[flang] Don't implicitly host-associate unknown EQUIVALENCE designators

When resolving names in a specification part, unknown names that appear
in a specification expression before any local declaration are assumed
to be implicitly declared objects in the host scope.  Objects in
EQUIVALENCE sets are not part of specification expressions, so ensure
that they do not receive this treatment; besides being wrong and
unimplementable, it will lead to a later crash during offset assignment.

Differential Revision: https://reviews.llvm.org/D159030

flang/lib/Semantics/resolve-names.cpp

@@ -7277,7 +7277,7 @@ const parser::Name *DeclarationVisitor::ResolveName(const parser::Name &name) {
 // be wrong we report an error later in CheckDeclarations().
 bool DeclarationVisitor::CheckForHostAssociatedImplicit(
     const parser::Name &name) {
-  if (!inSpecificationPart_) {
+  if (!inSpecificationPart_ || inEquivalenceStmt_) {
     return false;
   }
   if (name.symbol) {

flang/test/Semantics/offsets03.f90

@@ -29,7 +29,7 @@ subroutine mc         !CHECK: Subprogram scope: mc size=12 alignment=1
 
 ! Common block: objects are in order from COMMON statement and not part of module
 module md                   !CHECK: Module scope: md size=1 alignment=1
-  integer(1) :: i 
+  integer(1) :: i
   integer(2) :: d1          !CHECK: d1, PUBLIC size=2 offset=8:
   integer(4) :: d2          !CHECK: d2, PUBLIC size=4 offset=4:
   integer(1) :: d3          !CHECK: d3, PUBLIC size=1 offset=0:
@@ -65,3 +65,13 @@ module me
   equivalence(a1,a3(1)),(a2,a3(2))
   common /common8/ a1, a2   ! CHECK: common8 size=8 offset=0: CommonBlockDetails alignment=4:
 end
+
+! Ensure EQUIVALENCE of undeclared names in internal subprogram doesn't
+! attempt to host-associate
+subroutine host1
+ contains
+  subroutine internal
+    common /b/ x(4)  ! CHECK: x (Implicit) size=16 offset=0: ObjectEntity type: REAL(4) shape: 1_8:4_8
+    equivalence(x,y) ! CHECK: y (Implicit) size=4 offset=0: ObjectEntity type: REAL(4)
+  end
+end