[flang] Extension: allow DATA to precede declaration under IMPLICIT NONE(TYPE)

It is not standard conforming under IMPLICIT NONE(TYPE) for a name to
appear in a DATA statement prior to its explicit type declaration,
but it is benign, supported in other compilers, and attested in real
applications.  Support it with an optional portability warning.

Fixes GitHub LLVM bug llvm#63783.

Author: klausler

flang/docs/Extensions.md

@@ -283,6 +283,8 @@ end
 * When a name is brought into a scope by multiple ways,
   such as USE-association as well as an `IMPORT` from its host,
   it's an error only if the resolution is ambiguous.
+* An entity may appear in a `DATA` statement before its explicit
+  type declaration under `IMPLICIT NONE(TYPE)`.
 
 ### Extensions supported when enabled by options
 

flang/include/flang/Common/Fortran-features.h

@@ -36,7 +36,8 @@ ENUM_CLASS(LanguageFeature, BackslashEscapes, OldDebugLines,
     ForwardRefImplicitNone, OpenAccessAppend, BOZAsDefaultInteger,
     DistinguishableSpecifics, DefaultSave, PointerInSeqType, NonCharacterFormat,
     SaveMainProgram, SaveBigMainProgramVariables,
-    DistinctArrayConstructorLengths, PPCVector, RelaxedIntentInChecking)
+    DistinctArrayConstructorLengths, PPCVector, RelaxedIntentInChecking,
+    ForwardRefImplicitNoneData)
 
 // Portability and suspicious usage warnings for conforming code
 ENUM_CLASS(UsageWarning, Portability, PointerToUndefinable,

flang/lib/Semantics/resolve-names.cpp

@@ -697,6 +697,7 @@ class ScopeHandler : public ImplicitRulesVisitor {
   bool CheckPossibleBadForwardRef(const Symbol &);
 
   bool inSpecificationPart_{false};
+  bool inDataStmtObject_{false};
   bool inEquivalenceStmt_{false};
 
   // Some information is collected from a specification part for deferred
@@ -2481,6 +2482,9 @@ void ScopeHandler::ApplyImplicitRules(
     // or object, it'll be caught later.
     return;
   }
+  if (inDataStmtObject_) {
+    return;
+  }
   if (!context().HasError(symbol)) {
     Say(symbol.name(), "No explicit type declared for '%s'"_err_en_US);
     context().SetError(symbol);
@@ -2654,7 +2658,7 @@ const DeclTypeSpec &ScopeHandler::MakeLogicalType(int kind) {
 }
 
 void ScopeHandler::NotePossibleBadForwardRef(const parser::Name &name) {
-  if (inSpecificationPart_ && name.symbol) {
+  if (inSpecificationPart_ && !inDataStmtObject_ && name.symbol) {
     auto kind{currScope().kind()};
     if ((kind == Scope::Kind::Subprogram && !currScope().IsStmtFunction()) ||
         kind == Scope::Kind::BlockConstruct) {
@@ -6281,6 +6285,12 @@ void DeclarationVisitor::SetType(
   }
   auto *prevType{symbol.GetType()};
   if (!prevType) {
+    if (symbol.test(Symbol::Flag::InDataStmt) && isImplicitNoneType() &&
+        context().ShouldWarn(
+            common::LanguageFeature::ForwardRefImplicitNoneData)) {
+      Say(name,
+          "'%s' appeared in a DATA statement before its type was declared under IMPLICIT NONE(TYPE)"_port_en_US);
+    }
     symbol.SetType(type);
   } else if (symbol.has<UseDetails>()) {
     // error recovery case, redeclaration of use-associated name
@@ -6642,6 +6652,7 @@ bool ConstructVisitor::Pre(const parser::DataStmtObject &x) {
   auto flagRestorer{common::ScopedSet(inSpecificationPart_, false)};
   common::visit(common::visitors{
                     [&](const Indirection<parser::Variable> &y) {
+                      auto restorer{common::ScopedSet(inDataStmtObject_, true)};
                       Walk(y.value());
                       const parser::Name &first{
                           parser::GetFirstName(y.value())};
@@ -7199,7 +7210,7 @@ const parser::Name *DeclarationVisitor::ResolveName(const parser::Name &name) {
     }
     return &name;
   }
-  if (isImplicitNoneType()) {
+  if (isImplicitNoneType() && !inDataStmtObject_) {
     Say(name, "No explicit type declared for '%s'"_err_en_US);
     return nullptr;
   }

flang/test/Semantics/resolve30.f90

@@ -1,4 +1,4 @@
-! RUN: %python %S/test_errors.py %s %flang_fc1
+! RUN: %python %S/test_errors.py %s %flang_fc1 -pedantic
 subroutine s1
   integer x
   block
@@ -37,3 +37,10 @@ subroutine s4
   !ERROR: Must have INTEGER type, but is REAL(4)
   data(b(j), j=1, 16) / 16 * 0.0 /
 end
+
+subroutine s5
+  implicit none
+  data x/1./
+  !PORTABILITY: 'x' appeared in a DATA statement before its type was declared under IMPLICIT NONE(TYPE)
+  real x
+end