[flang] Catch name resolution error due to global scoping (llvm#77683)

In
    CALL FOO
    PRINT *, ABS(FOO)
we currently resolve the first FOO to a global external subprogram, but
then the second FOO is treated as an implicitly typed local variable.
This happens because the name FOO is not present in the local scope.

Fix by adding FOO to the local scope using a place-holding
HostAssocDetails symbol whose existence prevents the creation of another
FOO in the local scope. The symbol stored in the parser::Name parse tree
nodes or used in typed expressions will all continue to point to the
global external subprogram.

Resolves llvm-test-suite/Fortran/gfortran/regression/pr71859.f90.

Author: klausler

flang/include/flang/Evaluate/characteristics.h

@@ -227,7 +227,7 @@ struct DummyDataObject {
       std::optional<std::string> *warning = nullptr) const;
   static std::optional<DummyDataObject> Characterize(
       const semantics::Symbol &, FoldingContext &);
-  bool CanBePassedViaImplicitInterface() const;
+  bool CanBePassedViaImplicitInterface(std::string *whyNot = nullptr) const;
   llvm::raw_ostream &Dump(llvm::raw_ostream &) const;
 
   TypeAndShape type;
@@ -248,7 +248,7 @@ struct DummyProcedure {
   bool operator!=(const DummyProcedure &that) const { return !(*this == that); }
   bool IsCompatibleWith(
       const DummyProcedure &, std::string *whyNot = nullptr) const;
-  bool CanBePassedViaImplicitInterface() const;
+  bool CanBePassedViaImplicitInterface(std::string *whyNot = nullptr) const;
   llvm::raw_ostream &Dump(llvm::raw_ostream &) const;
 
   CopyableIndirection<Procedure> procedure;
@@ -282,7 +282,7 @@ struct DummyArgument {
   void SetOptional(bool = true);
   common::Intent GetIntent() const;
   void SetIntent(common::Intent);
-  bool CanBePassedViaImplicitInterface() const;
+  bool CanBePassedViaImplicitInterface(std::string *whyNot = nullptr) const;
   bool IsTypelessIntrinsicDummy() const;
   bool IsCompatibleWith(const DummyArgument &, std::string *whyNot = nullptr,
       std::optional<std::string> *warning = nullptr) const;
@@ -325,7 +325,7 @@ struct FunctionResult {
     return std::get_if<TypeAndShape>(&u);
   }
   void SetType(DynamicType t) { std::get<TypeAndShape>(u).set_type(t); }
-  bool CanBeReturnedViaImplicitInterface() const;
+  bool CanBeReturnedViaImplicitInterface(std::string *whyNot = nullptr) const;
   bool IsCompatibleWith(
       const FunctionResult &, std::string *whyNot = nullptr) const;
 
@@ -377,7 +377,7 @@ struct Procedure {
     return !attrs.test(Attr::ImplicitInterface);
   }
   int FindPassIndex(std::optional<parser::CharBlock>) const;
-  bool CanBeCalledViaImplicitInterface() const;
+  bool CanBeCalledViaImplicitInterface(std::string *whyNot = nullptr) const;
   bool CanOverride(const Procedure &, std::optional<int> passIndex) const;
   bool IsCompatibleWith(const Procedure &, std::string *whyNot = nullptr,
       const SpecificIntrinsic * = nullptr,

flang/lib/Evaluate/characteristics.cpp

@@ -417,24 +417,45 @@ std::optional<DummyDataObject> DummyDataObject::Characterize(
   return std::nullopt;
 }
 
