diff --git a/flang/docs/Extensions.md b/flang/docs/Extensions.md
index 33109ae2d3042..24e07510b268d 100644
--- a/flang/docs/Extensions.md
+++ b/flang/docs/Extensions.md
@@ -96,6 +96,12 @@ end
 * `NULL()` without `MOLD=` is not allowed to be associated as an
   actual argument corresponding to an assumed-rank dummy argument;
   its rank in the called procedure would not be well-defined.
+* When an index variable of a `FORALL` or `DO CONCURRENT` is present
+  in the enclosing scope, and the construct does not have an explicit
+  type specification for its index variables, some weird restrictions
+  in F'2023 subclause 19.4 paragraphs 6 & 8 should apply.  Since this
+  compiler properly scopes these names, violations of these restrictions
+  elicit only portability warnings by default.
 
 ## Extensions, deletions, and legacy features supported by default
 
diff --git a/flang/include/flang/Common/Fortran-features.h b/flang/include/flang/Common/Fortran-features.h
index a6b19e9833fc5..dc50aa7f5c559 100644
--- a/flang/include/flang/Common/Fortran-features.h
+++ b/flang/include/flang/Common/Fortran-features.h
@@ -45,7 +45,7 @@ ENUM_CLASS(LanguageFeature, BackslashEscapes, OldDebugLines,
     MiscSourceExtensions, AllocateToOtherLength, LongNames, IntrinsicAsSpecific,
     BenignNameClash, BenignRedundancy, NullMoldAllocatableComponentValue,
     NopassScalarBase, MiscUseExtensions, ImpliedDoIndexScope,
-    DistinctCommonSizes)
+    DistinctCommonSizes, OddIndexVariableRestrictions)
 
 // Portability and suspicious usage warnings for conforming code
 ENUM_CLASS(UsageWarning, Portability, PointerToUndefinable,
diff --git a/flang/lib/Semantics/resolve-names.cpp b/flang/lib/Semantics/resolve-names.cpp
index 64fc7de120873..06e35d22fe788 100644
--- a/flang/lib/Semantics/resolve-names.cpp
+++ b/flang/lib/Semantics/resolve-names.cpp
@@ -6638,10 +6638,14 @@ void ConstructVisitor::ResolveIndexName(
   const parser::Name &name{std::get<parser::Name>(control.t)};
   auto *prev{FindSymbol(name)};
   if (prev) {
-    if (prev->owner().kind() == Scope::Kind::Forall ||
-        prev->owner() == currScope()) {
+    if (prev->owner() == currScope()) {
       SayAlreadyDeclared(name, *prev);
       return;
+    } else if (prev->owner().kind() == Scope::Kind::Forall &&
+        context().ShouldWarn(
+            common::LanguageFeature::OddIndexVariableRestrictions)) {
+      SayWithDecl(name, *prev,
+          "Index variable '%s' should not also be an index in an enclosing FORALL or DO CONCURRENT"_port_en_US);
     }
     name.symbol = nullptr;
   }
@@ -6651,22 +6655,26 @@ void ConstructVisitor::ResolveIndexName(
   } else if (!prev) {
     ApplyImplicitRules(symbol);
   } else {
-    const Symbol &prevRoot{prev->GetUltimate()};
-    // prev could be host- use- or construct-associated with another symbol
-    if (!prevRoot.has<ObjectEntityDetails>() &&
-        !prevRoot.has<AssocEntityDetails>()) {
-      Say2(name, "Index name '%s' conflicts with existing identifier"_err_en_US,
-          *prev, "Previous declaration of '%s'"_en_US);
-      context().SetError(symbol);
-      return;
+    // Odd rules in F'2023 19.4 paras 6 & 8.
+    Symbol &prevRoot{prev->GetUltimate()};
+    if (const auto *type{prevRoot.GetType()}) {
+      symbol.SetType(*type);
     } else {
-      if (const auto *type{prevRoot.GetType()}) {
-        symbol.SetType(*type);
-      }
-      if (prevRoot.IsObjectArray()) {
-        SayWithDecl(name, *prev, "Index variable '%s' is not scalar"_err_en_US);
-        return;
+      ApplyImplicitRules(symbol);
+    }
+    if (prevRoot.has<ObjectEntityDetails>() ||
+        ConvertToObjectEntity(prevRoot)) {
+      if (prevRoot.IsObjectArray() &&
+          context().ShouldWarn(
+              common::LanguageFeature::OddIndexVariableRestrictions)) {
+        SayWithDecl(name, *prev,
+            "Index variable '%s' should be scalar in the enclosing scope"_port_en_US);
       }
+    } else if (!prevRoot.has<CommonBlockDetails>() &&
+        context().ShouldWarn(
+            common::LanguageFeature::OddIndexVariableRestrictions)) {
+      SayWithDecl(name, *prev,
+          "Index variable '%s' should be a scalar object or common block if it is present in the enclosing scope"_port_en_US);
     }
   }
   EvaluateExpr(parser::Scalar{parser::Integer{common::Clone(name)}});
@@ -6839,7 +6847,10 @@ bool ConstructVisitor::Pre(const parser::DataStmtValue &x) {
 
