[CSGen] Interpolations: Collect all vars referenced in the body to preserve connection to context

Since "tap" bodies are now type-checked together with the context,
it's imperative that the variable that represents an interpolation
never gets disconnected from its context in the constraint system,
otherwise it wouldn't be possible to determine types for the
references.

Author: xedin

lib/Sema/CSGen.cpp

@@ -1304,14 +1304,26 @@ struct VarRefCollector : public ASTWalker {
 
         SmallVector<TypeVariableType *, 2> referencedVars;
 
-        // If tap expression is located in a closure, let's connect them
-        // because interpolation could use parameters.
         if (auto *tap = getAsExpr<TapExpr>(appendingExpr)) {
           auto *tapDC = tap->getVar()->getDeclContext();
+          // If tap expression is located in a closure, let's
+          // connect them because interpolation could use parameters.
           if (auto *closure = dyn_cast<ClosureExpr>(tapDC)) {
             referencedVars.push_back(
                 CS.getType(closure)->castTo<TypeVariableType>());
           }
+
+          // Collect all of the variable references that appear
+          // in the tap body, otherwise tap expression is going
+          // to get disconnected from the context.
+          if (auto *body = tap->getBody()) {
+            VarRefCollector refCollector(CS);
+
+            body->walk(refCollector);
+
+            referencedVars.append(refCollector.varRefs.begin(),
+                                  refCollector.varRefs.end());
+          }
         }
 
         // Must be Conversion; if it's Equal, then in semi-rare cases, the