SIL: Walk into the thrown error type when computing substituted generic signature

Author: slavapestov

lib/SIL/IR/AbstractionPattern.cpp

@@ -2597,14 +2597,26 @@ class SubstFunctionTypePatternVisitor
     if (yieldType) {
       substYieldType = visit(yieldType, yieldPattern);
     }
-    
+
+    CanType newErrorType;
+    if (auto optErrorType = func->getEffectiveThrownErrorType()) {
+      newErrorType = visit((*optErrorType)->getCanonicalType(),
+                           pattern.getFunctionThrownErrorType());
+    }
+
     auto newResultTy = visit(func.getResult(),
                              pattern.getFunctionResultType());
 
     llvm::Optional<FunctionType::ExtInfo> extInfo;
     if (func->hasExtInfo())
       extInfo = func->getExtInfo();
-    
+
+    if (newErrorType) {
+      if (!extInfo)
+        extInfo = FunctionType::ExtInfo();
+      extInfo = extInfo->withThrows(true, newErrorType);
+    }
+
     return CanFunctionType::get(FunctionType::CanParamArrayRef(newParams),
                                 newResultTy, extInfo);
   }