 bool ConstructVisitor::Pre(const parser::DoConstruct &x) {
   if (x.IsDoConcurrent()) {
-    PushScope(Scope::Kind::OtherConstruct, nullptr);
+    // The new scope has Kind::Forall for index variable name conflict
+    // detection with nested FORALL/DO CONCURRENT constructs in
+    // ResolveIndexName().
+    PushScope(Scope::Kind::Forall, nullptr);
   }
   return true;
 }
diff --git a/flang/test/Semantics/dosemantics12.f90 b/flang/test/Semantics/dosemantics12.f90
index 3adf310051261..1757ade4b7c8f 100644
--- a/flang/test/Semantics/dosemantics12.f90
+++ b/flang/test/Semantics/dosemantics12.f90
@@ -1,4 +1,4 @@
-! RUN: %python %S/test_errors.py %s %flang_fc1
+! RUN: %python %S/test_errors.py %s %flang_fc1 -pedantic
 ! Copyright (c) 2019, NVIDIA CORPORATION.  All rights reserved.
 !
 ! Licensed under the Apache License, Version 2.0 (the "License");
@@ -313,10 +313,10 @@ subroutine s9()
     end do
   end do
 
-  ! OK since the DO CONCURRENT index-name exists only in the scope of the
-  ! DO CONCURRENT construct
+  ! Technically non-conformant (F'2023 19.4 p8)
   do concurrent (ivar = 1:10)
     print *, "hello"
+    !PORTABILITY: Index variable 'ivar' should not also be an index in an enclosing FORALL or DO CONCURRENT
     do concurrent (ivar = 1:10)
       print *, "hello"
     end do
diff --git a/flang/test/Semantics/forall01.f90 b/flang/test/Semantics/forall01.f90
index 5a493d45c6540..a81eb9621e77c 100644
--- a/flang/test/Semantics/forall01.f90
+++ b/flang/test/Semantics/forall01.f90
@@ -1,4 +1,4 @@
-! RUN: %python %S/test_errors.py %s %flang_fc1
+! RUN: %python %S/test_errors.py %s %flang_fc1 -pedantic
 subroutine forall1
   real :: a(9)
   !ERROR: 'i' is already declared in this scoping unit
@@ -10,8 +10,7 @@ subroutine forall1
     a(i) = i
   end forall
   forall (j=1:8)
-    !ERROR: 'j' is already declared in this scoping unit
-    !ERROR: Cannot redefine FORALL variable 'j'
+    !PORTABILITY: Index variable 'j' should not also be an index in an enclosing FORALL or DO CONCURRENT
     forall (j=1:9)
     end forall
   end forall
@@ -75,7 +74,6 @@ subroutine forall4
   forall(i=1:10:zero) a(i) = i
 end
 
