[flang] Fine-tune function result equivalence checking (#70260)

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.

Author: klausler

flang/lib/Evaluate/characteristics.cpp

@@ -1069,13 +1069,32 @@ 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) {
-                return true;
+              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 {
+                const auto *ifaceLenParam{
+                    ifaceTypeShape->type().charLengthParamValue()};
+                const auto *actualLenParam{
+                    actualTypeShape->type().charLengthParamValue()};
+                if (ifaceLenParam && actualLenParam &&
+                    ifaceLenParam->isExplicit() !=
+                        actualLenParam->isExplicit()) {
+                  if (whyNot) {
+                    *whyNot =
+                        "explicit-length vs deferred-length character results";
+                  }
+                } else {
+                  return true;
+                }
               }
             }
           }

flang/lib/Semantics/check-declarations.cpp

@@ -3289,11 +3289,14 @@ 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]};

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

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