } else if (!context_.HasError(binding.symbol())) {
-            int passIndex{bindingChars->FindPassIndex(binding.passName())};
-            int overriddenPassIndex{
+            auto passIndex{bindingChars->FindPassIndex(binding.passName())};
+            auto overriddenPassIndex{
                 overriddenChars->FindPassIndex(overriddenBinding->passName())};
-            if (passIndex != overriddenPassIndex) {
-              SayWithDeclaration(*overridden,
-                  "A type-bound procedure and its override must use the same PASS argument"_err_en_US);
-            } else if (!bindingChars->CanOverride(
-                           *overriddenChars, passIndex)) {
-              SayWithDeclaration(*overridden,
-                  "A type-bound procedure and its override must have compatible interfaces"_err_en_US);
+            if (passIndex && overriddenPassIndex) {
+              if (*passIndex != *overriddenPassIndex) {
+                SayWithDeclaration(*overridden,
+                    "A type-bound procedure and its override must use the same PASS argument"_err_en_US);
+              } else if (!bindingChars->CanOverride(
+                             *overriddenChars, passIndex)) {
+                SayWithDeclaration(*overridden,
+                    "A type-bound procedure and its override must have compatible interfaces"_err_en_US);
+              }
             }
           }
         }