-! Note: this gets warnings but not errors
 subroutine forall5
   real, target :: x(10), y(10)
   forall(i=1:10)
@@ -93,6 +91,8 @@ subroutine forall5
   endforall
   do concurrent(i=1:10)
     x = y
+    !Odd rule from F'2023 19.4 p8
+    !PORTABILITY: Index variable 'i' should not also be an index in an enclosing FORALL or DO CONCURRENT
     !WARNING: FORALL index variable 'i' not used on left-hand side of assignment
     forall(i=1:10) x = y
   end do
@@ -116,17 +116,20 @@ subroutine forall7(x)
   real :: a(10)
   class(*) :: x
   associate (j => iarr(1))
+    !PORTABILITY: Index variable 'j' should be a scalar object or common block if it is present in the enclosing scope
     forall (j=1:size(a))
       a(j) = a(j) + 1
     end forall
   end associate
   associate (j => iarr(1) + 1)
+    !PORTABILITY: Index variable 'j' should be a scalar object or common block if it is present in the enclosing scope
     forall (j=1:size(a))
       a(j) = a(j) + 1
     end forall
   end associate
   select type (j => x)
   type is (integer)
+    !PORTABILITY: Index variable 'j' should be a scalar object or common block if it is present in the enclosing scope
     forall (j=1:size(a))
       a(j) = a(j) + 1
     end forall
diff --git a/flang/test/Semantics/resolve35.f90 b/flang/test/Semantics/resolve35.f90
index 17034ebc2f0f3..2947b225978d1 100644
--- a/flang/test/Semantics/resolve35.f90
+++ b/flang/test/Semantics/resolve35.f90
@@ -1,4 +1,4 @@
-! RUN: %python %S/test_errors.py %s %flang_fc1
+! RUN: %python %S/test_errors.py %s %flang_fc1 -pedantic
 ! Construct names
 
 subroutine s1
@@ -21,11 +21,17 @@ subroutine s3
   real :: a(10,10), b(10,10)
   type y; end type
   integer(8) :: x
-  !ERROR: Index name 'y' conflicts with existing identifier
+  !PORTABILITY: Index variable 'y' should be a scalar object or common block if it is present in the enclosing scope
+  !ERROR: Must have INTEGER type, but is REAL(4)
   forall(x=1:10, y=1:10)
+    !ERROR: Must have INTEGER type, but is REAL(4)
+    !ERROR: Must have INTEGER type, but is REAL(4)
     a(x, y) = b(x, y)
   end forall
-  !ERROR: Index name 'y' conflicts with existing identifier
+  !PORTABILITY: Index variable 'y' should be a scalar object or common block if it is present in the enclosing scope
+  !ERROR: Must have INTEGER type, but is REAL(4)
+  !ERROR: Must have INTEGER type, but is REAL(4)
+  !ERROR: Must have INTEGER type, but is REAL(4)
   forall(x=1:10, y=1:10) a(x, y) = b(x, y)
 end
 
@@ -45,7 +51,7 @@ subroutine s4
     !ERROR: Must have INTEGER type, but is REAL(4)
     a(y) = b(y)
   end forall
-  !ERROR: Index variable 'i' is not scalar
+  !PORTABILITY: Index variable 'i' should be scalar in the enclosing scope
   forall(i=1:10)
     a(i) = b(i)
   end forall
@@ -55,7 +61,9 @@ subroutine s6
   integer, parameter :: n = 4
   real, dimension(n) :: x
   data(x(i), i=1, n) / n * 0.0 /
-  !ERROR: Index name 't' conflicts with existing identifier
+  !PORTABILITY: Index variable 't' should be a scalar object or common block if it is present in the enclosing scope
+  !ERROR: Must have INTEGER type, but is REAL(4)
+  !ERROR: Must have INTEGER type, but is REAL(4)
   forall(t=1:n) x(t) = 0.0
 contains
   subroutine t
diff --git a/flang/test/Semantics/resolve99.f90 b/flang/test/Semantics/resolve99.f90
index a2dd41cefd0e2..e56022b61bfd8 100644
--- a/flang/test/Semantics/resolve99.f90
+++ b/flang/test/Semantics/resolve99.f90
@@ -1,4 +1,4 @@
-! RUN: %python %S/test_errors.py %s %flang_fc1
+! RUN: %python %S/test_errors.py %s %flang_fc1 -pedantic
 ! Tests for the index-name of a FORALL statement
 
