[flang] Allow redudant attributes on use-/host- associated names

[klausler]

Summary

Constraint C815 in F'2018 allows a name to acquire an attribute at
most once per scope. For some attributes, the attribute may have
already been inherited, and the compiler was emitting a bogus error
message for a redundant application of the same attribute in another
scope.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Semantics/resolve-names.cpp

 }
 Symbol &ScopeHandler::MakeHostAssocSymbol(
     const parser::Name &name, const Symbol &hostSymbol) {
   Symbol &symbol{*NonDerivedTypeScope()
                       .try_emplace(name.source, HostAssocDetails{hostSymbol})
                       .first->second};
   name.symbol = &symbol;
   symbol.attrs() = hostSymbol.attrs(); // TODO: except PRIVATE, PUBLIC?
+  // These attributes can be redundantly reapplied without error
+  // on the host-associated name, at most once (C815).
+  symbol.implicitAttrs() =
+      symbol.attrs() & Attrs{Attr::ASYNCHRONOUS, Attr::VOLATILE};
   symbol.flags() = hostSymbol.flags();
   return symbol;
 }
 Symbol &ScopeHandler::CopySymbol(const SourceName &name, const Symbol &symbol) {
   CHECK(!FindInScope(name));
   return MakeSymbol(currScope(), name, symbol.attrs());
 }
 

flang/test/Semantics/resolve119.f90

+! RUN: %python %S/test_errors.py %s %flang_fc1
+! C815: an attribute may be applied at most once per scope
+module m
+  real a1, a2, v1, v2
+  asynchronous a1
+  asynchronous a2
+  !ERROR: ASYNCHRONOUS attribute was already specified on 'a2'
+  asynchronous a2
+  volatile v1
+  volatile v2
+  !ERROR: VOLATILE attribute was already specified on 'v2'
+  volatile v2
+ contains
+  subroutine modsub
+    asynchronous a1
+    asynchronous a2
+    !ERROR: ASYNCHRONOUS attribute was already specified on 'a2'
+    asynchronous a2
+    volatile v1
+    volatile v2
+    !ERROR: VOLATILE attribute was already specified on 'v2'
+    volatile v2
+    block
+      asynchronous a1
+      asynchronous a2
+      !ERROR: ASYNCHRONOUS attribute was already specified on 'a2'
+      asynchronous a2
+      volatile v1
+      volatile v2
+      !ERROR: VOLATILE attribute was already specified on 'v2'
+      volatile v2
+    end block
+  end
+end
+
+subroutine s
+  use m
+  asynchronous a1
+  asynchronous a2
+  !ERROR: ASYNCHRONOUS attribute was already specified on 'a2'
+  asynchronous a2
+  volatile v1
+  volatile v2
+  !ERROR: VOLATILE attribute was already specified on 'v2'
+  volatile v2
+  block
+    asynchronous a1
+    asynchronous a2
+    !ERROR: ASYNCHRONOUS attribute was already specified on 'a2'
+    asynchronous a2
+    volatile v1
+    volatile v2
+    !ERROR: VOLATILE attribute was already specified on 'v2'
+    volatile v2
+  end block
+ contains
+  subroutine internal
+    asynchronous a1
+    asynchronous a2
+    !ERROR: ASYNCHRONOUS attribute was already specified on 'a2'
+    asynchronous a2
+    volatile v1
+    volatile v2
+    !ERROR: VOLATILE attribute was already specified on 'v2'
+    volatile v2
+    block
+      asynchronous a1
+      asynchronous a2
+      !ERROR: ASYNCHRONOUS attribute was already specified on 'a2'
+      asynchronous a2
+      volatile v1
+      volatile v2
+      !ERROR: VOLATILE attribute was already specified on 'v2'
+      volatile v2
+    end block
+  end
+end
+