[flang] Fix crash from undetected program error. (#159847)

When a function or subroutine interface block conflicts with a name
already in scope, emit an error and avoid a crash.

Fixes #159554.

Author: klausler

flang/lib/Semantics/resolve-names.cpp

@@ -955,7 +955,7 @@ class SubprogramVisitor : public virtual ScopeHandler, public InterfaceVisitor {
   bool HandlePreviousCalls(const parser::Name &, Symbol &, Symbol::Flag);
   const Symbol *CheckExtantProc(const parser::Name &, Symbol::Flag);
   // Create a subprogram symbol in the current scope and push a new scope.
-  Symbol &PushSubprogramScope(const parser::Name &, Symbol::Flag,
+  Symbol *PushSubprogramScope(const parser::Name &, Symbol::Flag,
       const parser::LanguageBindingSpec * = nullptr,
       bool hasModulePrefix = false);
   Symbol *GetSpecificFromGeneric(const parser::Name &);
@@ -4492,10 +4492,13 @@ bool SubprogramVisitor::HandleStmtFunction(const parser::StmtFunctionStmt &x) {
         "'%s' has not been declared as an array or pointer-valued function"_err_en_US);
     return false;
   }
-  auto &symbol{PushSubprogramScope(name, Symbol::Flag::Function)};
-  symbol.set(Symbol::Flag::StmtFunction);
-  EraseSymbol(symbol); // removes symbol added by PushSubprogramScope
-  auto &details{symbol.get<SubprogramDetails>()};
+  Symbol *symbol{PushSubprogramScope(name, Symbol::Flag::Function)};
+  if (!symbol) {
+    return false;
+  }
+  symbol->set(Symbol::Flag::StmtFunction);
+  EraseSymbol(*symbol); // removes symbol added by PushSubprogramScope
+  auto &details{symbol->get<SubprogramDetails>()};
   for (const auto &dummyName : std::get<std::list<parser::Name>>(x.t)) {
     ObjectEntityDetails dummyDetails{true};
     if (auto *dummySymbol{FindInScope(currScope().parent(), dummyName)}) {
@@ -5124,19 +5127,22 @@ bool SubprogramVisitor::BeginSubprogram(const parser::Name &name,
       }
     }
   }
-  Symbol &newSymbol{
+  Symbol *newSymbol{
       PushSubprogramScope(name, subpFlag, bindingSpec, hasModulePrefix)};
+  if (!newSymbol) {
+    return false;
+  }
   if (moduleInterface) {
-    newSymbol.get<SubprogramDetails>().set_moduleInterface(*moduleInterface);
+    newSymbol->get<SubprogramDetails>().set_moduleInterface(*moduleInterface);
     if (moduleInterface->attrs().test(Attr::PRIVATE)) {
-      SetImplicitAttr(newSymbol, Attr::PRIVATE);
+      SetImplicitAttr(*newSymbol, Attr::PRIVATE);
     } else if (moduleInterface->attrs().test(Attr::PUBLIC)) {
-      SetImplicitAttr(newSymbol, Attr::PUBLIC);
+      SetImplicitAttr(*newSymbol, Attr::PUBLIC);
     }
   }
   if (entryStmts) {
     for (const auto &ref : *entryStmts) {
-      CreateEntry(*ref, newSymbol);
+      CreateEntry(*ref, *newSymbol);
     }
   }
   return true;
@@ -5243,12 +5249,16 @@ const Symbol *SubprogramVisitor::CheckExtantProc(
   return prev;
 }
 
-Symbol &SubprogramVisitor::PushSubprogramScope(const parser::Name &name,
+Symbol *SubprogramVisitor::PushSubprogramScope(const parser::Name &name,
     Symbol::Flag subpFlag, const parser::LanguageBindingSpec *bindingSpec,
     bool hasModulePrefix) {
   Symbol *symbol{GetSpecificFromGeneric(name)};
   const DeclTypeSpec *previousImplicitType{nullptr};
   SourceName previousName;
+  if (symbol && inInterfaceBlock() && !symbol->has<SubprogramDetails>()) {
+    SayAlreadyDeclared(name, *symbol);
+    return nullptr;
+  }
   if (!symbol) {
     if (bindingSpec && currScope().IsGlobal() &&
         std::get<std::optional<parser::ScalarDefaultCharConstantExpr>>(
@@ -5277,9 +5287,7 @@ Symbol &SubprogramVisitor::PushSubprogramScope(const parser::Name &name,
   if (subpFlag == Symbol::Flag::Function) {
     auto &funcResultTop{funcResultStack().Push(currScope(), name.source)};
     funcResultTop.previousImplicitType = previousImplicitType;
-    ;
     funcResultTop.previousName = previousName;
-    ;
   }
   if (inInterfaceBlock()) {
     auto &details{symbol->get<SubprogramDetails>()};
@@ -5305,7 +5313,7 @@ Symbol &SubprogramVisitor::PushSubprogramScope(const parser::Name &name,
       found && found->has<HostAssocDetails>()) {
     found->set(subpFlag); // PushScope() created symbol
   }
-  return *symbol;
+  return symbol;
 }
 
 void SubprogramVisitor::PushBlockDataScope(const parser::Name &name) {

flang/test/Semantics/bug159554.f90

@@ -0,0 +1,8 @@
+!RUN: %python %S/test_errors.py %s %flang_fc1
+use, intrinsic :: iso_c_binding
+interface c_funloc
+!ERROR: 'c_funloc' is already declared in this scoping unit
+  function c_funloc()
+  end function
+end interface
+end