[flang] Allow OpenMP declarations before type declarations

Skip resolving implicit types for OpenMP declarative directives, to
allow them to appear before type declarations, which is supported
by several compilers. This was discussed in
https://discourse.llvm.org/t/rfc-openmp-should-type-declaration-be-allowed-after-threadprivate/81345.

This fixes the semantic errors of
#106021.

Author: luporl

flang/lib/Semantics/resolve-names.cpp

@@ -720,6 +720,7 @@ class ScopeHandler : public ImplicitRulesVisitor {
 
   bool inSpecificationPart_{false};
   bool deferImplicitTyping_{false};
+  bool skipImplicitTyping_{false};
   bool inEquivalenceStmt_{false};
 
   // Some information is collected from a specification part for deferred
@@ -758,6 +759,10 @@ class ScopeHandler : public ImplicitRulesVisitor {
     }
   }
 
+  void SkipImplicitTyping(bool skip) {
+    deferImplicitTyping_ = skipImplicitTyping_ = skip;
+  }
+
 private:
   Scope *currScope_{nullptr};
   FuncResultStack funcResultStack_{*this};
@@ -1506,6 +1511,25 @@ class OmpVisitor : public virtual DeclarationVisitor {
   void Post(const parser::OmpEndCriticalDirective &) {
     messageHandler().set_currStmtSource(std::nullopt);
   }
+  bool Pre(const parser::OpenMPThreadprivate &) {
+    SkipImplicitTyping(true);
+    return true;
+  }
+  void Post(const parser::OpenMPThreadprivate &) { SkipImplicitTyping(false); }
+  bool Pre(const parser::OpenMPDeclareTargetConstruct &) {
+    SkipImplicitTyping(true);
+    return true;
+  }
+  void Post(const parser::OpenMPDeclareTargetConstruct &) {
+    SkipImplicitTyping(false);
+  }
+  bool Pre(const parser::OpenMPDeclarativeAllocate &) {
+    SkipImplicitTyping(true);
+    return true;
+  }
+  void Post(const parser::OpenMPDeclarativeAllocate &) {
+    SkipImplicitTyping(false);
+  }
 };
 
 bool OmpVisitor::NeedsScope(const parser::OpenMPBlockConstruct &x) {
@@ -2556,8 +2580,10 @@ void ScopeHandler::ApplyImplicitRules(
     return;
   }
   if (const DeclTypeSpec * type{GetImplicitType(symbol)}) {
-    symbol.set(Symbol::Flag::Implicit);
-    symbol.SetType(*type);
+    if (!skipImplicitTyping_) {
+      symbol.set(Symbol::Flag::Implicit);
+      symbol.SetType(*type);
+    }
     return;
   }
   if (symbol.has<ProcEntityDetails>() && !symbol.attrs().test(Attr::EXTERNAL)) {

flang/test/Semantics/OpenMP/declarative-directive.f90 renamed to flang/test/Semantics/OpenMP/declarative-directive01.f90

@@ -0,0 +1,56 @@
+! RUN: %flang -fsyntax-only -fopenmp %s 2>&1
+
+! Check that OpenMP declarative directives can be used with objects that have
+! an incomplete type.
+
+subroutine test_decl
+  ! OMPv5.2 5.2 threadprivate
+  ! OMPv5.2 6.5 allocate
+  implicit none
+  save :: x1, y1
+  !$omp threadprivate(x1)
+  !$omp allocate(y1)
+  integer :: x1, y1
+
+  ! OMPv5.2 7.7 declare-simd
+  external :: simd_func
+  !$omp declare simd(simd_func)
+  logical :: simd_func
+
+  ! OMPv5.2 7.8.1 declare-target
+  allocatable :: j
+  !$omp declare target(j)
+  save :: j
+  real(kind=8) :: j(:)
+
+  ! OMPv5.2 5.5.11 declare-reduction - crashes
+  !external :: my_add_red
+  !!$omp declare reduction(my_add_red : integer : my_add_red(omp_out, omp_in)) &
+  !!$omp&  initializer(omp_priv=0)
+  !integer :: my_add_red
+end subroutine
+
+subroutine test_decl2
+  save x1, y1
+  !$omp threadprivate(x1)
+  !$omp allocate(y1)
+  integer :: x1, y1
+
+  ! implicit decl
+  !$omp threadprivate(x2)
+  !$omp allocate(y2)
+  save x2, y2
+end subroutine
+
+module m1
+  ! implicit decl
+  !$omp threadprivate(x, y, z)
+  integer :: y
+  real :: z
+
+contains
+  subroutine sub
+    !$omp parallel copyin(x, y, z)
+    !$omp end parallel
+  end subroutine
+end module

flang/test/Semantics/OpenMP/declarative-directive02.f90

@@ -6,21 +6,16 @@
 
 module test_0
     implicit none
-!ERROR: The given DECLARE TARGET directive clause has an invalid argument
 !ERROR: No explicit type declared for 'no_implicit_materialization_1'
 !$omp declare target(no_implicit_materialization_1)
 
-!ERROR: The given DECLARE TARGET directive clause has an invalid argument
 !ERROR: No explicit type declared for 'no_implicit_materialization_2'
 !$omp declare target link(no_implicit_materialization_2)
 
-!ERROR: The given DECLARE TARGET directive clause has an invalid argument
 !WARNING: The usage of TO clause on DECLARE TARGET directive has been deprecated. Use ENTER clause instead.
 !ERROR: No explicit type declared for 'no_implicit_materialization_3'
 !$omp declare target to(no_implicit_materialization_3)
 
-!ERROR: The given DECLARE TARGET directive clause has an invalid argument
-!ERROR: No explicit type declared for 'no_implicit_materialization_3'
 !$omp declare target enter(no_implicit_materialization_3)
 
 INTEGER :: data_int = 10