diff --git a/flang/docs/Extensions.md b/flang/docs/Extensions.md
--- a/flang/docs/Extensions.md
+++ b/flang/docs/Extensions.md
@@ -79,6 +79,11 @@
   kind 4, because the grammar of Fortran expressions parses it as a
   negation of a literal constant, not a negative literal constant.
   This compiler accepts it with a portability warning.
+* Construct names like `loop` in `loop: do j=1,n` are defined to
+  be "local identifiers" and should be distinct in the "inclusive
+  scope" -- i.e., not scoped by `BLOCK` constructs.
+  As most (but not all) compilers implement `BLOCK` scoping of construct
+  names, so does f18, with a portability warning.
 
 ## Extensions, deletions, and legacy features supported by default
 
diff --git a/flang/lib/Semantics/resolve-names.cpp b/flang/lib/Semantics/resolve-names.cpp
--- a/flang/lib/Semantics/resolve-names.cpp
+++ b/flang/lib/Semantics/resolve-names.cpp
@@ -553,6 +553,7 @@
   // Search for name in a derived type scope and its parents.
   Symbol *FindInTypeOrParents(const Scope &, const parser::Name &);
   Symbol *FindInTypeOrParents(const parser::Name &);
+  Symbol *FindInScopeOrBlockConstructs(const Scope &, SourceName);
   Symbol *FindSeparateModuleProcedureInterface(const parser::Name &);
   void EraseSymbol(const parser::Name &);
   void EraseSymbol(const Symbol &symbol) { currScope().erase(symbol.name()); }
@@ -2369,6 +2370,20 @@
 Symbol *ScopeHandler::FindInTypeOrParents(const parser::Name &name) {
   return FindInTypeOrParents(currScope(), name);
 }
+Symbol *ScopeHandler::FindInScopeOrBlockConstructs(
+    const Scope &scope, SourceName name) {
+  if (Symbol * symbol{FindInScope(scope, name)}) {
+    return symbol;
+  }
+  for (const Scope &child : scope.children()) {
+    if (child.kind() == Scope::Kind::BlockConstruct) {
+      if (Symbol * symbol{FindInScopeOrBlockConstructs(child, name)}) {
+        return symbol;
+      }
+    }
+  }
+  return nullptr;
+}
 
 void ScopeHandler::EraseSymbol(const parser::Name &name) {
   currScope().erase(name.source);
@@ -6556,8 +6571,19 @@
 }
 
 bool ConstructVisitor::CheckDef(const std::optional<parser::Name> &x) {
-  if (x) {
-    MakeSymbol(*x, MiscDetails{MiscDetails::Kind::ConstructName});
+  if (x && !x->symbol) {
+    // Construct names are not scoped by BLOCK in the standard, but many,
+    // but not all, compilers do treat them as if they were so scoped.
+    if (Symbol * inner{FindInScope(currScope(), *x)}) {
+      SayAlreadyDeclared(*x, *inner);
+    } else {
+      if (Symbol *
+          other{FindInScopeOrBlockConstructs(InclusiveScope(), x->source)}) {
+        SayWithDecl(*x, *other,
+            "The construct name '%s' should be distinct at the subprogram level"_port_en_US);
+      }
+      MakeSymbol(*x, MiscDetails{MiscDetails::Kind::ConstructName});
+    }
   }
   return true;
 }
diff --git a/flang/test/Semantics/OpenACC/acc-branch.f90 b/flang/test/Semantics/OpenACC/acc-branch.f90
--- a/flang/test/Semantics/OpenACC/acc-branch.f90
+++ b/flang/test/Semantics/OpenACC/acc-branch.f90
@@ -53,6 +53,7 @@
   ! Exit branches out of parallel construct, attached to an OpenACC parallel construct.
   thisblk: BLOCK
     fortname: if (.true.) then
+      !PORTABILITY: The construct name 'name1' should be distinct at the subprogram level
       name1: do k = 1, N
         !$acc parallel
         !ERROR: EXIT to construct 'fortname' outside of PARALLEL construct is not allowed