[flang] Check for BIND(C) name conflicts with alternate entries (llvm#156563)

Added IsAlternateEntry() and modified IsExternalProcedureDefinition() to
also check for alternate entries.

(IsExternalProcedureDefinition() is called from
CheckHelper::CheckGlobalName(), which checks for duplicate global
symbols.)

Fixes llvm#62778

Author: eugeneepshteyn

flang/include/flang/Evaluate/tools.h

@@ -1531,6 +1531,12 @@ class Scope;
 // point to its subprogram.
 const Symbol *GetMainEntry(const Symbol *);
 
+inline bool IsAlternateEntry(const Symbol *symbol) {
+  // If symbol is not alternate entry symbol, GetMainEntry() returns the same
+  // symbol.
+  return symbol && GetMainEntry(symbol) != symbol;
+}
+
 // These functions are used in Evaluate so they are defined here rather than in
 // Semantics to avoid a link-time dependency on Semantics.
 // All of these apply GetUltimate() or ResolveAssociations() to their arguments.

flang/lib/Semantics/check-declarations.cpp

@@ -2950,7 +2950,7 @@ static bool IsSubprogramDefinition(const Symbol &symbol) {
 
 static bool IsExternalProcedureDefinition(const Symbol &symbol) {
   return IsBlockData(symbol) ||
-      (IsSubprogramDefinition(symbol) &&
+      ((IsSubprogramDefinition(symbol) || IsAlternateEntry(&symbol)) &&
           (IsExternal(symbol) || symbol.GetBindName()));
 }
 

flang/test/Semantics/bind-c01.f90

@@ -29,3 +29,11 @@ subroutine foo() bind(c, name="x6")
 end subroutine
 subroutine foo() bind(c, name="x7")
 end subroutine
+
+subroutine entries()
+
+entry e1() bind(C, name="e")
+
+!ERROR: Two entities have the same global name 'e'
+entry e2() bind(C, name="e")
+end subroutine