[flang] Fix false errors in function result derived type checking

flang/lib/Semantics/resolve-names.cpp

@@ -2695,9 +2695,16 @@ void ArraySpecVisitor::PostAttrSpec() {
 
 FuncResultStack::~FuncResultStack() { CHECK(stack_.empty()); }
 
+// True when either type is absent, or if they are both present and are
+// equivalent for interface compatibility purposes.
 static bool TypesMismatchIfNonNull(
     const DeclTypeSpec *type1, const DeclTypeSpec *type2) {
-  return type1 && type2 && *type1 != *type2;
+  if (auto t1{evaluate::DynamicType::From(type1)}) {
+    if (auto t2{evaluate::DynamicType::From(type2)}) {
+      return !t1->IsEquivalentTo(*t2);
+    }
+  }
+  return false;
 }
 
 void FuncResultStack::CompleteFunctionResultType() {

flang/test/Semantics/global03.f90

@@ -0,0 +1,73 @@
+! RUN: %python %S/test_errors.py %s %flang_fc1 -Werror
+! Catch discrepancies between implicit result types and a global definition,
+! allowing for derived types and type equivalence.
+
+module m
+  type t1
+    integer n
+  end type
+  type t2
+    real a
+  end type
+  type t3
+    sequence
+    integer n
+  end type
+end
+
+function xfunc1()
+  use m
+  type(t1) xfunc1
+  xfunc1%n = 123
+end
+
+function yfunc1()
+  use m
+  type(t1) yfunc1
+  yfunc1%n = 123
+end
+
+function zfunc1()
+  type t3
+    sequence
+    integer n
+  end type
+  type(t3) zfunc1
+  zfunc1%n = 123
+end
+
+program main
+  use m
+  implicit type(t1) (x)
+  implicit type(t2) (y)
+  implicit type(t3) (z)
+  print *, xfunc1() ! ok
+  print *, xfunc2() ! ok
+!ERROR: Implicit declaration of function 'yfunc1' has a different result type than in previous declaration
+  print *, yfunc1()
+  print *, yfunc2()
+  print *, zfunc1() ! ok
+  print *, zfunc2() ! ok
+end
+
+function xfunc2()
+  use m
+  type(t1) xfunc2
+  xfunc2%n = 123
+end
+
+function yfunc2()
+  use m
+!ERROR: Function 'yfunc2' has a result type that differs from the implicit type it obtained in a previous reference
+  type(t1) yfunc2
+  yfunc2%n = 123
+end
+
+function zfunc2()
+  type t3
+    sequence
+    integer n
+  end type
+  type(t3) zfunc2
+  zfunc2%n = 123
+end