[flang] Fine-tune function result equivalence checking

[klausler]

When a separate module function's definition has a redundant interface -- it's defined with MODULE FUNCTION, not MODULE PROCEDURE -- the check for result type equivalence needs to allow for character lengths that are the results of specification expressions. At present, identical-looking length specification expression don't compare equal, since they can refer to distinct dummy argument symbols. Ensure just that they are both constant or not, and if constant, that the lengths have the same value.

[llvmbot]

@llvm/pr-subscribers-flang-semantics

Author: Peter Klausler (klausler)

Changes

When a separate module function's definition has a redundant interface -- it's defined with MODULE FUNCTION, not MODULE PROCEDURE -- the check for result type equivalence needs to allow for character lengths that are the results of specification expressions. At present, identical-looking length specification expression don't compare equal, since they can refer to distinct dummy argument symbols. Ensure just that they are both constant or not, and if constant, that the lengths have the same value.

---

Full diff: https://github.com/llvm/llvm-project/pull/70260.diff

4 Files Affected:

• (modified) flang/lib/Evaluate/characteristics.cpp (+12-6)
• (modified) flang/lib/Semantics/check-declarations.cpp (+8-5)
• (modified) flang/test/Semantics/separate-mp02.f90 (+15-2)
• (modified) flang/test/Semantics/separate-mp03.f90 (+1-1)

diff --git a/flang/lib/Evaluate/characteristics.cpp b/flang/lib/Evaluate/characteristics.cpp
index ac61e72f428a97e..bce0ec3f080b51f 100644
--- a/flang/lib/Evaluate/characteristics.cpp
+++ b/flang/lib/Evaluate/characteristics.cpp
@@ -1060,12 +1060,18 @@ bool FunctionResult::IsCompatibleWith(
                 actual.IsAssumedLengthCharacter()) {
               return true;
             } else {
-              const auto *ifaceLenParam{
-                  ifaceTypeShape->type().charLengthParamValue()};
-              const auto *actualLenParam{
-                  actualTypeShape->type().charLengthParamValue()};
-              if (ifaceLenParam && actualLenParam &&
-                  *ifaceLenParam == *actualLenParam) {
+              auto len{ToInt64(ifaceTypeShape->LEN())};
+              auto actualLen{ToInt64(actualTypeShape->LEN())};
+              if (len.has_value() != actualLen.has_value()) {
+                if (whyNot) {
+                  *whyNot = "constant-length vs non-constant-length character "
+                            "results";
+                }
+              } else if (len && *len != *actualLen) {
+                if (whyNot) {
+                  *whyNot = "character results with distinct lengths";
+                }
+              } else {
                 return true;
               }
             }
diff --git a/flang/lib/Semantics/check-declarations.cpp b/flang/lib/Semantics/check-declarations.cpp
index 2c2866d590ae5a4..3fcef83595d693f 100644
--- a/flang/lib/Semantics/check-declarations.cpp
+++ b/flang/lib/Semantics/check-declarations.cpp
@@ -3285,11 +3285,14 @@ void SubprogramMatchHelper::Check(
     Say(symbol1, symbol2,
         "Module subprogram '%s' and its corresponding interface body are not both BIND(C)"_err_en_US);
   }
-  if (proc1->functionResult && proc2->functionResult &&
-      *proc1->functionResult != *proc2->functionResult) {
-    Say(symbol1, symbol2,
-        "Return type of function '%s' does not match return type of"
-        " the corresponding interface body"_err_en_US);
+  if (proc1->functionResult && proc2->functionResult) {
+    std::string whyNot;
+    if (!proc1->functionResult->IsCompatibleWith(
+            *proc2->functionResult, &whyNot)) {
+      Say(symbol1, symbol2,
+          "Result of function '%s' is not compatible with the result of the corresponding interface body: %s"_err_en_US,
+          whyNot);
+    }
   }
   for (int i{0}; i < nargs1; ++i) {
     const Symbol *arg1{args1[i]};
diff --git a/flang/test/Semantics/separate-mp02.f90 b/flang/test/Semantics/separate-mp02.f90
index fd9c4c3cc18f98b..9eeca0886f93dc4 100644
--- a/flang/test/Semantics/separate-mp02.f90
+++ b/flang/test/Semantics/separate-mp02.f90
@@ -272,10 +272,10 @@ module function f3()
   !OK
   real module function f1()
   end
-  !ERROR: Return type of function 'f2' does not match return type of the corresponding interface body
+  !ERROR: Result of function 'f2' is not compatible with the result of the corresponding interface body: function results have distinct types: INTEGER(4) vs REAL(4)
   integer module function f2()
   end
-  !ERROR: Return type of function 'f3' does not match return type of the corresponding interface body
+  !ERROR: Result of function 'f3' is not compatible with the result of the corresponding interface body: function results have incompatible attributes
   module function f3()
     real :: f3
     pointer :: f3
@@ -334,3 +334,16 @@ module subroutine sub2(s)
     character(len=1) s
   end subroutine
 end submodule
+
+module m10
+  interface
+    module character(2) function f()
+    end function
+  end interface
+end module
+submodule(m10) sm10
+ contains
+  !ERROR: Result of function 'f' is not compatible with the result of the corresponding interface body: function results have distinct types: CHARACTER(KIND=1,LEN=3_8) vs CHARACTER(KIND=1,LEN=2_8)
+  module character(3) function f()
+  end function
+end submodule
diff --git a/flang/test/Semantics/separate-mp03.f90 b/flang/test/Semantics/separate-mp03.f90
index 33bf1cf8e414fd5..f6c02c5add3fdaa 100644
--- a/flang/test/Semantics/separate-mp03.f90
+++ b/flang/test/Semantics/separate-mp03.f90
@@ -81,7 +81,7 @@ integer module function f1(x)
   !ERROR: 'notf2' was not declared a separate module procedure
   module procedure notf2
   end procedure
-  !ERROR: Return type of function 'f3' does not match return type of the corresponding interface body
+  !ERROR: Result of function 'f3' is not compatible with the result of the corresponding interface body: function results have distinct types: REAL(4) vs INTEGER(4)
   module function f3(x) result(res)
     real :: res
     real, intent(in) :: x

[clementval]

LGTM