From 1b2e89b4fce76f7a0250f199ba9c2dc99aa84bc1 Mon Sep 17 00:00:00 2001
From: Peter Klausler <pklausler@nvidia.com>
Date: Tue, 21 Oct 2025 14:21:19 -0700
Subject: [PATCH] [flang] Adjust needless warning

When an external procedure has an explicit interface in one scope,
and an implicit interface in another, and there's at least one call
to it from which dummy argument information can be inferred, don't
emit a warning about potential incompatibility if the only difference
in their characteristics is that one of their interfaces was implicit.
---
 flang/lib/Semantics/check-declarations.cpp |  2 +-
 flang/test/Semantics/bug1491.f90           | 21 +++++++++++++++++++++
 flang/test/Semantics/null-init.f90         |  2 +-
 3 files changed, 23 insertions(+), 2 deletions(-)
 create mode 100644 flang/test/Semantics/bug1491.f90

diff --git a/flang/lib/Semantics/check-declarations.cpp b/flang/lib/Semantics/check-declarations.cpp
index 31e246cf0ab03..3e21bc6f218a7 100644
--- a/flang/lib/Semantics/check-declarations.cpp
+++ b/flang/lib/Semantics/check-declarations.cpp
@@ -1781,7 +1781,7 @@ void CheckHelper::CheckExternal(const Symbol &symbol) {
         if (auto previousChars{Characterize(previous)}) {
           std::string whyNot;
           if (!chars->IsCompatibleWith(*previousChars,
-                  /*ignoreImplicitVsExplicit=*/false, &whyNot)) {
+                  /*ignoreImplicitVsExplicit=*/true, &whyNot)) {
             if (auto *msg{Warn(common::UsageWarning::ExternalInterfaceMismatch,
                     "The external interface '%s' is not compatible with an earlier definition (%s)"_warn_en_US,
                     symbol.name(), whyNot)}) {
diff --git a/flang/test/Semantics/bug1491.f90 b/flang/test/Semantics/bug1491.f90
new file mode 100644
index 0000000000000..ccc6eea31a1d4
--- /dev/null
+++ b/flang/test/Semantics/bug1491.f90
@@ -0,0 +1,21 @@
+!RUN: %python %S/test_errors.py %s %flang_fc1 -Werror -pedantic
+module m
+  interface
+    integer function foo1()
+    end function
+    integer function foo2(j)
+    end function
+    integer function foo3()
+    end function
+  end interface
+end module
+
+subroutine test()
+  integer, external :: foo1
+!WARNING: The external interface 'foo2' is not compatible with an earlier definition (distinct numbers of dummy arguments) [-Wexternal-interface-mismatch]
+  integer, external :: foo2
+  integer, external :: foo3
+  call bar(foo1())
+  call bar(foo2())
+  call baz(foo3)
+end subroutine
diff --git a/flang/test/Semantics/null-init.f90 b/flang/test/Semantics/null-init.f90
index d01ad75a75a11..a88cefe136a5e 100644
--- a/flang/test/Semantics/null-init.f90
+++ b/flang/test/Semantics/null-init.f90
@@ -37,7 +37,7 @@ module m6
 
 module m7
   interface
-    !WARNING: The external interface 'null' is not compatible with an earlier definition (incompatible procedure attributes: ImplicitInterface) [-Wexternal-interface-mismatch]
+    !WARNING: The external interface 'null' is not compatible with an earlier definition (function results have incompatible attributes) [-Wexternal-interface-mismatch]
     function null() result(p)
       integer, pointer :: p
     end function