-bool DummyDataObject::CanBePassedViaImplicitInterface() const {
+bool DummyDataObject::CanBePassedViaImplicitInterface(
+    std::string *whyNot) const {
   if ((attrs &
           Attrs{Attr::Allocatable, Attr::Asynchronous, Attr::Optional,
               Attr::Pointer, Attr::Target, Attr::Value, Attr::Volatile})
           .any()) {
+    if (whyNot) {
+      *whyNot = "a dummy argument has the allocatable, asynchronous, optional, "
+                "pointer, target, value, or volatile attribute";
+    }
     return false; // 15.4.2.2(3)(a)
   } else if ((type.attrs() &
                  TypeAndShape::Attrs{TypeAndShape::Attr::AssumedShape,
                      TypeAndShape::Attr::AssumedRank,
                      TypeAndShape::Attr::Coarray})
                  .any()) {
+    if (whyNot) {
+      *whyNot = "a dummy argument is assumed-shape, assumed-rank, or a coarray";
+    }
     return false; // 15.4.2.2(3)(b-d)
   } else if (type.type().IsPolymorphic()) {
+    if (whyNot) {
+      *whyNot = "a dummy argument is polymorphic";
+    }
     return false; // 15.4.2.2(3)(f)
   } else if (cudaDataAttr) {
+    if (whyNot) {
+      *whyNot = "a dummy argument has a CUDA data attribute";
+    }
     return false;
   } else if (const auto *derived{GetDerivedTypeSpec(type.type())}) {
-    return derived->parameters().empty(); // 15.4.2.2(3)(e)
+    if (derived->parameters().empty()) { // 15.4.2.2(3)(e)
+      return true;
+    } else {
+      if (whyNot) {
+        *whyNot = "a dummy argument has derived type parameters";
+      }
+      return false;
+    }
   } else {
     return true;
   }
@@ -493,8 +514,12 @@ bool DummyProcedure::IsCompatibleWith(
   return true;
 }
 
-bool DummyProcedure::CanBePassedViaImplicitInterface() const {
+bool DummyProcedure::CanBePassedViaImplicitInterface(
+    std::string *whyNot) const {
   if ((attrs & Attrs{Attr::Optional, Attr::Pointer}).any()) {
+    if (whyNot) {
+      *whyNot = "a dummy procedure is optional or a pointer";
+    }
     return false; // 15.4.2.2(3)(a)
   }
   return true;
@@ -895,11 +920,11 @@ common::Intent DummyArgument::GetIntent() const {
       u);
 }
 
-bool DummyArgument::CanBePassedViaImplicitInterface() const {
+bool DummyArgument::CanBePassedViaImplicitInterface(std::string *whyNot) const {
   if (const auto *object{std::get_if<DummyDataObject>(&u)}) {
-    return object->CanBePassedViaImplicitInterface();
+    return object->CanBePassedViaImplicitInterface(whyNot);
   } else if (const auto *proc{std::get_if<DummyProcedure>(&u)}) {
-    return proc->CanBePassedViaImplicitInterface();
+    return proc->CanBePassedViaImplicitInterface(whyNot);
   } else {
     return true;
   }
@@ -970,13 +995,23 @@ bool FunctionResult::IsAssumedLengthCharacter() const {
   }
 }
 
