From fe55e42eff2466d1fffae30cf24037636f44b04c Mon Sep 17 00:00:00 2001
From: Paul Scoropan <1paulscoropan@gmail.com>
Date: Mon, 6 Feb 2023 19:09:10 +0000
Subject: [PATCH] [Flang] [Semantics] More descript warning message for same
 named external statements and interfaces

For issue: https://github.com/llvm/llvm-project/issues/56605. Previous error messages are not descriptive of problem. Issuing warnings instead that do not halt compilation but offer better description of problem.

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D143426
---
 flang/lib/Semantics/resolve-names.cpp | 31 +++++++++++++++++++++------
 flang/test/Semantics/resolve20.f90    |  2 +-
 2 files changed, 25 insertions(+), 8 deletions(-)

diff --git a/flang/lib/Semantics/resolve-names.cpp b/flang/lib/Semantics/resolve-names.cpp
index f4d3d88abff14..12e4f4dd2d994 100644
--- a/flang/lib/Semantics/resolve-names.cpp
+++ b/flang/lib/Semantics/resolve-names.cpp
@@ -3911,11 +3911,19 @@ bool SubprogramVisitor::HandlePreviousCalls(
     // ENTRY's name.  We have to replace that symbol in situ to avoid the
     // obligation to rewrite symbol pointers in the parse tree.
     if (!symbol.test(subpFlag)) {
-      Say2(name,
-          subpFlag == Symbol::Flag::Function
-              ? "'%s' was previously called as a subroutine"_err_en_US
-              : "'%s' was previously called as a function"_err_en_US,
-          symbol, "Previous call of '%s'"_en_US);
+      // External statements issue an explicit EXTERNAL attribute.
+      if (symbol.attrs().test(Attr::EXTERNAL) &&
+          !symbol.implicitAttrs().test(Attr::EXTERNAL)) {
+        // Warn if external statement previously declared.
+        Say(name,
+            "EXTERNAL attribute was already specified on '%s'"_warn_en_US);
+      } else {
+        Say2(name,
+            subpFlag == Symbol::Flag::Function
+                ? "'%s' was previously called as a subroutine"_err_en_US
+                : "'%s' was previously called as a function"_err_en_US,
+            symbol, "Previous call of '%s'"_en_US);
+      }
     }
     EntityDetails entity;
     if (proc->type()) {
@@ -4333,8 +4341,17 @@ bool DeclarationVisitor::Pre(const parser::ExternalStmt &x) {
   for (const auto &name : x.v) {
     auto *symbol{FindSymbol(name)};
     if (!ConvertToProcEntity(DEREF(symbol))) {
-      SayWithDecl(
-          name, *symbol, "EXTERNAL attribute not allowed on '%s'"_err_en_US);
+      // Check if previous symbol is an interface.
+      if (auto *details{symbol->detailsIf<SubprogramDetails>()}) {
+        if (details->isInterface()) {
+          // Warn if interface previously declared.
+          Say(name,
+              "EXTERNAL attribute was already specified on '%s'"_warn_en_US);
+        }
+      } else {
+        SayWithDecl(
+            name, *symbol, "EXTERNAL attribute not allowed on '%s'"_err_en_US);
+      }
     } else if (symbol->attrs().test(Attr::INTRINSIC)) { // C840
       Say(symbol->name(),
           "Symbol '%s' cannot have both INTRINSIC and EXTERNAL attributes"_err_en_US,
diff --git a/flang/test/Semantics/resolve20.f90 b/flang/test/Semantics/resolve20.f90
index b1d73341b57b1..a3d240dbc5982 100644
--- a/flang/test/Semantics/resolve20.f90
+++ b/flang/test/Semantics/resolve20.f90
@@ -48,7 +48,7 @@ subroutine forward
   external :: a, b, c, d
   !ERROR: EXTERNAL attribute not allowed on 'm'
   external :: m
-  !ERROR: EXTERNAL attribute not allowed on 'foo'
+  !WARNING: EXTERNAL attribute was already specified on 'foo'
   external :: foo
   !ERROR: EXTERNAL attribute not allowed on 'bar'
   external :: bar