[flang] Fix bug merging generics on USE

When we use-associate the same generic name from two different modules
they are merged together. If the same specific procedure occurs in both
generics it should only occur once in the merged one.

Similarly, it's not an error if they both have a derived type or
specific procedure named the same as the generic, as long as they are
the same symbol.

Fixes flang-compiler/f18#733.

Original-commit: flang-compiler/f18@d37db07691
Reviewed-on: https://github.com/flang-compiler/f18/pull/741
Tree-same-pre-rewrite: false

flang/lib/semantics/symbol.cc

@@ -179,15 +179,19 @@ Symbol *GenericDetails::CheckSpecific() {
 
 void GenericDetails::CopyFrom(const GenericDetails &from) {
   if (from.specific_) {
-    CHECK(!specific_);
+    CHECK(!specific_ || specific_ == from.specific_);
     specific_ = from.specific_;
   }
   if (from.derivedType_) {
-    CHECK(!derivedType_);
+    CHECK(!derivedType_ || derivedType_ == from.derivedType_);
     derivedType_ = from.derivedType_;
   }
-  auto &procs{from.specificProcs_};
-  specificProcs_.insert(specificProcs_.end(), procs.begin(), procs.end());
+  for (const Symbol *symbol : from.specificProcs_) {
+    auto it{std::find(specificProcs_.begin(), specificProcs_.end(), symbol)};
+    if (it == specificProcs_.end()) {
+      specificProcs_.push_back(symbol);
+    }
+  }
 }
 
 // The name of the kind of details for this symbol.

flang/test/semantics/resolve17.f90

@@ -175,3 +175,31 @@ subroutine s8
   !ERROR: Reference to 'g' is ambiguous
   g = 1
 end
+
+module m9a
+  interface g
+    module procedure s1
+    module procedure g
+  end interface
+contains
+  subroutine g()
+  end
+  subroutine s1(x)
+    integer :: x
+  end
+end module
+module m9b
+  use m9a
+  interface g
+    module procedure s2
+  end interface
+contains
+  subroutine s2(x)
+    real :: x
+  end
+end module
+! Merge use-associated generics that have the same symbol (s1)
+subroutine s9
+  use m9a
+  use m9b
+end