-bool FunctionResult::CanBeReturnedViaImplicitInterface() const {
+bool FunctionResult::CanBeReturnedViaImplicitInterface(
+    std::string *whyNot) const {
   if (attrs.test(Attr::Pointer) || attrs.test(Attr::Allocatable)) {
+    if (whyNot) {
+      *whyNot = "the function result is a pointer or allocatable";
+    }
     return false; // 15.4.2.2(4)(b)
   } else if (cudaDataAttr) {
+    if (whyNot) {
+      *whyNot = "the function result has CUDA attributes";
+    }
     return false;
   } else if (const auto *typeAndShape{GetTypeAndShape()}) {
     if (typeAndShape->Rank() > 0) {
+      if (whyNot) {
+        *whyNot = "the function result is an array";
+      }
       return false; // 15.4.2.2(4)(a)
     } else {
       const DynamicType &type{typeAndShape->type()};
@@ -986,31 +1021,52 @@ bool FunctionResult::CanBeReturnedViaImplicitInterface() const {
           return true;
         } else if (const auto *param{type.charLengthParamValue()}) {
           if (const auto &expr{param->GetExplicit()}) {
-            return IsConstantExpr(*expr); // 15.4.2.2(4)(c)
+            if (IsConstantExpr(*expr)) { // 15.4.2.2(4)(c)
+              return true;
+            } else {
+              if (whyNot) {
+                *whyNot = "the function result's length is not constant";
+              }
+              return false;
+            }
           } else if (param->isAssumed()) {
             return true;
           }
         }
+        if (whyNot) {
+          *whyNot = "the function result's length is not known to the caller";
+        }
         return false;
       case TypeCategory::Derived:
-        if (!type.IsPolymorphic()) {
+        if (type.IsPolymorphic()) {
+          if (whyNot) {
+            *whyNot = "the function result is polymorphic";
+          }
+          return false;
+        } else {
           const auto &spec{type.GetDerivedTypeSpec()};
           for (const auto &pair : spec.parameters()) {
             if (const auto &expr{pair.second.GetExplicit()}) {
               if (!IsConstantExpr(*expr)) {
+                if (whyNot) {
+                  *whyNot = "the function result's derived type has a "
+                            "non-constant parameter";
+                }
                 return false; // 15.4.2.2(4)(c)
               }
             }
           }
           return true;
         }
-        return false;
       default:
         return true;
       }
     }
   } else {
-    return false; // 15.4.2.2(4)(b) - procedure pointer
+    if (whyNot) {
+      *whyNot = "the function result has unknown type or shape";
+    }
+    return false; // 15.4.2.2(4)(b) - procedure pointer?
   }
 }
 
@@ -1341,20 +1397,30 @@ std::optional<Procedure> Procedure::FromActuals(const ProcedureDesignator &proc,
   return callee;
 }
 
-bool Procedure::CanBeCalledViaImplicitInterface() const {
-  // TODO: Pass back information on why we return false
-  if (attrs.test(Attr::Elemental) || attrs.test(Attr::BindC)) {
+bool Procedure::CanBeCalledViaImplicitInterface(std::string *whyNot) const {
+  if (attrs.test(Attr::Elemental)) {
+    if (whyNot) {
+      *whyNot = "the procedure is elemental";
+    }
+    return false; // 15.4.2.2(5,6)
+  } else if (attrs.test(Attr::BindC)) {
+    if (whyNot) {
+      *whyNot = "the procedure is BIND(C)";
+    }
     return false; // 15.4.2.2(5,6)
   } else if (cudaSubprogramAttrs &&
       *cudaSubprogramAttrs != common::CUDASubprogramAttrs::Host &&
       *cudaSubprogramAttrs != common::CUDASubprogramAttrs::Global) {
+    if (whyNot) {
+      *whyNot = "the procedure is CUDA but neither HOST nor GLOBAL";
+    }
     return false;
   } else if (IsFunction() &&
-      !functionResult->CanBeReturnedViaImplicitInterface()) {
+      !functionResult->CanBeReturnedViaImplicitInterface(whyNot)) {
     return false;
   } else {
     for (const DummyArgument &arg : dummyArguments) {
-      if (!arg.CanBePassedViaImplicitInterface()) {
+      if (!arg.CanBePassedViaImplicitInterface(whyNot)) {
         return false;
       }
     }

flang/lib/Semantics/expression.cpp

@@ -3088,21 +3088,18 @@ const Assignment *ExpressionAnalyzer::Analyze(
 }
 
 static bool IsExternalCalledImplicitly(
-    parser::CharBlock callSite, const ProcedureDesignator &proc) {
-  if (const auto *symbol{proc.GetSymbol()}) {
-    return symbol->has<semantics::SubprogramDetails>() &&
-        symbol->owner().IsGlobal() &&
-        (!symbol->scope() /*ENTRY*/ ||
-            !symbol->scope()->sourceRange().Contains(callSite));
-  } else {
-    return false;
-  }
+    parser::CharBlock callSite, const Symbol *symbol) {
+  return symbol && symbol->owner().IsGlobal() &&
+      symbol->has<semantics::SubprogramDetails>() &&
+      (!symbol->scope() /*ENTRY*/ ||
+          !symbol->scope()->sourceRange().Contains(callSite));
 }
 
