[flang] Emit error on impossible-to-implement construct

flang/docs/Extensions.md

@@ -557,6 +557,17 @@ end
   generic intrinsic function's inferred result type does not
   match an explicit declaration.  This message is a warning.
 
+* There is no restriction in the standard against assigning
+  to a whole polymorphic allocatable under control of a `WHERE`
+  construct or statement, but there is no good portable
+  behavior to implement and the standard isn't entirely clear
+  what it should mean.
+  (Other compilers allow it, but the results are never meaningful;
+  some never change the type, some change the type according to
+  the value of the last mask element, some treat these
+  assignment statements as no-ops, and the rest crash during compilation.)
+  The compiler flags this case as an error.
+
 ## Standard features that might as well not be
 
 * f18 supports designators with constant expressions, properly

flang/lib/Semantics/assignment.cpp

@@ -41,7 +41,6 @@ class AssignmentContext {
   void PopWhereContext();
   void Analyze(const parser::AssignmentStmt &);
   void Analyze(const parser::PointerAssignmentStmt &);
-  void Analyze(const parser::ConcurrentControl &);
   SemanticsContext &context() { return context_; }
 
 private:
@@ -76,6 +75,11 @@ void AssignmentContext::Analyze(const parser::AssignmentStmt &stmt) {
         whole{evaluate::UnwrapWholeSymbolOrComponentDataRef(lhs)}) {
       if (IsAllocatable(whole->GetUltimate())) {
         flags.set(DefinabilityFlag::PotentialDeallocation);
+        if (IsPolymorphic(*whole) && whereDepth_ > 0) {
+          Say(lhsLoc,
+              "Assignment to whole polymorphic allocatable '%s' may not be nested in a WHERE statement or construct"_err_en_US,
+              whole->name());
+        }
       }
     }
     if (auto whyNot{WhyNotDefinable(lhsLoc, scope, flags, lhs)}) {

flang/test/Semantics/bug133669.f90

@@ -0,0 +1,51 @@
+!RUN: %python %S/test_errors.py %s %flang_fc1
+module m
+ contains
+  subroutine s(x, y, mask)
+    class(*), allocatable, intent(in out) :: x(:), y(:)
+    logical, intent(in) :: mask(:)
+    select type(x)
+    type is(integer)
+      print *, 'before, x is integer', x
+    type is(real)
+      print *, 'before, x is real', x
+    class default
+      print *, 'before, x has some other type'
+    end select
+    select type(y)
+    type is(integer)
+      print *, 'y is integer', y
+    type is(real)
+      print *, 'y is real', y
+    end select
+    print *, 'mask', mask
+    !ERROR: Assignment to whole polymorphic allocatable 'x' may not be nested in a WHERE statement or construct
+    where(mask) x = y
+    select type(x)
+    type is(integer)
+      print *, 'after, x is integer', x
+    type is(real)
+      print *, 'after, x is real', x
+    class default
+      print *, 'before, x has some other type'
+    end select
+    print *
+  end
+end
+
+program main
+  use m
+  class(*), allocatable :: x(:), y(:)
+  x = [1, 2]
+  y = [3., 4.]
+  call s(x, y, [.false., .false.])
+  x = [1, 2]
+  y = [3., 4.]
+  call s(x, y, [.false., .true.])
+  x = [1, 2]
+  y = [3., 4.]
+  call s(x, y, [.true., .false.])
+  x = [1, 2]
+  y = [3., 4.]
+  call s(x, y, [.true., .true.])
+end program main