[flang] fix ICE with ignore_tkr(tk) character in explicit interface (#140885)

Some MPI libraries use character dummies + ignore(TKR) to allow passing
any kind of buffer.

This was meant to already be handled by #108168
However, when the library interface also had an argument requiring an
explicit interface, `builder.convertWithSemantics` was not allowed to properly deal
with the actual/dummy type mismatch and generated bad IR causing errors like:
`'fir.convert' op invalid type conversion'!fir.ref' / '!fir.boxchar\<1\>'`.

This restriction was artificial, lowering should just handle any cases
allowed by semantics. Just remove it.

flang/lib/Lower/ConvertCall.cpp

@@ -486,7 +486,6 @@ Fortran::lower::genCallOpAndResult(
 
   // Deal with potential mismatches in arguments types. Passing an array to a
   // scalar argument should for instance be tolerated here.
-  bool callingImplicitInterface = caller.canBeCalledViaImplicitInterface();
   for (auto [fst, snd] : llvm::zip(caller.getInputs(), funcType.getInputs())) {
     // When passing arguments to a procedure that can be called by implicit
     // interface, allow any character actual arguments to be passed to dummy
@@ -518,10 +517,17 @@ Fortran::lower::genCallOpAndResult(
         // Do not attempt any reboxing here that could break this.
         bool legacyLowering =
             !converter.getLoweringOptions().getLowerToHighLevelFIR();
+        // When dealing with a dummy character argument (fir.boxchar), the
+        // effective argument might be a non-character raw pointer. This may
+        // happen when calling an implicit interface that was previously called
+        // with a character argument, or when calling an explicit interface with
+        // an IgnoreTKR dummy character arguments. Allow creating a fir.boxchar
+        // from the raw pointer, which requires a non-trivial type conversion.
+        const bool allowCharacterConversions = true;
         bool isVolatile = fir::isa_volatile_type(snd);
         cast = builder.createVolatileCast(loc, isVolatile, fst);
         cast = builder.convertWithSemantics(loc, snd, cast,
-                                            callingImplicitInterface,
+                                            allowCharacterConversions,
                                             /*allowRebox=*/legacyLowering);
       }
     }
@@ -1446,6 +1452,8 @@ static PreparedDummyArgument preparePresentUserCallActualArgument(
   // cause the fir.if results to be assumed-rank in case of OPTIONAL dummy,
   // causing extra runtime costs due to the unknown runtime size of assumed-rank
   // descriptors.
+  // For TKR dummy characters, the boxchar creation also happens later when
+  // creating the fir.call .
   preparedDummy.dummy =
       builder.createConvert(loc, dummyTypeWithActualRank, addr);
   return preparedDummy;

flang/test/Lower/HLFIR/ignore-type-f77-character.f90

@@ -8,6 +8,13 @@ module test_char_tk
     !dir$ ignore_tkr(tkrdm) c
     end subroutine
   end interface
+  interface
+    subroutine foo_requires_explicit_interface(c, i)
+    character(1)::c(*)
+    !dir$ ignore_tkr(tkrdm) c
+    integer, optional :: i
+    end subroutine
+  end interface
 contains
   subroutine test_normal()
     character(1) :: c(10)
@@ -32,4 +39,14 @@ contains
 !CHECK:           %[[VAL_5:.*]] = fir.convert %{{.*}} : (!fir.ref<!fir.array<10xf32>>) -> !fir.ref<!fir.char<1,?>>
 !CHECK:           %[[VAL_6:.*]] = fir.emboxchar %[[VAL_5]], %c0{{.*}}: (!fir.ref<!fir.char<1,?>>, index) -> !fir.boxchar<1>
 !CHECK:           fir.call @_QPfoo(%[[VAL_6]]) fastmath<contract> : (!fir.boxchar<1>) -> ()
+
+  subroutine test_requires_explicit_interface(x, i)
+    real :: x(10)
+    integer :: i
+    call foo_requires_explicit_interface(x, i)
+  end subroutine
+!CHECK-LABEL:   func.func @_QMtest_char_tkPtest_requires_explicit_interface(
+!CHECK:           %[[VAL_5:.*]] = fir.convert %{{.*}} : (!fir.ref<!fir.array<10xf32>>) -> !fir.ref<!fir.char<1,?>>
+!CHECK:           %[[VAL_6:.*]] = fir.emboxchar %[[VAL_5]], %c0{{.*}}: (!fir.ref<!fir.char<1,?>>, index) -> !fir.boxchar<1>
+!CHECK:           fir.call @_QPfoo_requires_explicit_interface(%[[VAL_6]], %{{.*}})
 end module