 module m1
@@ -31,7 +31,7 @@ subroutine constructAssoc()
     integer, dimension(4) :: table
     integer :: localVar
     associate (assocVar => localVar)
-      ! assocVar is construct associated with localVar
+      !PORTABILITY: Index variable 'assocvar' should be a scalar object or common block if it is present in the enclosing scope
       FORALL (assocVar=1:4) table(assocVar) = 343
     end associate
   end subroutine constructAssoc
@@ -44,7 +44,9 @@ end subroutine commonSub
 
   subroutine mismatch()
     integer, dimension(4) :: table
-    !ERROR: Index name 'typename' conflicts with existing identifier
+    !PORTABILITY: Index variable 'typename' should be a scalar object or common block if it is present in the enclosing scope
+    !ERROR: Must have INTEGER type, but is REAL(4)
+    !ERROR: Must have INTEGER type, but is REAL(4)
     FORALL (typeName=1:4) table(typeName) = 343
   end subroutine mismatch
 end program indexName
diff --git a/flang/test/Semantics/symbol09.f90 b/flang/test/Semantics/symbol09.f90
index 06dd4cdf7d925..98cd1d954c3e7 100644
--- a/flang/test/Semantics/symbol09.f90
+++ b/flang/test/Semantics/symbol09.f90
@@ -25,10 +25,10 @@ subroutine s2
  real a(10)
  !DEF: /s2/i ObjectEntity INTEGER(4)
  integer i
- !DEF: /s2/OtherConstruct1/i ObjectEntity INTEGER(4)
+ !DEF: /s2/Forall1/i ObjectEntity INTEGER(4)
  do concurrent(i=1:10)
   !REF: /s2/a
-  !REF: /s2/OtherConstruct1/i
+  !REF: /s2/Forall1/i
   a(i) = i
  end do
  !REF: /s2/i
@@ -104,14 +104,14 @@ subroutine s6
  integer(kind=8) j
  !DEF: /s6/a ObjectEntity INTEGER(4)
  integer :: a(5) = 1
- !DEF: /s6/OtherConstruct1/i ObjectEntity INTEGER(4)
- !DEF: /s6/OtherConstruct1/j (LocalityLocal) HostAssoc INTEGER(8)
- !DEF: /s6/OtherConstruct1/k (Implicit, LocalityLocalInit) HostAssoc INTEGER(4)
-  !DEF: /s6/OtherConstruct1/a (LocalityShared) HostAssoc INTEGER(4)
+ !DEF: /s6/Forall1/i ObjectEntity INTEGER(4)
+ !DEF: /s6/Forall1/j (LocalityLocal) HostAssoc INTEGER(8)
+ !DEF: /s6/Forall1/k (Implicit, LocalityLocalInit) HostAssoc INTEGER(4)
+ !DEF: /s6/Forall1/a (LocalityShared) HostAssoc INTEGER(4)
  do concurrent(integer::i=1:5)local(j)local_init(k)shared(a)
-  !REF: /s6/OtherConstruct1/a
-  !REF: /s6/OtherConstruct1/i
-  !REF: /s6/OtherConstruct1/j
+  !REF: /s6/Forall1/a
+  !REF: /s6/Forall1/i
+  !REF: /s6/Forall1/j
   a(i) = j+1
  end do
 end subroutine