 std::optional<characteristics::Procedure> ExpressionAnalyzer::CheckCall(
     parser::CharBlock callSite, const ProcedureDesignator &proc,
     ActualArguments &arguments) {
-  bool treatExternalAsImplicit{IsExternalCalledImplicitly(callSite, proc)};
+  bool treatExternalAsImplicit{
+      IsExternalCalledImplicitly(callSite, proc.GetSymbol())};
   const Symbol *procSymbol{proc.GetSymbol()};
   std::optional<characteristics::Procedure> chars;
   if (procSymbol && procSymbol->has<semantics::ProcEntityDetails>() &&
@@ -3138,10 +3135,15 @@ std::optional<characteristics::Procedure> ExpressionAnalyzer::CheckCall(
   }
   bool ok{true};
   if (chars) {
-    if (treatExternalAsImplicit && !chars->CanBeCalledViaImplicitInterface()) {
-      Say(callSite,
-          "References to the procedure '%s' require an explicit interface"_err_en_US,
-          DEREF(procSymbol).name());
+    std::string whyNot;
+    if (treatExternalAsImplicit &&
+        !chars->CanBeCalledViaImplicitInterface(&whyNot)) {
+      if (auto *msg{Say(callSite,
+              "References to the procedure '%s' require an explicit interface"_err_en_US,
+              DEREF(procSymbol).name())};
+          msg && !whyNot.empty()) {
+        msg->Attach(callSite, "%s"_because_en_US, whyNot);
+      }
     }
     const SpecificIntrinsic *specificIntrinsic{proc.GetSpecificIntrinsic()};
     bool procIsDummy{procSymbol && IsDummy(*procSymbol)};

flang/lib/Semantics/resolve-names.cpp

@@ -7767,14 +7767,26 @@ void ResolveNamesVisitor::HandleProcedureName(
     if (!symbol->attrs().test(Attr::INTRINSIC)) {
       if (CheckImplicitNoneExternal(name.source, *symbol)) {
         MakeExternal(*symbol);
+        // Create a place-holder HostAssocDetails symbol to preclude later
+        // use of this name as a local symbol; but don't actually use this new
+        // HostAssocDetails symbol in expressions.
+        MakeHostAssocSymbol(name, *symbol);
+        name.symbol = symbol;
       }
     }
     CheckEntryDummyUse(name.source, symbol);
     SetProcFlag(name, *symbol, flag);
   } else if (CheckUseError(name)) {
     // error was reported
   } else {
-    symbol = &Resolve(name, symbol)->GetUltimate();
+    symbol = &symbol->GetUltimate();
+    if (!name.symbol ||
+        (name.symbol->has<HostAssocDetails>() && symbol->owner().IsGlobal() &&
+            (symbol->has<ProcEntityDetails>() ||
+                (symbol->has<SubprogramDetails>() &&
+                    symbol->scope() /*not ENTRY*/)))) {
+      name.symbol = symbol;
+    }
     CheckEntryDummyUse(name.source, symbol);
     bool convertedToProcEntity{ConvertToProcEntity(*symbol)};
     if (convertedToProcEntity && !symbol->attrs().test(Attr::EXTERNAL) &&

flang/test/Semantics/call24.f90

@@ -27,18 +27,22 @@ subroutine test()
   ! descriptor involved, copy-in/copy-out...)
 
   !ERROR: References to the procedure 'foo' require an explicit interface
+  !BECAUSE: a dummy argument has the allocatable, asynchronous, optional, pointer, target, value, or volatile attribute
   call foo(a_pointer)
 
   ! This call would be error if the interface was explicit here.
 
