[flang] Fix crash resolving interface procedure type.

flang/include/flang/Semantics/symbol.h

@@ -1126,6 +1126,9 @@ inline const DeclTypeSpec *Symbol::GetTypeImpl(int depth) const {
           [&](const HostAssocDetails &x) {
             return x.symbol().GetTypeImpl(depth);
           },
+          [&](const GenericDetails &x) {
+            return x.specific() ? x.specific()->GetTypeImpl(depth) : nullptr;
+          },
           [](const auto &) -> const DeclTypeSpec * { return nullptr; },
       },
       details_);

flang/test/Semantics/Inputs/generic-shadows-specific-a.f90

@@ -0,0 +1,21 @@
+! these modules must be read from  module files
+module m1
+  interface f ! reference must be to generic
+    module procedure f ! must have same name as generic interface
+  end interface
+ contains
+  character function f() ! must be character
+    f = 'q'
+  end
+end
+module m2
+   use m1
+end
+module m3
+   use m2 ! must be m2, not m1
+ contains
+   subroutine mustExist() ! not called, but must exist
+     character x
+     x = f()
+   end
+end

flang/test/Semantics/generic-shadows-specific-b.f90

@@ -0,0 +1,13 @@
+! Check that expected code produced with no crash.
+subroutine reproducer()
+  use m2
+  use m3
+  character x
+  x = f()
+end
+
+! RUN: %flang_fc1 -fsyntax-only %S/Inputs/generic-shadows-specific-a.f90
+! RUN: bbc -emit-fir -o - %s | FileCheck %s
+
+! CHECK-LABEL: func.func @_QPreproducer
+! CHECK: fir.call @_QMm1Pf