[flang][CUDA] Fix crash in name resolution for CUDA function (#108616)

When a function result type appears on a FUNCTION statement after some
CUDA attributes, there wasn't always valid program source location
information attached to the function result variable information stack.
Ensure that some relevant source information is always available.

Author: klausler

flang/lib/Semantics/resolve-names.cpp

@@ -474,15 +474,15 @@ class FuncResultStack {
   ~FuncResultStack();
 
   struct FuncInfo {
-    explicit FuncInfo(const Scope &s) : scope{s} {}
+    FuncInfo(const Scope &s, SourceName at) : scope{s}, source{at} {}
     const Scope &scope;
+    SourceName source;
     // Parse tree of the type specification in the FUNCTION prefix
     const parser::DeclarationTypeSpec *parsedType{nullptr};
     // Name of the function RESULT in the FUNCTION suffix, if any
     const parser::Name *resultName{nullptr};
     // Result symbol
     Symbol *resultSymbol{nullptr};
-    std::optional<SourceName> source;
     bool inFunctionStmt{false}; // true between Pre/Post of FunctionStmt
   };
 
@@ -492,7 +492,9 @@ class FuncResultStack {
   void CompleteTypeIfFunctionResult(Symbol &);
 
   FuncInfo *Top() { return stack_.empty() ? nullptr : &stack_.back(); }
-  FuncInfo &Push(const Scope &scope) { return stack_.emplace_back(scope); }
+  FuncInfo &Push(const Scope &scope, SourceName at) {
+    return stack_.emplace_back(scope, at);
+  }
   void Pop();
 
 private:
@@ -3799,11 +3801,13 @@ bool SubprogramVisitor::Pre(const parser::PrefixSpec &x) {
   if (const auto *parsedType{std::get_if<parser::DeclarationTypeSpec>(&x.u)}) {
     if (FuncResultStack::FuncInfo * info{funcResultStack().Top()}) {
       if (info->parsedType) { // C1543
-        Say(currStmtSource().value(),
+        Say(currStmtSource().value_or(info->source),
             "FUNCTION prefix cannot specify the type more than once"_err_en_US);
       } else {
         info->parsedType = parsedType;
-        info->source = currStmtSource();
+        if (auto at{currStmtSource()}) {
+          info->source = *at;
+        }
       }
     } else {
       Say(currStmtSource().value(),
@@ -3978,6 +3982,9 @@ bool SubprogramVisitor::Pre(const parser::FunctionStmt &) {
   FuncResultStack::FuncInfo &info{DEREF(funcResultStack().Top())};
   CHECK(!info.inFunctionStmt);
   info.inFunctionStmt = true;
+  if (auto at{currStmtSource()}) {
+    info.source = *at;
+  }
   return BeginAttrs();
 }
 bool SubprogramVisitor::Pre(const parser::EntryStmt &) { return BeginAttrs(); }
@@ -4507,7 +4514,7 @@ Symbol &SubprogramVisitor::PushSubprogramScope(const parser::Name &name,
   symbol->set(subpFlag);
   PushScope(Scope::Kind::Subprogram, symbol);
   if (subpFlag == Symbol::Flag::Function) {
-    funcResultStack().Push(currScope());
+    funcResultStack().Push(currScope(), name.source);
   }
   if (inInterfaceBlock()) {
     auto &details{symbol->get<SubprogramDetails>()};