   !ERROR: References to the procedure 'foo' require an explicit interface
+  !BECAUSE: a dummy argument has the allocatable, asynchronous, optional, pointer, target, value, or volatile attribute
   call foo(an_array)
 
   !ERROR: References to the procedure 'bar' require an explicit interface
+  !BECAUSE: a dummy procedure is optional or a pointer
   !WARNING: If the procedure's interface were explicit, this reference would be in error
   !BECAUSE: Actual argument associated with procedure pointer dummy argument 'a_pointer=' must be a pointer unless INTENT(IN)
   call bar(sin)
 
   !ERROR: References to the procedure 'baz' require an explicit interface
+  !BECAUSE: a dummy procedure is optional or a pointer
   call baz(sin)
 end subroutine

flang/test/Semantics/call25.f90

@@ -1,4 +1,4 @@
-! RUN: not %flang -fsyntax-only 2>&1 %s | FileCheck %s
+! RUN: not %flang -fsyntax-only -pedantic 2>&1 %s | FileCheck %s
 module m
  contains
   subroutine subr1(f)

flang/test/Semantics/local-vs-global.f90

@@ -74,6 +74,7 @@ program test
   call block_data_before_2
   call explicit_before_1(1.)
   !ERROR: References to the procedure 'explicit_before_2' require an explicit interface
+  !BECAUSE: a dummy argument has the allocatable, asynchronous, optional, pointer, target, value, or volatile attribute
   call explicit_before_2(1.)
   !WARNING: If the procedure's interface were explicit, this reference would be in error
   !BECAUSE: Dummy argument 'a=' (#1) is not OPTIONAL and is not associated with an actual argument in this procedure reference
@@ -83,6 +84,7 @@ program test
   call implicit_before_2
   print *, explicit_func_before_1(1.)
   !ERROR: References to the procedure 'explicit_func_before_2' require an explicit interface
+  !BECAUSE: a dummy argument has the allocatable, asynchronous, optional, pointer, target, value, or volatile attribute
   print *, explicit_func_before_2(1.)
   !WARNING: If the procedure's interface were explicit, this reference would be in error
   !BECAUSE: Dummy argument 'a=' (#1) is not OPTIONAL and is not associated with an actual argument in this procedure reference
@@ -96,6 +98,7 @@ program test
   call block_data_after_2
   call explicit_after_1(1.)
   !ERROR: References to the procedure 'explicit_after_2' require an explicit interface
+  !BECAUSE: a dummy argument has the allocatable, asynchronous, optional, pointer, target, value, or volatile attribute
   call explicit_after_2(1.)
   !WARNING: If the procedure's interface were explicit, this reference would be in error
   !BECAUSE: Dummy argument 'a=' (#1) is not OPTIONAL and is not associated with an actual argument in this procedure reference
@@ -105,6 +108,7 @@ program test
   call implicit_after_2
   print *, explicit_func_after_1(1.)
   !ERROR: References to the procedure 'explicit_func_after_2' require an explicit interface
+  !BECAUSE: a dummy argument has the allocatable, asynchronous, optional, pointer, target, value, or volatile attribute
   print *, explicit_func_after_2(1.)
   !WARNING: If the procedure's interface were explicit, this reference would be in error
   !BECAUSE: Dummy argument 'a=' (#1) is not OPTIONAL and is not associated with an actual argument in this procedure reference

flang/test/Semantics/reshape.f90

@@ -56,7 +56,7 @@ program reshaper
   !ERROR: Size of 'shape=' argument must not be greater than 15
   CALL ext_sub(RESHAPE([(n, n=1,20)], &
     [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]))
-  !WARNING: Reference to the procedure 'ext_sub' has an implicit interface that is distinct from another reference: incompatible dummy argument #1: incompatible dummy data object shapes
+  !ERROR: Reference to the procedure 'ext_sub' has an implicit interface that is distinct from another reference: incompatible dummy argument #1: incompatible dummy data object shapes
   !ERROR: 'shape=' argument must not have a negative extent
   CALL ext_sub(RESHAPE([(n, n=1,20)], [1, -5, 3]))
   !ERROR: 'order=' argument has unacceptable rank 2